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### Honors Chemistry

Measurement

What is chemistry?

- Chemistry is the study of all things and the changes they can undergo.
- Chemistry is called a central science because it overlaps so many sciences.
- Chemical – is any substance with a definite composition.

Chemists use the scientific method as a systematic approach to gather knowledge.

- Observation
- Question
- Hypothesis
- Experiment
- Conclusion
- All hypotheses must be testable in order to be a valid hypothesis.

Types of Observations

- Qualitative: Describes something using the 5 senses
- Quantitative: Uses numbers in the description
- Quantity – something that has magnitude, size, or amount.
- Unit – a quantity adopted as a standard of measurement

Experiment

- Natural Law – Describes how nature behaves
- Theory – Explains why nature behaves the way it does
- A theory and a hypothesis are both explanations, but a theory is an explanation formed after much experimentation.

Variables in a Experiment

- Independent Variable - You control
- Dependent Variable – Variable factor – what is being tested
- Experimental Control – Factor that remains constant for comparison

Factors in an Experiment

- Independent:most regular variable – goes on the X-axis
- Dependent:what you are testing – goes on the Y-axis
- Experimental Control:part of the experiment that stays

the same.

Dependent variable

“Y” axis

Independent variable

“X” axis

Measurement in Chemistry

Measurement is a key ingredient in ALL sciences, especially chemistry.

- Scientific Notation
- Accuracy and Precision
- Significant Figures
- Measurement Devices
- Metric System
- Dimensional Analysis

Scientific Notation is a shorthand way of expressing a number.Consists of two factors:

- Coefficient - a number between 1 and 10 (only 1 digit to the LEFT of the decimal point)
- Base - a power of 10 “power of 10” shows the number of 10’s that are to be multiplied together
- Examples on the number line:

1x102 4x101 1x100

1x10-10 1x10-1

Adding and Subtracting(without calculator)

- Exponents must be the same
- If number gets bigger, exponent gets smaller
- If number gets smaller, exponent gets larger

(8 x 10-2) + (3 x 10-4) - (2 x 10-3)

(80 x 10-3) + (0.3 x 10-3) – (2 x 10-3) =

78.3 x 10-3 = 7.83 x 10-2

Multiplication(without calculator)

- Multiply number and add exponents (base 10 remains the same)

(6 x 10-6)(8 x 103) =

48 x 10-3 4.8 x 10-2

(6 x 10-3)2 =

36 x 10-6 = 3.6 x 10-5

Division(without calculator)

- Divide number and subtract exponents (base 10 remains the same)

(7.2 x 10-8)÷(8 x 10-5) =

0.9 x 10-3 9 x 10-4

Cube Root

- Make number a whole number, take cube root of number, multiply exponent by 1/3.

(2.7 x 10-8)1/3 =

(27 x 10-9)1/3 =

3 x 10-3

Square Root

- Make number a whole number, take square root of number, multiply exponent by ½.

(1.44 x 10-6)1/2 =

(144 x 10-8)1/2 =

12 x 10-4 = 1.2 x 10-3

1st Commandment of Chemistry: KNOW THY CALCULATOR!

Find the “EE” key – it may be a 2nd function!

If you have a graphing calculator look for the following keys:

Find the (-) key.

1st Law of Chemistry:

Know Thy Calculator!

Look at the calculator that is similar to yours…

Find the “(-)” or the “+/-” key.

Uncertainty in Measurement

- Measurements are uncertain because:
- 1) Instruments are not free from error.
- 2) Measuring involves some estimation.
- Precision –when the instrument gives you about the same results under similar conditions. The smaller the increments of measurement an instrument has, the more precise it can be.
- Accuracy – when the experimental value is close to the actual value.
- % Error = experimental– acceptedvalue x 100

accepted value

What is the goal for a game of darts?

Hitting the Bulls Eye!

Label the following data as accurate, precise, neither, or both.

- 1) 200g, 1g, 40g
- Neither
- 2) 78g, 80g, 79g
- Precise
- 3) 16g, 14g, 17g
- Accurate and Precise

How to read a triple beam balance

28.570 g

Ohaus Triple Beam Balance Tutorial

Reading A Triple Beam Balance Tutorial

How to read a triple beam balance

109.076 g

Ohaus Triple Beam Balance Tutorial

Reading A Triple Beam Balance Tutorial

Significant Figures and Digits

- A prescribed decimal that determines the amount of rounding off to be done base on the precision of the experiment.
- ALWAYS ESTIMATE 1 DIGIT MORE THAN THE INSTRUMENT MEASURES.
- Significant digits include measured digits and the estimated digit.
- Exact Numbers – Do not involve estimation
- ex. 12 in = 1 ft

VI. Significant Digits

- Use Atlantic-Pacific Rule – imagine a US map

decimal

point

decimal

point

Pacific

Atlantic

resent

bsent

1100

4 significant digits

1100.

8 significant digits

11.010000

2 significant digits

0.025

5 significant digits

0.00035000

1,000,100

5 significant digits

Decimal Absent Start counting with the 1st nonzero digit and count all the rest.

Decimal Present Start counting with the 1st nonzero digit and count all the rest.

