CHEM 1412 Practice exam 1
Chapter 13-15
1. Which of the following concentration unit will be affected by temperature?
(A) mass percent
(B) molarity
(C) mole fraction (D) molality
2. Which of the following action will reduce the solubility of a gas in water?
(A) pressure
(B) molar mass of the gas (C) acidic or basic gas (D) temperature
Answer questions Q3-Q6 based on the following information
A solution of 12.00 g of an unknown nondissociating compound dissolved in 200.0 g of benzene
freezes at 3.45oC. The Kf of benzene is 5.07oC/m. The freezing point of benzene is 5.45oC.
3. What is the mass percent of the solution?
12.00g/(12.00+200.0)g=5.66%
4. What is the molality of the unknown solution?
ΔTf = Kf m, ΔTf = T f ◦ – Tf = 5.45-3.45=2.00 oC, m= ΔTf /Kf= 2.00 oC/(5.07oC/m)=0.394m
5. How many moles of unknown in the solution?
The molality m = moles of solute/1kg solvent, in 200.0g of solvent, there are 200.0g x
(1kg/1000g) x 0.394mol/ kg solvent=0.0789 mol
6. What is the molar mass of the unknown?
M= mass/n=12.00g/0.0789 mol=152.09 g/mol
7. The osmotic pressure of 0.050 M MgSO4 at 25 C is 1.60 atm. What is the Van't Hoff factor?
(A)1.9
(B)1.3
(C)2.5
(D)2
 = iMRT, i= /MRT=1.60 atm/[0.050Mx(0.082atm·L/K·mol)x298K]=1.3
8. The freezing point of 21.2 g of NaCl in 135 mL of water (Kf=1.86◦C/m) is _____.
(A)
–5.00 ◦C
(B)
–4.60 ◦C
(C)
–9.99 ◦C
(D)
–11.5 ◦C
9. The rate at which hydrogen reacts in the reaction N2 + 3H2  2NH3 is 0.074 Ms–1. The rate
of formation of ammonia is _____ Ms–1.
A)
0.148
B)
0.049
C)
0.111
D)
0.222
10. The reaction 2N2O5(g)  2N2O4(g) + O2(g) obeys the rate law: rate = k[N2O5]. In which the
specific rate constant is 0.00840 s-1. If 2.50 moles of N2O5 were placed in a 5.0-liter container at
that temperature, how many moles of N2O5 would remain after 1.00 minute?
ln(mol of At/2.50) = -(0.00840 s-1)x60.
Mole of At= 1.51 moles
11. The rate of the reaction A B when [A] = 0.2 M is 3.2 x 10–2 Ms–1. If the reaction is second
order in A, the rate constant is _____ M–1s–1.
(A)1.6
(B)0.8
(C)34
(D)0.24
12. The gas-phase decomposition of NOBr is second order in [NOBr], with k = 0.810 M-1.s-1 at
10oC. We start with 4.00 x 10-3 M NOBr in a flask at 10oC. How many seconds does it take to
use up 1.50 x 10-3 M of this NOBr?
1/(2.50 x 10-3 M) = (0.810 M-1.s-1 )t + 1/(4.00 x 10-3 M)
t =185.19 seconds
13. The reaction 4PH3(g)  P4(g) + 6H2(g) is a first-order reaction with a half-life of 35 s. The
rate constant of the reaction is _____ s–1.
(A)1.12 x 10–4
(B)1.98 x 10–2
(C) 1.98  10–3
(D)3.16 x 10–2
14. The following mechanism for the reaction between H2 and CO to form formaldehyde, H2CO,
has been proposed:
Step 1
H2  2H
(fast, equilibrium)
Step 2
H + CO  HCO
(slow)
Step 3
H + HCO  H2CO (fast)
(a) Which step is the rate-determining step? Step 2
(b) Write the balanced equation for the overall reaction. H2 + CO  H2CO
(c) Identify any intermediates in this proposed mechanism. H and HCO
15. Write the equilibrium law according to Kc for the following reaction:
CaCO3(s) + 2HCl(aq)  Ca2+(aq) + 2Cl-(aq) + H2O(l) + CO2(g)
Kc = [Ca2+(aq)][Cl-(aq)]2[CO2(g)]/[HCl(aq)]2
16. Consider the reaction
the
at equilibrium if the
A) 2.7 atm
B) 0.085 atm
C) 11 atm
D) 5.3 atm
E) 7.5 atm
At 1273 K, the Kp value is 167.5. What is
is 0.17 atm at this temperature?
17. Kc for the reaction
is 4.28 x 1022 at 928 ºC. Kp for the reaction at the same temperature is _____.
A)5.26 x 10-18
B)4.34 x 1020
C)3.56 x 10-15
D)7.49 x 1019
18. Consider the following reaction:
2HF(g)
H2(g) + F2(g)
(K = 1.00
10–2)
Given 3.00 mole of HF(g), 0.18 mole of H2(g), and 0.5 mole of F2(g) are mixed in a 5.00 L flask,
determine the reaction quotient, Q.
(A) Q = 0.01
(B) Q = 0.04
(C) Q = 0.05
(D) Q = 0.08
19. The equilibrium constant, Kp, for the reaction
is 1.26 at 250°C. The reaction is started with PCl5, PCl3 and Cl2 at 0.01, 0.2 and 0.5 atm at
250°C. When the reaction comes to equilibrium, the pressure of _____.
(A)
PCl3 must have increased
(B)
Cl2 must have increased
(C)
PCl5 must have decreased
(D)
PCl5 must have increased
20. 2.0 moles of N2(g) and 4.0 moles of H2(g) were added to a 1.0-liter container and the
following reaction then occurred:
3H2(g) + N2(g)
2NH3(g)
The equilibrium concentration of NH3(g) = 0.3 moles/liter at 700.°C. The value for K at 700.°C
for the formation of ammonia is:
A) 1.9  10–1
B) 2.8  10–2
C) 1.1  10–3
D) 3.8  10–3
21. 2.50 mole of NOCl was heated in a 2.0 L vessel at 400°C.At equilibrium it was found that
36.0% of the NOCl had dissociated. 2NOCl(g)  2NO(g) +Cl2(g)
The equilibrium constant, Kc of the reaction is
(A)0.0472
(B)0.0712
(C)0.63
( D)5.93
22. Consider the equilibrium: 2SO2(g) + O2(g)  2SO3(g) Ho < 0
How will each of the following affect an equilibrium mixture of the three gases? Shift to left,
right or no change.
(a) O2(g) is added to the system R
(b) Remove O2(g) from the system L
(c) The reaction mixture is heated L
(d) The volume of the reaction vessel is doubled L
(e) A catalyst is added to the mixture N
(f) The pressure of the system is increased by adding a noble gas R