City College, Chemistry Department
Chemistry 10401, Spring 2017, sections H*, Prof. T. Lazaridis
First Midterm exam, 3/6/2017
Last Name: _____________________________________________
First Name: _____________________________________________
Note: There are 9 questions in this exam (check both sides of each sheet). Fill in your
answer in the blank space provided immediately following each question. Half a point will
be subtracted every time you report a numerical result with an incorrect number of
significant figures. Good luck!
Useful constants/equations
NA = 6.022 × 1023
R = 0.0821 L.atm/mol.K=8.314 J/molK
1 atm = 760 torr
Clausius-Clapeyron: lnP=-ΔΗvap/RT + C
1. a. (5) Which substance would you expect to have the higher boiling point,
carbon dioxide (CO2) or formaldehyde (CH2O)? Which type of interactions
are responsible for this?
Formaldehyde. It is polar and experiences dipole-dipole interactions.
CO2 is nonpolar.
b. (5) Arrange Xe,Ar and Kr in order of increasing London dispersion forces
Ar<Kr<Xe
(dispersion forces increase with atomic size)
2. (10) The vapor pressure of diethyl ether is 400 torr at 17.9 oC and 760 torr
at 34.6 oC. Determine its enthalpy of vaporization.
Use the Clausius-Clapeyron Eq. with T1=273.15+17.9, T2=273.15+34.6,
P1=400 torr , P2=760 torr:
ln(P1/P2)=-ΔHvap/R (1/T1-1/T2) => ΔΗvap = 28.6 kJ/mol
3. (10) The phase diagram for sulfur is shown below.
a) (3) How many triple points does sulfur have?
Three
b) (3) What is the state of sulfur at 25 oC and 1 atm? Rhombic
c) (4) What will you see if you start from the conditions at (b) above and
gradually increase the temperature to 160 oC? At what temperature(s) will
you see changes in state?
At 114 oC it will convert to monoclinic and at 119 oC to a liquid
4. (13) The vapor pressure of pure water at 25 oC is 23.76 torr. A dilute
aqueous solution of urea (a nonvolatile, nonionic solute) has vapor pressure
22.50 torr at the same temperature. What is the molality of urea in this
solution? (Molar mass of water is 18.02 g/mol)
Raoult’s law: Pwat = Pvap,wat * Xwat => Xwat = 0.9470
Xurea=1-Xwat= 0.0530
100 moles of solution will contain 94.70 mol water and 5.30 mol urea.
94.70 mol water X18.02 g/mol = 1706 g = 1.706 Kg
molality = 5.30 mol/1.706 Kg= 3.107
5. (10) The solubility of CO2 in water is 3.1X10-2 M at 25 oC and CO2 partial
pressure of 1.00 atm. Calculate the molarity of CO2 in sparkling water
bottled at 25 oC and a CO2 partial pressure of 4.0 atm.
Henry’s law: c=k*p => c2/c1 = p2/p1 = 4 => c2= 0.12 M
6. (12) A solution containing 3.00 g of an unknown compound dissolved in
60.0 g of water boils at 100.28 oC. Calculate the molar mass of the unknown
compound. (Kb of water is 0.512 oC Kg/mol)
ΔTb=0.28 = Kb*m => m=0.55 mol/Kg = Nmol/0.0600 Kg
=> Nmol=0.033
MM= 3.00 g/0.033 mol= 91 g/mol
7. (12) In the first-order reaction
SO2Cl2 (g) à SO2 (g) + Cl2 (g)
10.0% of the reactant decomposes in 80.0 min.
a) (6) Calculate the reaction rate constant
10.0% decomposes, thus 90.0% remains
Integrated rate law for 1st order reactions: [A]/[A]o=exp(-kt) =>
ln (90.0/100) = -k*80.0 min => k= 1.32X10-3 min-1
b) (6) How many minutes will it take for a 5.00 mmol sample to decompose
to 3.50 mmol?
ln(3.50/5.00) = -kt. Use k from part (a) to find t=270 min
8. (12) The rate constant for a second order reaction is 0.105 M-1s-1 at 759 K
and 0.343 M-1s-1 at 791 K. Calculate the activation energy for this reaction
and the preexponential factor A.
ln(k1/k2) = Ea/R (1/T2 – 1/T1) => Ea= 185 kJ/mol
Use this Ea value in:
k1= Aexp(-Ea/RT1) => A= 5.32 X1011 M-1s-1
9. (11) The rate law for the reaction
2Fe2+ (aq) + Cl2 (aq) à 2Cl- (aq) + 2Fe3+ (aq)
is r=k[Fe2+][ Cl2]. Propose a reaction mechanism consistent with this rate
law.
-
Step 1 (slow)
Fe2+ (aq) + Cl2 (aq) à Cl (aq) + Cl (aq) +Fe3+ (aq)
Step 2 (fast):
Fe2+ (aq) + Cl (aq) à Cl- (aq) + Fe3+ (aq)