Keys to the Study of Chemistry
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Keys to the Study of Chemistry. Ashton T. Griffin Wayne Community College Chapter 1.1-1.6 in Silberberg 5 th and 6 th editions. Goals & Objectives. The student will be able to identify the name and symbol of the first 36 elements on the periodic table. (I-1)

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Keys to the study of chemistry

Keys to the Study of Chemistry

  • Ashton T. Griffin

  • Wayne Community College

  • Chapter 1.1-1.6 in Silberberg 5th and 6th editions.


Keys to the study of chemistry

Goals & Objectives

  • The student will be able to identify the name and symbol of the first 36 elements on the periodic table. (I-1)

  • The student will understand the common units of length, volume, mass, and temperature and their numerical prefixes. (1.5)


Keys to the study of chemistry

Goals & Objectives

  • The student will understand the meaning of uncertainty in measurements and the use of significant figures and rounding. (1.6)

  • The student will understand the distinction between accuracy and precision and between systematic and random error. (1.6)


Keys to the study of chemistry

Master these Skills

  • The student will be able to:

  • Use conversion factors in calculations (1.4; SP 1.3-1.5)

  • Find the density from mass and volume (SP 1.6)

  • Convert between the Kelvin, Celsius, and Fahrenheit temperature scales (SP 1.7)


Keys to the study of chemistry

Master these Skills

  • The student will be able to:

  • Determine the number of significant figures (SP 1.8) and rounding to the correct number of digits. (SP 1.9)


Keys to the study of chemistry

Chemistry

Chemistry is the study of matter,

its properties,

the changes that matter undergoes,

and

the energy associated with these changes.


Keys to the study of chemistry

Definitions

Matter

anything that has both mass and volume

- the “stuff” of the universe: books, planets, trees, professors, students

Composition

the types and amounts of simpler substances that make up a sample of matter

Properties

the characteristics that give each substance a unique identity


Keys to the study of chemistry

Physical Properties

properties a substance shows by itself without interacting with another substance

- color, melting point, boiling point, density

Chemical Properties

properties a substance shows as it interacts with, or transforms into, other substances

- flammability, corrosiveness


Keys to the study of chemistry

Figure 1.1

The distinction between physical and chemical change.


Keys to the study of chemistry

Table 1.1 Some Characteristic Properties of Copper


Keys to the study of chemistry

Elements

  • The simplest forms of matter

  • Cannot be separated by chemical means into simpler stable substances

  • Represented by symbols on the Periodic Table

  • Learn the names and symbols for first 36 elements (I-1)


Keys to the study of chemistry

The States of Matter

A solid has a fixed shape and volume. Solids may be hard or soft, rigid or flexible.

A liquid has a varying shape that conforms to the shape of the container, but a fixed volume. A liquid has an upper surface.

A gas has no fixed shape or volume and therefore does not have a surface.


Figure 1 2

Figure 1.2

The physical states of matter.


Keys to the study of chemistry

Temperature and Change of State

  • A change of state is a physical change.

    • Physical form changes, composition does not.

  • Changes in physical state are reversible

    • by changing the temperature.

  • A chemical change cannot simply be reversed by a change in temperature.


Keys to the study of chemistry

PROBLEM:

Decide whether each of the following processes is primarily a physical or a chemical change, and explain briefly:

(a) Frost forms as the temperature drops on a humid winter night.

(b) A cornstalk grows from a seed that is watered and fertilized.

(c) A match ignites to form ash and a mixture of gases.

(d) Perspiration evaporates when you relax after jogging.

(e) A silver fork tarnishes slowly in air.

PLAN:

“Does the substance change composition or just change form?”

Sample Problem 1.2

Distinguishing Between Physical and Chemical Change


Keys to the study of chemistry

Sample Problem 1.2

SOLUTION:

(a) Frost forms as the temperature drops on a humid winter night.

physical change

(b) A cornstalk grows from a seed that is watered and fertilized.

chemical change

(c) A match ignites to form ash and a mixture of gases.

chemical change

(d) Perspiration evaporates when you relax after jogging.

physical change

(e) A silver fork tarnishes slowly in air.

chemical change


Keys to the study of chemistry

Energy in Chemistry

Energy is the ability to do work.

Potential Energy

is energy due to the position of an object.

Kinetic Energy

is energy due to the movement of an object.

