homework chapter 0 - 0, 1, 2, 4 chapter 1 – 15, 16, 19, 20, 29, 31, 34

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)Parts per million (PPM)

PPMPPM Parts per million is a convenient way to

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

solution g1,000,000m

solute mg = cppm

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

PPMPPM

solution g1,000,000m

solute mg = cppm

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

Density of solution = Density of water=Density of solution = Density of water=1.0 g/ml1.0 g/ml

solution g1,000,000m

solute mg = cppm

solution 1g

solution 1000mg

solution 1ml

solution 1g

solution ml 1000

solute mg = cppm

Question Question Converting PPM to MolarityConverting 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 M”, is it permissible to dump this solution down the drain?

Volunteers??Volunteers??

Preparation of Stock Solutions

Solids Liquids

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).

L1

Cu mmol 8.00 2

mmol

mol

1000

1

L

mol

1

Cu 1000.8 23

L

mol

1

Cu 1000.8 23

224

Cu 1

5CuSO 1

mol

OHmolL

OHSmol

1

5OCu 1000.8 243

L

OHSmol

1

5OCu 1000.8 243

L5000.0 OHSmol 243 5OCu 1000.4

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).

OHSmol 243 5OCu 1000.4

OHS

OHSg

24

24

5OCu mol 1

5OCu 69.249

OHSg 24 5OCu 999.0

Thus …

Add ______g CuSO4.5H2O

Into a volumetric flask

Add about _____ ml of waterSwirl to dissolveAnd 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:

M V = M Vi i f f

Question

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

M V = M Vi i f f mlmM 020.25mM=V8 i

mM

ml

8

020.25mM=Vi

0.625mL=

From a liquid – consider concentrated HCl

A more difficult example

Prepare a 500.0 mL of 1 M HCl.

Wt %

MW

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 solutionHCl=1.19 g/ml

MW=36.46 g/mol

%HCl (w/w)=37%

Solution 100

HCl 37

1

1000

solution

solution 19.1

g

g

L

ml

mL

g

MassHCl per Liter L

g HCl 1040.4 2

Molarity

L

g HCl 1040.4 2

HCl 46.36

HCl 1

g

mol M1.12

Dilution

Determined concentration of stock is ______ M HCl. We want a 500.0 mL solution that is 1M.

ffii VM=VM mLM 0.0051M=V1.21 i

M

mL

1.21

0.0051M=Vi

41.322mL=

NOTE

Care must be exercised when handling strong acids!!(Always, Always add acid to water)

Add about 300 ml of water firstThen add acidDilute to mark

Homework

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

34

Chapter 3Chapter 3Experimental ErrorExperimental Error

And propagation of And propagation of uncertaintyuncertainty

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

)(

)(

mlvolume

gmass

ml

g1018.4

What is its uncertainty?

Significant Figures (cont’d)

The last measured digit always has some uncertainty.

3-1 Significant Figures3-1 Significant Figures

What is meant by significant figures?

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:a. 3.0130 meters b. 6.8 daysc. 0.00104 poundsd. 350 milese. 9 students

“Rules”

1. All non-zero digits are significant2. Zeros:

a. Leading Zeros are not significantb. Captive Zeros are significantc. Trailing Zeros are significant

3. Exact numbers have no uncertainty

(e.g. counting numbers)

Reading a “scale”

What is the “value”?

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

3-23-2Significant Figures in ArithmeticSignificant 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!

3-23-2Significant Figures in ArithmeticSignificant Figures in Arithmetic

Summations: When performing addition and subtraction

report the answer to the same number of decimal places as the term with the fewest decimal places

+10.001+ 5.32+ 6.130

?21.45121.451 ___ 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 1061.151307 x 106

3-23-2Significant Figures in ArithmeticSignificant 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

3-23-2Logarithms and AntilogarithmsLogarithms and Antilogarithms

From math class:log(100) = 2Or log(102) = 2But what about significant figures?

Between -5 and -4

3-23-2Logarithms and AntilogarithmsLogarithms 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?

log (3.39 x 10-5) = -4.469800302log (3.39 x 10-5) = log (3.39 x 10-5) = ____ sig figs

3-23-2Logarithms and AntilogarithmsLogarithms and Antilogarithms

Try the following:Antilog 4.37 =234422.3442 x 104

___ sigs___ sigs

“Rules”

Logarithms and antilogs1. 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 error3-4. 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.

EstimatedEstimated

Treated statisticallyTreated statistically

Can be detected and Can be detected and eliminated or lessenedeliminated or lessened

Absolute and Relative Absolute and Relative UncertaintyUncertainty

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 Absolute and Relative UncertaintyUncertainty

Relative uncertainty compares the size Relative uncertainty compares the size of the absolute uncertainty with its of the absolute uncertainty with its associated measurement. 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

tmeasuremen of magnitude

yuncertaint absolutety UncertainRelative 002.0

12.35

0.02

3-5. Estimating Random 3-5. Estimating Random Error (absolute uncertainty)Error (absolute uncertainty)

Consider the summation:

+ 0.50 (+ 0.02)+4.10 (+ 0.03) -1.97 (+ 0.05)

2.63 (+ ?)

Sy = + 0.06

...222 cbay ssss

3-5. Estimating Random 3-5. Estimating Random ErrorError

Consider the following operation:

?)(010406.0)04.0(97.1

)0001.0(0050.0)02.0(10.4

...222

c

s

b

s

a

s

y

scbay

222

97.1

04.0

0050.0

0001.0

10.4

02.0

y

sy

02897.0y

syysy 02897.0 0.010406 =

Try this one

)4.0(3.42)5(1030)10(820

)001.0(050.0)2.0(6.11)2.0(3.14

3-5. Estimating Random 3-5. Estimating Random ErrorError

For exponents

a

sx

y

s

ay

For

ay

x

aS is ain y uncertaint

3-5. Estimating Random 3-5. Estimating Random ErrorError

Logarithms antilogs

a

ss

ay

For

ay 434.0

S is ain y uncertaint

log

a

ay sy

s

aantiy

For

303.2

S is ain y uncertaint

log

a

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 + ?

a

ss

ay

ay 434.0

S is ain y uncertaint

log

a

00.2

02.0434.0ys

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

ay sy

s

aantiy

303.2

S is ain y uncertaint

log

a

yss ay 303.2

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

)(

)(

mlvolume

gmass

ml

g1018.4

What is its uncertainty?

The minute paper

Please answer each question in 1 or 2 sentences

1) What was the most useful or meaningful thing you learned during this session?

2) What question(s) remain uppermost in your mind as we end this session?