1
Chem 265
October Mid Term Quiz
2012
Name:
[2]
1.
Circle the acidic gas.
NH3
O2
N2 CO2
What percentage of this gas is typically present in dry air?
0.039%
[2]
Circle the basic gas.
NH3
O2
N2 CO2
What is the product when this gas dissolves in water?
NH4OH
[2]
2.
What is the pH of clean rain?
5.5
What information is obtained from a BOD (biological oxygen demand) test?
The oxygen required for biological degradation of the organic matter by
microorganisms
[4]
3.
What do we
carbonate?
see
happening when acid is added to a solution or suspension of
Gas bubbles off and solid carbonate dissolves
Write equations showing the stepwise reactions involved. Use HCl and sodium
carbonate, but you do not need to include the spectator ions in the equations.
H+ + CO32- ⇋H CO3-
H CO3- + H+ ⇋ H2 CO3
H2 CO3 ⇋ CO2 + H2O
2
[2]
Why does the composition of a solution of sodium hydroxide change over time if
it is stored in a plastic bottle?
Carbon dioxide diffuses in and reacts with the hydroxide and carbonates are
formed.
[5]
4. (a) What is the ionic srength of a solution containing 0.030 M NaOH and 0.020 M
K2SO42µ = ½(Σcizi2) = ½ (0.030*12 +0.030*12 +0.040*12 + 0.020*22)
= ½ (0.180) = 0.090 M
(b) What is the pH of the solution?
Table of Activity coefficients for aqueous solutions at 25o.
Ion
Ionic strength (µ,M)
0.001
0.005
0.01
0.05
+
H
0.967
0.933
0.914
0.86
OH0.964
0.926
0.900
0.81
0.1
0.83
0.76
γH+ =0.83 γOH- = 0.76
pH = -log [H+] γH+
[H+] γH = 1.0 x10-14/[OH-] γOH- = 1.0 x10-14/0.030*0.76 = 4.386 x10-13
pH = 12.36
3
(c) What is the pH calculated to be if you neglect activities?
pH = 12.48
[3]
5.
Write the equation relating activity and concentration.
A = γc
Does activity increase or decrease as the ionic strength is increased?
Decrease
Is the following statement true or false:
The activity of the ions in a 2.0 M solution of HCl is much bigger than 2.0 M.
True
[4]
6.
A sample of a bulk lot, containing particles of polymer and salt in a ratio of 2:8, was
taken.
The sample contained 106 particles.
All particles were the same size of 0.35 mm.
What is the expected number and relative standard deviation for the sampling of the
polymer particles?
Let n = number of particles in total sample.
n = 106
2
Fraction of polymer particles = 10 = 0.2 = p
The number of polymer particles in the sample will be np = 0.2*106 or 2 x 105
𝜎 = √npq = √106 ∗ 0.2 ∗ 0.8 = √1.6 ∗ 105 = 4 ∗ 102
𝜎
Relative standard deviation = 𝑛𝑝 =
4.0∗102 ∗100%
2∗105
= 0.2%
4
[2]
What are two ways we can decease the sampling error when sampling a dry
material which we know contains several different components?
Take more sample
Grind to make more smaller particles
[1]
7.
Which acid do we use to volatilize silicates?
HF
[1]
Which acid do we use to dissolve gold?
Aqua regia
[1]
Name a base which can be used as a flux to dissolve inorganic material.
Sodium carbonate
[4]
8. Write the Ka and Kb reactions of NaHCO3.
HCO3- + H2O ⇋ H2 CO3 + OH- Kb
HCO3- + H2O ⇋ CO3-2 + H3O+
[5]
Ka
9. Calculate the molar solubility of PbI2 in a solution that is 0.020 M in NaI.
Ksp for PbI2 = 7.9 x 10-9
Ksp for PbI2 = 7.9 x 10-9 = [Pb2+][I-]2
Let s = the solubility Then [Pb2+] = s
[I-] = 2s + 0.020 M
Assume 2s + 0.020 M ≈0.020 M
7.9 x 10-9 = s*0.0202 𝑠 =
7.9 𝑥10−9
0.0202
= 1.98 x 10-5 ≈ 2.0 x 10-5 M
Check assumption: 2 x 2.0 x 10-5 M << 0.020 M Correct.
5
[6]
10. Write the charge and mass balance for a solution made by dissolving 0.2 mol CaBr2
in 1 L water to give Ca2+, Br-, CaBr+, and CaOH+.
Charge
2[Ca2+] +[H+] + [CaBr+] + [CaOH+] = [OH-] +[Br-]
Mass balance
.
0.2 M = [Ca2+] + [CaBr+] + [CaOH+]
0.4 M = [CaBr+] + [Br-]
[2]
11.
Calculate log Kf for the formation of a nickel triethylenediamine complex.
We will represent ethylenediamine by en.
Ni2+ + en ⇋ Ni(en)2+
log K1 = 7.52
Ni(en)2++ en ⇋ Ni(en)22+
log K2 = 6.32
Ni(en)22+ + en ⇋ Ni(en)32+
log K3 = 4.49
Log Kf = sum of logs given = 18.33
[1]
12.
Why does a precipitate form when you first add sodium hydroxide to a solution of
zinc, but then disappears when you keep adding more hydroxide?
Zinc hydroxide forms which is insoluble but as more hydroxide is added we get a
soluble comlex of Zn(OH)42+ forming and ppt dissolves.
[2]
13. Distinguish between precision and accuracy. ( Just need notes – not complete
sentences).
Precision: reproducibility
Accuracy: how close to the real answer you are
6
[3]
14. Students were measuring the concentration of ethanol in their favorite beverage. The
following results were obtained: 20.2 %, 20.8 %, 21.6 %, 19.9 %, 19.4 %, 16.8 %, 20.0
%, 19.2 %, 21.3 %.
Can we discard the outlier?
Table of Critical values of G for rejection of outlier.
Number of observations
G (95% confidence )
4
1.463
5
1.672
6
1.822
7
1.938
8
2.032
9
2.110
10
2.176
11
2.234
Av = 21.1% s = 2.2 %
Gcalc =
|𝑞𝑢𝑒𝑠𝑡𝑖𝑜𝑛𝑎𝑏𝑙𝑒 𝑣𝑎𝑙𝑢𝑒−𝑥̅ |
𝑠
=
16.8−19.9
1.4
= 2.2
Gcalc > Gtable , so point should be discarded.
7