Total Energy = Potential Energy + Kinetic Energy


Keys to the study of chemistry

Energy Changes

Lower energy states are more stable and are favored over higher energy states.

Energy is neither created nor destroyed

– it is conserved

– and can be converted from one form to another.


Keys to the study of chemistry

Observations

Natural phenomena and measured events; can be stated as a natural law if universally consistent.

Hypothesis

Tentative proposal that explains observations.

Hypothesis is revised if experimental results do not support it.

Procedure to test hypothesis; measures one variable at a time.

Experiment

Set of conceptual assumptions that explains data from accumulated experiments; predicts related phenomena.

Model (Theory)

Model is altered if predicted events do not support it.

Further Experiment

Tests predictions based on model

Figure 1.6

The scientific approach to understanding nature.


Keys to the study of chemistry

350 mi

50 mi

=

or 50 mi.h-1

7 h

1 h

Chemical Problem Solving

  • All measured quantities consist of

    • a number and a unit.

  • Units are manipulated like numbers:

    • 3 ft x 4 ft = 12 ft2


Keys to the study of chemistry

1 mi

5280 ft

5280 ft

5280 ft

Conversion Factors

A conversion factoris a ratio of equivalent quantities

used to express a quantity in different units.

The relationship 1 mi = 5280 ft

gives us the conversion factor:

=

= 1


Keys to the study of chemistry

PROBLEM:

The height of the Angel Falls is 3212 ft. Express this quantity in miles (mi) if 1 mi = 5280 ft.

PLAN:

Set up the conversion factor so that ft will cancel and the answer will be in mi.

1 mi

5280 ft

A conversion factor is chosen and set up so that all units cancel except those required for the answer.

SOLUTION:

3212 ft x

= 0.6083 mi


Keys to the study of chemistry

Clarify the known and unknown.

Suggest steps from known to unknown.

Prepare a visual summary of steps that includes conversion factors, equations, known variables.

Systematic Approach to Solving Chemistry Problems

  • State Problem

  • Plan

  • Solution

  • Check

  • Comment

  • Follow-up Problem


Keys to the study of chemistry

PROBLEM:

To wire your stereo equipment, you need 325 centimeters (cm) of speaker wire that sells for $0.15/ft. What is the price of the wire?

PLAN:

We know the length (in cm) of wire and cost per length ($/ft). We have to convert cm to inches and inches to feet. Then we can find the cost for the length in feet.

length (cm) of wire

length (in) of wire

length (ft) of wire

Price ($) of wire

Sample Problem 1.3

Converting Units of Length

2.54 cm = 1 in

12 in = 1 ft

1 ft = $0.15


Keys to the study of chemistry

= 325 cm x

= 128 in

= 128 in x

= 10.7 ft x

= 10.7 ft

1 in

2.54 cm

$ 0.15

1 ft

1 ft

12 in

Sample Problem 1.3

SOLUTION:

Length (in) = length (cm) x conversion factor

Length (ft) = length (in) x conversion factor

Price ($) = length (ft) x conversion factor

= $ 1.60


Keys to the study of chemistry

Table 1. 2 SI Base Units


Keys to the study of chemistry

Units -- Metric System

  • Masskilogram(kg), gram(g)

  • Lengthmeter(m), centimeter(cm)

  • Volumecubic meter(m3),

    cubic centimeter (cm3)

    liter(L) = 1000 cm3 (exact)

    milliliter(mL) = 1 cm3 (exact)

  • Timesecond(s)

  • Temperature Kelvin(K) Celsius (C)


Keys to the study of chemistry

Additional SI Units

  • Current – Ampere

  • Amount of Substance – Mole

  • Luminous Intensity – Candela

  • Four of these units are of particular interest to chemist.


Keys to the study of chemistry

The Second

  • Initially the second was tied to the Earth’s rotation. 1/86,400th of the mean solar day.

  • In 1967, the second was based on the cesium-133 atomic clock.


Keys to the study of chemistry

The Meter

  • In 1791, the meter was defined to be one ten-millionth of the length of the meridian passing through Paris from the equator to the North Pole.

  • In 1889, a platinum-iridium bar was inscribed with two lines – this became the standard for the meter.


Keys to the study of chemistry

The Meter (continued)

  • In 1960, the meter was based on the wavelength of krypton-86 radiation.

  • Finally in 1983, the meter was re-defined as the length traveled by light in exactly 1/299,792,458 of a second.


