Name:
Chem 332
Analytical Chemistry
Exam I
Show all work for partial credit
1. (10 points) Massachusetts limits the amount of lead in drinking water to .015 mg/L.
Express this concentration in Molarity, ppm, and ppb.
.015 mg = .000015g
.000015g x 1 mole/207.2 g = 72.4 x10-9M
1 liter water = 1000 g
.000015g/1000g = 15x10-9
15x10-9 x106 ppm = .015 ppm
15x10-9 x109 ppb = 15 ppb
2. A. (5 points) I am making a 0.1M NaCl (molar mass 58.44) solution for an
experiment. To measure the mass of the NaCl, I will use the bulk balance that is only
accurate to ±0.01g. I place a piece of paper on the balance and set the tare to read
0.00. I then put the NaCl on the balance until it reads 5.84 g. What it the uncertainty in
this mass?
5.84±.01 g -0.00±.01 = 5.84
Subtraction error = sqrt(.012 + .012) = .014
5.84±.014g
B. (5 points) The volume of the solution I am making is 1L. To measure this volume I
will use a large graduated cylinder that can measure volume to ± 10 mL. What is the
uncertainty in my final calculated molarity?
(5.84g/58.44 g/mol)/1L = .1M
Will ignore the error in the molar mass
This is a division problem
relative error = sqrt((.014/5.84)2 + (10/1000)2)
=.0102
.0102 X .1 M = .001M
.100±.001M
3. Statistics
Below is the data for % á-acids in hops from our class:
% á-acids in hops
Toluene
Extract
Methanol
Extract
8.94
10.78
7.43
8.56
7.96
10.44
7.52
6.84
4.02
7.34
7.43
9.46
7.46
7.99
A. (10 points) What is the average, standard deviation, and 95% Confidence interval for
the % á-acids in the Toluene extract?
Average : 7.25
Standard Deviation: 1.56
CI = 7.25 ±ts/sqrt(n)
t=2.45
7.25± 2.45(1.56)/sqrt(7)
7.25±1.41 5.84-8.66
B. (5 points) Could the 4.02 value in the set of toluene extract data be rejected using
the Q test? (You can use the Grubbs test instead if you want)
Q
Grubbs
Gap = 7.43-4.02 =3.41
|7.25-4.02|/1.56
Interval = 8.94-4.02=4.92
2.07
Q = .693
>1.938
> .51, reject
Reject
C. (No points, but needed for next questions) What is the average, and standard
deviation for the % á-acids in the Methanol extract?
Average = 8.77 s = 1.51
D. (5 points) Use the t-test to determine if the mean you get for the % á- in the Toluene
is statistically different than the mean you get for the % á- for the methanol extract.
(Use a 99% level for statistical significance)
spooled = sqrt((6(1.562) + 6(1.51)2)/12) = 1.53
t=|8.77-7.25|/1.53 sqrt(7x7)/(7+7) = 1.86 < T table (3.169) so not statistically
different
4. (10 points) Ag+ at 10-100 ppb can be used to disinfect swimming pools. One way to
maintain an appropriate concentration of Ag+ is to add a slightly soluble silver salt to the
pool. Calculate the ppb of Ag+ in a saturated solution of AgBr (Ksp = 5.0x10-13)
5.0x10-13 = [Ag+][Cl-] =X2
X=[Ag+] = sqrt(5.0x10-13) = 7.1x10-7 M
For grins (and 3 points extra credit) an Olympic sized pool has a volume of 2.5x106
liters. How much silver is there in a pool of this size?
2.5x106 x 7.1x10-7 = 1.7675 moles
1.7675 moles x 107.9 g/mole = 191 g silver
(2 points extra credit) The current price of silver is $21.43/ounce. How many $ of silver
are in this pool?)
191 g x 1lbs/453.59 g x 16 oz/lbs x $21.43/oz = $144.38
5A. (3 points) Calculate the pH of a .5M solution of Hypochlorous acid, HOCl?
(Ka 3.0x10-8)
Ka=[A-][H+]/[HA]
3.0x10-8 = X2/.5-X; will assume .5-X ~ .5
X = sqrt(.5(3.0x10-8) = .000122
Assumption that .5-.0001~.5 is good
pH=-log(.000122)
=3.91
5B. (3 points) What is the Kb of NaOCl?
KW =KA@KB
1x10-14=3.0x10-8@KB
KB =1x10-14/3x10-8 = 3.33x10-7
5C. (4 points) What is the pH of .5M NaOCl?
KB = [HB+][OH-]/[B]
3.33x10-7=X2/.5-X; will assume that .5-X ~ .5
3.33x10-7=X2/.5; 3.33x10-7@.5=X2
X=sqrt(3.33x10-7@.5) = .00041
Assumption that .5-.0004~.5 is good
pOH = -log(.0004) = 3.39; pH = 14-3.39 = 10.61