Assignment 9 - Solutions
Chem 1000A
Practice Assignment
1. (a) Draw the Maxwell-Boltzmann distribution (number of molecules versus molecular speed)
for Ar and Xe. (b) Calculate the rms speed for both gases at 25 °C.
(a) The graph would look similar to Figure 12.18 in your text book. The blue curve would be the
one for Xe and the red curve the one for Ar.
(b) (ū2)1/2 = (3RT/M)1/2
M(Ar) = 39.948 g mol-1
M(Xe) = 131.29 g mol-1
T = 25 °C = (273.15 + 25) K = 298.15 K
rms speed(Ar) = (3 × 8.314510 J K-1 mol-1 × 298.15 K/131.29 g mol-1)1/2 = (56.645 J g-1) 1/2 =
(56.645 kg m2 s-2 g-1) ½ = (56.645 ×103 g m2 s-2 g-1) ½ = (56645 m2 s-2) ½ = 238.00 m s-1
rms speed(Xe) = (3 × 8.314510 J K-1 mol-1 × 298.15 K/39.948 g mol-1)1/2 = (186.16 J g-1) 1/2 =
(186.16 kg m2 s-2 g-1) ½ = (186.16 ×103 g m2 s-2 g-1) ½ = (186160 m2 s-2) ½ = 431.47 m s-1
2. Which of the following acids are weak Brønstedt acids?
(a) HCl
is a strong Brønstedt acid
(b) H3PO4
is a weak Brønstedt acid
(c) HSO4is a weak Brønstedt acid
(d) B(OH)3
is not a Brønstedt acid, but a Lewis acid
(e) Ca(OH)2
is not a Brønstedt acid, but a base
3. Draw the structure of the following compounds.
(a) butanal
CH3CH2CH2CHO
(b) methanoic acid
HCOOH
(c) 2-butanol
CH3CH(OH)CH2CH3
(d) 3-hexyne
CH3CH2C≡CCH2CH3
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4. You wish to determine the weight percent of copper in a copper-containing alloy. After
dissolving a sample of an alloy in acid, and excess of KI is added and the Cu2+ and I- ions
undergo the reaction
2Cu2+(aq) + 5I-(aq) → 2CuI(s) + I3-(aq)
The liberated I3- is titrated with sodium thiosulfate according to the equation
I3-(aq) + 2S2O32-(aq) → S4O62-(aq) + 3I-(aq)
If 26.32 mL of 0.101 M Na2S2O3 is required for titration to the equivalence point, what is the
weight percent of Cu in 0.251 g of the alloy?
1. calculate moles of S2O32- used:
26.32 mL × 0.101 mol L-1 = 26.32 × 10-3 L × 0.101 mol L-1 = 2.66 mmol
2. calculate how many moles of I3- were present in the solution that has been titrated.
2.66 mmol S2O32- × (1 mol I3-/2 mol S2O32-) = 1.33 mmol I33. calculate how many moles of Cu2+ were consumed to produce 1.33 mmol I3-.
1.33 mmol I3- × (2 mol Cu2+/2 mol I3-) = 2.66 mmol Cu2+
4. calculate how many grams of Cu2+ (and Cu) you had.
M(Cu) = 63.546 g mol-1
m(Cu) = 2.66 × 10-3 mol × 63.546 g mol-1 = 0.169 g
5. calculate the weight percent of Cu in the alloy.
0.169 g/0.251g × 100 % = 67.3%
The weight percent of copper in the copper-containing alloy is 67.3%.
5. You have an aqueous solution of Fe2(SO4)3. Which products do you get when you combine it
with the following solutions. Provide reaction equations.
(a) NH3(aq)
Ammonia is a base, therefore, it will generate OH- anions in aqueous solution. The OH- anions
will react with Fe2+ and precipitate Fe(OH)2.
Fe2(SO4)3(aq) + 6NH3(aq) + 6H2O → 2Fe(OH)3 + 3(NH4)2SO4(aq)
(b) NaCl(aq)
Fe2(SO4)3(aq) + NaCl(aq) → no reaction
(c) Pb(CH3COO)2(aq)
PbSO4 precipitates.
Fe2(SO4)3(aq) + 3Pb(CH3COO)2(aq) → 3PbSO4(s) + 2Fe(CH3COO)3 (aq)
(ū2)1/2 = (3RT/M)1/2
R = 8.314510 J K-1 mol-1
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