Keys to the study of chemistry

The Kilogram

  • In 1799, a platinum-iridium cylinder was fabricated to represent the mass of a cubic deciliter of water at 4 C. In new standard was created in 1879. Due to the changing nature its mass, it was suggested in 2005 that the kilogram be redefined in terms of “fixed constants of nature”.


Keys to the study of chemistry

The Kilogram Standard


Keys to the study of chemistry

The Mole

  • Since the 1960’s, the mole has been based on the number of atoms in 12.0 g of carbon-12 or 6.022 x 1023 atoms.

  • New attempts to define the mole include using a new standard Si-28.

  • New attempts will continue.


Keys to the study of chemistry

Table 1.3

Common Decimal Prefixes Used with SI Units


Keys to the study of chemistry

Units-- Metric System

  • Use numerical prefixes for larger or smaller units:

    Mega (M)1000000 times unit (106)

    kilo (k)1000 times unit (103)

    centi (c) 0.01 times unit(10-2)

    milli (m) 0.001 times unit (10-3)

    Micro (µ) 0.000001 times unit (10-6)


Keys to the study of chemistry

Table 1.4 Common SI-English Equivalent Quantities


Keys to the study of chemistry

Figure 1.8

Common laboratory volumetric glassware.


Keys to the study of chemistry

PROBLEM:

A graduated cylinder contains 19.9 mL of water. When a small piece of galena, an ore of lead, is added, it sinks and the volume increases to 24.5 mL. What is the volume of the piece of galena in cm3 and in L?

volume (mL) before and after

subtract

volume (mL) of galena

PLAN:

The volume of the galena is equal to the difference in the volume of the water before and after the addition.

volume (cm3)

of galena

volume (L)

of galena

Sample Problem 1.4

Converting Units of Volume

1 mL = 1 cm3

1 mL = 10-3 L


Keys to the study of chemistry

4.6 mL x

4.6 mL x

1 cm3

1 mL

10-3 L

1 mL

Sample Problem 1.4

SOLUTION:

(24.5 - 19.9) mL = volume of galena = 4.6 mL

= 4.6 cm3

= 4.6 x 10-3 L


Keys to the study of chemistry

PROBLEM:

Many international computer communications are carried out by optical fibers in cables laid along the ocean floor. If one strand of optical fiber weighs 1.19 x 10-3 lb/m, what is the mass (in kg) of a cable made of six strands of optical fiber, each long enough to link New York and Paris (8.94 x 103 km)?

PLAN:

The sequence of steps may vary but essentially we need to find the length of the entire cable and convert it to mass.

length (km) of fiber

length (m) of fiber

mass (lb) of fiber

Mass (kg) of cable

mass (lb) of cable

Sample Problem 1.5

Converting Units of Mass

1 km = 103 m

1 m = 1.19 x 10-3 lb

2.205 lb = 1 kg

6 fibers = 1 cable


Keys to the study of chemistry

8.84 x 103 km x

= 8.84 x 106 m

1.19 x 10-3 lb

1 m

1.05 x 104 lb

1 fiber

6.30 x 104 lb

1 cable

8.84 x 106 m x

= 1.05 x 104 lb

103 m

1 km

1 kg

2.205 lb

6 fibers

1 cable

x

x

= 6.30 x 104 lb/cable

Sample Problem 1.5

SOLUTION:

= 2.86 x 104 kg/cable


Keys to the study of chemistry

Units -- Metric System

  • Numerical Prefixes:

    • 12.5 m = _______ cm

    • 1.35 kg = _______ g

    • 0.0256 mm = _______ µm

    • 89.7 megahertz = _______ hertz

      (1 hertz = 1 cycle per second)


Keys to the study of chemistry

Derived Quantities

  • Frequency (cycles/s, hertz)

  • Density (mass/volume, g/cm3)

  • Speed (distance/time, m/s)

  • Acceleration (distance/(time)2, m/s2)

  • Force (mass x acceleration, kg•m/s2, newton)

  • Pressure (force/area, kg/(m•s2), pascal)

  • Energy (force x distance, kg•m2/s2, joule)


Keys to the study of chemistry

Figure 1.9

Some interesting quantities of length (A), volume (B), and mass (C).


Keys to the study of chemistry

density =

mass

volume

Density

At a given temperature and pressure, the density of a substance is a characteristic physical property and has a specific value.


Keys to the study of chemistry

Table 1.5

Densities of Some Common Substances*

*At room temperature (20°C) and normal atmospheric pressure (1atm).