Significant Digits in Addition and Subtraction

- Add or subtract numbers
- Answer can only be as exact as the least exact number. (Look at the decimal place)
- Ex. 4.1 cm + 0.07cm
- 4.17 cm
- 4.2 cm

Significant Digits and Multiplication and Division

- Multiply and Divide the numbers.
- Round answer to the same number of significant digits as the number with the fewest significant digits.
- Ex. 7.079 cm / 0.535 cm
- 13.2317757
- 13.2

Atmospheric pressure is measured with a barometer. This is a glass tube sealed at one end and filled with Hg.

Using a Manometera device used to measure pressure

- Reading a Manometer
- Barometer containing Hg

Temperature ConversionsCelsius and Kelvin

- K = °C + 273
- °C = K - 273
- Zero Point on Kelvin Scale – Absolute Zero
- 0 K and -273 °C
- Kinetic energy is energy of motion. Temperature is a measure of kinetic energy. Since the temperature at absolute zero is a true zero, there is no particle motion Therefore, nothing can exist at absolute zero.

Measurements: basic to all sciences & all are comparisons to a standard

- English – still used in US
- Metric – devised in the late 1700’s in France
- SI – Le SystèmeInternationaled’Unités
- Modern metric system (1960)
- Based on 7 base units
- Base units are modified by prefixes

SI Base Units

meter (m)

- Length
- Mass (SI standard unit)
- Time
- Temperature
- Amount of a substance mole (mol)
- Electric current ampere (A)
- Luminous intensity candela (cd)

kilogram (kg)

second (s)

Kelvin (K)

The Meter

- The original standard for the meter was kept in a safe in France.
- The meter stick is a replica of that standard.
- A meter is made up of 100 centimeters and 1000 millimeters.
- Lasers are now used to determine the standard for a meter.

Mass is the amount of matter in an object.

1 cm3 of water = 1 gram.

The standard kilogram is kept under lock and key in Washington, DC and other cities around the world.

The GramDerived Units

- Area: 2-D
- L x W (m2)
- Volume: 3-D
- Solid - L x W x H (m3)
- Liquid or irregular shaped object - graduated cylinder (L or cm3)
- Density
- mass/volume (kg/m3)

The Liter

- The liter is 1000 mL
- 10cm x 10cm x 10cm
- 1 liter= 1000 cm3 = 1 dm3
- 1 milliliter = 1 cm3 = 1 cc = 20 drops

=

Conversions between units

- Factor-label method or dimensional analysis – based on using unit equalities

60 s = 1 min

60 s OR 1 min

1 min 60 s

Example 1: 3.6 x 104 s = ? days

3.6 x 104 s

1 hr

60 min

1 day

24 hr

1 min

60 s

=

0.42 days = 4.2 x 10-1 days

1 min

__________________

60 s

1 hr

__________________

60 min

1 day

__________________

24 hr

3.6 x 104 s

x

x

x

=

Example 3: A room measures 12 feet by 15 feet. Calculate the minimum number of square yards of carpet needed to cover this area.

180 ft2

1 yd2

= 20 yd2

9 ft2

A closer look at density

- Physical = A characteristic of a substance that does not involve a chemical change
- Examples: texture, state of matter, density, hardness, boiling point
- Density = The ratio of the mass of a substance to the volume of the substance.
- D = mass / volume

Density of an Irregular solid:

1- Find the mass of the object

2- Find the volume if the

object by water displacement!

The characteristic plot for a Direct Relationship is a straight line graph.

- Indirect Relationship
- The characteristic plot for an Inverse Relationship is a curve of the type illustrated here. As one of the variables increases, the other decreases. Note: It is not a straight line sloping downward.

Examples

- Determine the density of aluminum from the analysis of data from 5 samples.
- 54.0-g sample has a volume of 20.0 mL
- 14.0-g sample has a volume of 5.0 mL
- 41.0-g sample has a volume of 15.0 mL
- 27.0-g sample has a volume of 10.0 mL
- 19.0-g sample has a volume of 7.0 mL

HINT: Graph the data with volume as the independent variable.

Find the slope of the line!

- Convert the density of benzene, 0.8787 g/cm3, to kg/m3.

878.7 kg/m3

- Calculate the density of mercury if 1.00 x 102 g occupies a volume of 7.36 cm3.

13.6 g/cm3

Density Graph

BACK

Energy Transfer

- Heat-energy that is transferred from one object to another due to a difference in temperature. (symbol for heat = q)
- Temperature = a measure of the average kinetic energy of the particles in a substance. Temperature is an intensive property, and heat is an extensive property.
- Thermochemistry – the study of heat changes in a chemical reaction.
- Heat vs. Temperature

Calorimetry

- Calorimetry is the study of heat flow and measurement.
- Calorimetry experiments determine the heats of reactions by making accurate measurements of temperature changes produced by a calorimeter.

Heat Capacity – amount of heat needed to raise the temperature of an object 1°C.

- Specific Heat – amount of heat needed to raise 1g of a substance 1°C.

-Symbol for specific heat is C.

Heat and Temperature

- Formula for heat absorbed for released:

q = C x m x ∆T

- Remember: Specific Heat of Water =

4.184 J/g· °C

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