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# Homework - PowerPoint PPT Presentation

Homework. Chapter 0 - 0, 1, 2, 4 Chapter 1 – 15, 16, 19, 20, 29, 31, 34. Question: . What is the molarity of a 10% (w/v) solution of glucose?. Parts per million (PPM). PPM.

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## Homework

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Homework
• Chapter 0 - 0, 1, 2, 4
• Chapter 1 – 15, 16, 19, 20, 29, 31, 34
Question:

What is the molarity of a 10% (w/v) solution of glucose?

### Parts per million (PPM)

PPM
• Parts per million is a convenient way to express dilute concentrations. Historically, 1 mg per liter or per 1000 ml is referred to as 1 ppm. However, this is not really the case, as parts per million should be expressed as:

Show that the above equation is equivalent to mg per liter.

PPM

For dilute solutions, the density of the solution will be the same as water.

Density of solution = Density of water=

1.0 g/ml

Question Converting PPM to Molarity

The town of Canton prohibits the dumping of copper solutions that have concentrations greater than 0.3969 ppm. When cleaning the quant lab, Dr. Skeels found a bottle labeled “copper standard - 7 mM”, is it permissible to dump this solution down the drain?

Volunteers??

Solution preparation cont’d

Describe the Preparation of a 500.0 mL of a solution that contains 8.00 mM Cu2+ using CuSO4.5H2O (MW 149.69).

Solution preparation cont’d

Describe the Preparation of a 500.0 mL of a solution that contains 8.00 mM Cu2+ using CuSO4.5H2O (MW 149.69).

Thus …

Swirl to dissolve

And fill to the _____ ml mark

Question
• Using the 8 mM Cu2+ solution, prepare 20 mL of a 0.25 mM Cu2+ solution.
Dilutions
• To make dilutions of a solution, the following equation should be employed:
Question

Using the 8 mM Cu2+ solution, prepare 20 mL of a 0.25 mM solution.

### From a liquid – consider concentrated HCl

A more difficult example
• Prepare a 500.0 mL of 1 M HCl.

MW

Wt %

Density

Try it out …

Consider it in two steps:

(1) Determine concentration of Stock

(2) Make dilution

(1) Concentration of Stock
• Must find grams of HCl per liter of solution

dHCl=1.19 g/ml

%HCl (w/w)=37%

MW=36.46 g/mol

Mass

HCl per

Liter

Molarity

Dilution
• Determined concentration of stock is ______ M HCl. We want a 500.0 mL solution that is 1M.
NOTE
• Care must be exercised

when handling strong acids!!

(Always, Always add acid to water)

Dilute to mark

Homework
• Chapter 0 - 0, 1, 2, 4
• Chapter 1 – 15, 16, 19, 20, 29, 31, 34

### Chapter 3

Experimental Error

And propagation of uncertainty

Suppose

You determine the density of some mineral by measuring its mass

• 4.635 +0.002 g

And then measured its volume

• 1.13 + 0.05 ml

What is its uncertainty?

Significant Figures (cont’d)
• The last measured digit always has some uncertainty.
3-1 Significant Figures
• What is meant by significant figures?

Significant figures: minimum number of digits required to express a value in scientific notation without loss of accuracy.

Examples
• How many sig. figs in:
• 3.0130 meters
• 6.8 days
• 0.00104 pounds
• 350 miles
• 9 students
“Rules”
• All non-zero digits are significant
• Zeros:
• Leading Zeros are not significant
• Captive Zeros are significant
• Trailing Zeros are significant
• Exact numbers have no uncertainty

(e.g. counting numbers)

What is the “value”?

When reading the scale of any apparatus, try to estimate to the nearest tenth of a division.

3-2Significant Figures in Arithmetic
• We often need to estimate the uncertainty of a result that has been computed from two or more experimental data, each of which has a known sample uncertainty.

Significant figures can provide a marginally good way to express uncertainty!

3-2Significant Figures in Arithmetic
• Summations:
• When performing addition and subtraction report the answer to the same number of decimal places as the term with the fewestdecimal places

+10.001

+ 5.32

+ 6.130

21.451

21.451

___ decimal places

?

Try this one

1.632 x 105

4.107 x 103

0.984 x 106

0.1632 x 106

0.004107 x 106

0.984 x 106

+

+

1.151307 x 106

1.151307 x 106

3-2Significant Figures in Arithmetic
• Multiplication/Division:
• When performing multiplication or division report the answer to the same number of sig figs as the least precise term in the operation

16.315 x 0.031 =

?

0.505765

___ sig figs

___ sig figs

____ sig figs

3-2Logarithms and Antilogarithms

From math class:

log(100) = 2

Or log(102) = 2

3-2Logarithms and Antilogarithms

Let’s consider the following:

An operation requires that you take the log of 0.0000339. What is the log of this number?

-4.469800302

log (3.39 x 10-5) =

log (3.39 x 10-5) =

log (3.39 x 10-5) =

Between -5 and -4

____ sig figs

3-2Logarithms and Antilogarithms
• Try the following:

Antilog 4.37 =

2.3442 x 104

23442

___ sigs

“Rules”
• Logarithms and antilogs

1. In a logarithm, keep as many digits to the right of the decimal point as there are sig figs in the original number.

2. In an anti-log, keep as many digits are there are digits to the right of the decimal point in the original number.

3-4. Types of error
• Error – difference between your answer and the ‘true’ one. Generally, all errors are of one of three types.
• Systematic (aka determinate) – problem with the method, all errors are of the same magnitude and direction (affect accuracy)
• Random – (aka indeterminate) causes data to be scattered more or less symmetrically around a mean value. (affect precision)
• Gross. – occur only occasionally, and are often large.

Can be detected and eliminated or lessened

Estimated

Treated statistically

Absolute and Relative Uncertainty
• Absolute uncertainty expresses the margin of uncertainty associated with a measurement.

Consider a calibrated buret which has an uncertainty + 0.02 ml. Then, we say that the absolute uncertainty is + 0.02 ml

Absolute and Relative Uncertainty
• Relative uncertainty compares the size of the absolute uncertainty with its associated measurement.

Consider a calibrated buret which has an uncertainty is + 0.02 ml. Find the relative uncertainty is 12.35 + 0.02, we say that the relative uncertainty is

3-5. Estimating Random Error (absolute uncertainty)
• Consider the summation:

+ 0.50 (+ 0.02)

+4.10 (+ 0.03)

-1.97 (+ 0.05)

Sy = + 0.06

2.63 (+ ?)

3-5. Estimating Random Error
• Consider the following operation:

0.010406 =

3-5. Estimating Random Error
• Logarithms antilogs
Question
• Calculate the absolute standard deviation for a the pH of a solutions whose hydronium ion concentration is

2.00 (+ 0.02) x 10-4

pH = 3.6990 + ?

Question
• Calculate the absolute value for the hydronium ion concentration for a solution that has a pH of 7.02 (+ 0.02)

[H+] = 0.954992 (+ ?) x 10-7

Suppose

You determine the density of some mineral by measuring its mass

• 4.635 +0.002 g

And then measured its volume

• 1.13 + 0.05 ml

What is its uncertainty?

The minute paper