Keys to the study of chemistry

lengths (mm) of sides

PROBLEM:

Lithium, a soft, gray solid with the lowest density of any metal, is a key component of advanced batteries. A slab of lithium weighs 1.49x103 mg and has sides that are 20.9 mm by 11.1 mm by 11.9 mm. Find the density of lithium in g/cm3.

mass (mg) of Li

lengths (cm) of sides

mass (g) of Li

PLAN:

Density is expressed in g/cm3 so we need the mass in g and the volume in cm3.

density (g/cm3) of Li

volume (cm3)

Sample Problem 1.6

Calculating Density from Mass and Length

10 mm = 1 cm

103 mg = 1 g

multiply lengths

divide mass by volume


Keys to the study of chemistry

1 g

103 mg

1 cm

10 mm

1.49x103 mg x

= 1.49 g

20.9 mm x

= 2.09 cm

density of Li =

1.49 g

2.76 cm3

Sample Problem 1.6

SOLUTION:

Similarly the other sides will be 1.11 cm and 1.19 cm, respectively.

Volume = 2.09 x 1.11 x 1.19 = 2.76 cm3

= 0.540 g/cm3


Keys to the study of chemistry

Dimensional Analysis

  • Derived quantities - Density

    Determine the density of a substance(g/ml) if 742g of it occupies 97.3 cubic centimeters.

    Determine the volume of a liquid having a density of 1.32 g/mL required to have 125 g of the liquid.


Keys to the study of chemistry

Figure 1.10

Some interesting temperatures.


Keys to the study of chemistry

Freezing and boiling points of water in the Celsius, Kelvin (absolute) and Fahrenheit scales.

Figure 1.11


Keys to the study of chemistry

Table 1.6 The Three Temperature Scales


Keys to the study of chemistry

Temperature Scales

Kelvin ( K ) - The “absolute temperature scale” begins at absolute zero and has only positive values. Note that the kelvin is not used with the degree sign (°).

Celsius ( oC ) - The Celsius scale is based on the freezing and boiling points of water. This is the temperature scale used most commonly around the world. The Celsius and Kelvin scales use the same sizedegree although their starting points differ.

Fahrenheit ( oF ) –The Fahrenheit scale is commonly used in the US. The Fahrenheit scale has a different degree size and different zero points than both the Celsius and Kelvin scales.


Keys to the study of chemistry

T (in °F) = T (in °C) + 32

5

T (in °C) = [T (in °F) – 32]

9

9

5

Temperature Conversions

T (in K) = T (in oC) + 273.15

T (in oC) = T (in K) - 273.15


Keys to the study of chemistry

PROBLEM:

A child has a body temperature of 38.7°C, and normal body temperature is 98.6°F. Does the child have a fever? What is the child’s temperature in kelvins?

PLAN:

We have to convert °C to °F to find out if the child has a fever. We can then use the °C to Kelvin relationship to find the temperature in Kelvin.

(38.7 °C) + 32 = 101.7 °F

9

5

Sample Problem 1.7

Converting Units of Temperature

SOLUTION:

Converting from °C to °F

Yes, the child has a fever.

Converting from °C to K

38.7 °C + 273.15 = 311.8 K


Keys to the study of chemistry

Significant Figures

Every measurement includes some uncertainty. The rightmost digit of any quantity is always estimated.

The recorded digits, both certain and uncertain, are called significant figures.

The greater the number of significant figures in a quantity, the greater its certainty.


Keys to the study of chemistry

Figure 1.12

The number of significant figures in a measurement.


Keys to the study of chemistry

Determining Which Digits are Significant

  • Make sure the measured quantity has a decimal point.

  • Start at the left and move right until you reach the first nonzero digit.

  • Count that digit and every digit to its right as significant.

All digits are significant

- except zeros that are used only to position the decimal point.


Keys to the study of chemistry

  • Zeros that end a number are significant

    • whether they occur before or after the decimal point

    • as long as a decimal point is present.

  • 1.030 mL has 4 significant figures.

  • 5300. L has 4 significant figures.

  • If no decimal point is present

    • zeros at the end of the number are not significant.

  • 5300 L has only 2 significant figures.


Keys to the study of chemistry

PROBLEM:

For each of the following quantities, underline the zeros that are significant figures (sf), and determine the number of significant figures in each quantity. For (d) to (f), express each in exponential notation first.

(a) 0.0030 L

(b) 0.1044 g

(c) 53,069 mL

(d) 0.00004715 m

(e) 57,600. s

(f) 0.0000007160 cm3

PLAN:

We determine the number of significant figures by counting digits, paying particular attention to the position of zeros in relation to the decimal point, and underline zeros that are significant.

Sample Problem 1.8

Determining the Number of Significant Figures


Keys to the study of chemistry

Sample Problem 1.8

SOLUTION:

(a) 0.0030 L has 2 sf

(b) 0.1044 g has 4 sf

(c) 53,069 mL has 5 sf

(d) 0.00004715 m = 4.715x10-5 m has 4 sf

(e) 57,600. s = 5.7600x104 s has 5 sf

(f) 0.0000007160 cm3 = 7.160x10-7 cm3 has 4 sf


Keys to the study of chemistry

Rules for Significant Figures in Calculations

1. For multiplication and division. The answer contains the same number of significant figures as there are in the measurement with the fewest significant figures.

Multiply the following numbers:

9.2 cm x 6.8 cm x 0.3744 cm

= 23.4225 cm3 =23cm3


Keys to the study of chemistry

Multiplication and Division of Inexact Numbers

  • The result can have no more sig. figs. than the least number of sig. figs. used to obtain the result.

    4.242 x 1.23 = 5.21766 5.22

    12.24/2.0 = 6.12 6.1


Keys to the study of chemistry

Multiplication and Division of Inexact and Exact Numbers

  • Use of exact conversion factors retains the number of sig figs in the measured (inexact) value.

    22.36 inches x 2.54 centimeters per inch= 56.80 centimeters

  • Conversion factors involving powers of ten are always exact.

    1 kilometer = 1000 meters

    3.5 kilometers = 3.5 x 103 meters


Keys to the study of chemistry

83.5 mL

+ 23.28 mL

865.9 mL

- 2.8121 mL

Rules for Significant Figures in Calculations

2. For addition and subtraction. The answer has the same number of decimal places as there are in the measurement with the fewest decimal places.

Example: adding two volumes

106.78 mL = 106.8 mL

Example: subtracting two volumes

863.0879 mL = 863.1 mL


Keys to the study of chemistry

Addition and Subtraction of Inexact Numbers

  • Result will have a digit as far to the right as all the numbers have a digit in common

    2.02 8.7397

    1.234-2.123

    + 3.6923 6.6167

    6.9463

    6.95 6.617


Keys to the study of chemistry

Rules for Rounding Off Numbers

1.If the digit removed is more than 5,the preceding number increases by 1.

5.379 rounds to 5.38 if 3 significant figures are retained.

2.If the digit removed is less than 5, the preceding number is unchanged.

0.2413 rounds to 0.241 if 3 significant figures are retained.


Keys to the study of chemistry

3.If the digit removed is 5 followed by zeros or with no following digits, the preceding number increases by 1 if it is odd and remains unchanged if it is even.

17.75 rounds to 17.8, but 17.65 rounds to 17.6.

If the 5 is followed by other nonzero digits, rule 1 is followed:

17.6500 rounds to 17.6, but 17.6513 rounds to 17.7

4. Be sure to carry two or more additional significant figures through a multistep calculation and round off the finalanswer only.


Keys to the study of chemistry

Rounding Off Numbers

  • Rule 1

  • If the first digit to be dropped is less than 5, that digit and all the digits that follow it are simply dropped.

  • Thus, 62.312 rounded off to 3 significant figures become 62.3.


Keys to the study of chemistry

Rounding Off Numbers

  • Rule 2

  • If the first digit to be dropped is a digit greater than 5, or a 5 followed by digits other than all zeros, the excess digits are all dropped and the last retained digit is increased in value by one unit.


Keys to the study of chemistry

Example of Rule 2

  • Thus 62.782 and 62.558 rounded off to 3 significant figures become, respectively, 62.8 and 62.6.


Keys to the study of chemistry

Rounding Off Numbers

  • Rule 3

  • If the first digit to be dropped is a 5 not followed by any other digit or a 5 followed only by zeros, an odd-even rule applies. Drop the 5 and any zeros that follow it and then:

  • Increase the last retained digit by one unit if it is odd and leave the last retained digit the same if it is even.


Keys to the study of chemistry

Example of Rule 3

  • Thus, 62.650 and 62.350 rounded to 3 significant figures become, respectively, 62.6 (even rule) and 62.4 (odd rule). The number zero as a last retained digit is always considered an even number; thus, 62.050 rounded to 3 significant figures becomes 62.0.


Keys to the study of chemistry

Rounding

  • Round each of the following numbers to 3 significant figures:

  • 12.36

  • 125.5

  • 89.2532

  • 58.22

  • 12586.365

  • 599.68


Keys to the study of chemistry

Figure 1.13

Significant figures and measuring devices.

The measuring device used determines the number of significant digits possible.


Keys to the study of chemistry

Exact Numbers

Exact numbershave no uncertainty associated with them.

Numbers may be exact by definition:

1000 mg= 1 g

60 min = 1 hr

2.54 cm = 1 in

Numbers may be exact by count:

exactly 26 letters in the alphabet

Exact numbers do not limit the number of significant digits in a calculation.


Keys to the study of chemistry

PROBLEM:

Perform the following calculations and round each answer to the correct number of significant figures:

1 g

4.80x104 mg

1000 mg

16.3521 cm2 - 1.448 cm2

(b)

(a)

11.55 cm3

7.085 cm

PLAN:

We use the rules for rounding presented in the text: (a) We subtract before we divide. (b) We note that the unit conversion involves an exact number.

Sample Problem 1.9

Significant Figures and Rounding


Keys to the study of chemistry

1 g

4.80x104 mg

1000 mg

16.3521 cm2 - 1.448 cm2

(b)

(a)

11.55 cm3

7.085 cm

48.0 g

14.904 cm2

=

=

11.55 cm3

7.085 cm

Sample Problem 1.9

SOLUTION:

= 2.104 cm

= 4.16 g/ cm3


Keys to the study of chemistry

Scientific Notation

  • Can be used to express very large or very small numbers

  • Expresses value as A x 10n

    1≥A<10, n is an integer

    14,345 = 1.4345 x 104

    0.009867 = 9.867 x 10-3


Keys to the study of chemistry

Scientific Notation

  • Is useful for handling significant digits

    Express 14,345 to 3 sig. figs.

    1.43 x 104

    Express 93,000,000 to 4 sig. Fig

    9.300 x 107

    Express 0.009867 to 2 sig. figs.

    9.9 x 10-3or 0.0099


Keys to the study of chemistry

Precision, Accuracy, and Error

Precision refers to how close the measurements in a series are to each other.

Accuracy refers to how close each measurement is to the actual value.

Systematic error produces values that are either all higher or all lower than the actual value.

This error is part of the experimental system.

Random error produces values that are both higher and lower than the actual value.


Figure 1 14

precise and accurate

precise but not accurate

Figure 1.14

Precision and accuracy in a laboratory calibration.


Keys to the study of chemistry

random error

systematic error

Precision and accuracy in the laboratory.

Figure 1.14

continued


Keys to the study of chemistry

Percentage Problems

Percent is the number of items of a specified type in a group of 100 total items.

Parts per hundred

Percent = number of items of interest x 100%

total items


Keys to the study of chemistry

Percentage Problems

A student answered 19 items correctly on a 23 point test. What was his score as a percentage?


Keys to the study of chemistry

Percentage Problems

Range as a percent of the average is a way to express precision.

% of average = (highest – lowest) x 100%

average

= (20.50 – 19.25) units x 100 % = 6.32% 19.78 units


Keys to the study of chemistry

Percentage Problems

A technician measured the breaking strength of three samples of plastic. His results were:

Run 1: 65.8 MPa

Run 2: 72.4 MPa

Run 3: 68.3 MPa

What was the range of his measurements as a percent of the average?

Note: 1 MPa = 145 pounds/in2


Keys to the study of chemistry

Percentage Problems

Percent difference is a way to express accuracy.

% difference = (measured – actual) x 100%

actual

= (19.78 – 20.00) units x 100% = –1.1%

20.00 units


Keys to the study of chemistry

Percentage Problems

A student determined the density of aluminum metal to be 2.64 g/cm3. The accepted value is 2.70 g/cm3. What is the percent differ-ence between her result and the accepted value?

Did she do a good job?


Keys to the study of chemistry

Percentage Problems

A student did three experiments to determine the density of rubbing alcohol. Her results were: 0.778 g/mL; 0.795 g/mL; 0.789 g/mL. What is her precision as % of average?

The true value is 0.785 g/mL. What is her accuracy?


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