Name:_____________
Chemistry 112
Second Hour Exam
Remember- Show all work for partial credit
1. (10 points) I am going to dissolve 50.0g of MgCl2 in 500 ml of water
What is the Molar concentration of MgCl2 in this solution?
MW of MgCl2 is 24.31+35.45(2) = 95.21g/mol
Mole MgCl2 = 50/95.21 = .5252
Molarity = mole/liters = .5252/.5 = 1.050M
What is the Molar concentration of Mg2+ in this solution?
MgCl2 6 Mg2+ + 2 Cl- so you get 1 moles of Mg2+ for each mole of MgCl2 you dissolved
Molarity Mg2+ = 1.050 Mol MgCl2 X 1 Mol Mg2+ /1 mol MgCl2 = 2.100M
What is the Molar concentration of Cl- in this solution?
MgCl2 6 Mg2+ + 2 Cl- so you get 2 moles of Cl- for each mole of MgCl2 you dissolved
Molarity MgCl2 = 1.050 Mol MgCl2 X 2 Mol Cl-/1 mol MgCl2 = 2.100M
2. (10 points) When the following solutions are mixed together, what precipitate (if any) will
form:
BaCl2(aq) + Na 2SO4(aq) BaSO4(s)
AgNO3(aq) + Na 3PO4(aq) Ag3PO4(s)
FeSO4(aq) + KCl(aq) N.R.
NaOH(aq) + K 2SO4(aq) N.R.
CoCl2(aq) + NaOH(aq) Co(OH)2
2
3. (10 points) Write balanced molecular, complete ionic and net ionic equations for the
following reactions:
Copper (II) sulfate and mercury(II) chloride:
Molecular: CuSO4(aq) + HgCl2(aq) W HgSO4(s) + CuCl2 (aq)
Complete ionic : Cu2+(aq)+SO42-(aq)+Hg 2+(aq)+2Cl-(aq) WHgSO4(s)+Cu2+(aq) + 2Cl-(aq)
Net ionic: SO42-(aq)+Hg 2+(aq) WHgSO4(s)+Cu2+(aq) + 2Cl-(aq)
Hydrobromic acid and aqueous magnesium hydroxide:
Molecular : 2HBr(aq) + Mg(OH)2(aq) W 2H2O (l) + MgBr2(aq)
Complete ionic: 2H+(aq)+2Br-(aq)+Mg+2(aq)+2OH-(aq) W2H2O (l) + Mg2+(aq)+ 2Br- (aq)
Net ionic: 1H+(aq)+1OH-(aq) W1H2O (l)
4. (10 points) Assign oxidation state for all atoms in the following compounds:
NH3
H=+1
N=-3
Li3 N
Li=+1
N=-3
N2
N=0
NO
O=-2
N=+2
NO2O=-2
N=+3
3
5. A (5 points) Balance the following redox reaction that occurs in acidic conditions:
Cu(s) + NO3-(aq) 6 Cu2+(aq) + NO(g)
Cu(s)6Cu+2
NO3- 6NO
+2
Cu(s)6Cu +2e
NO3- 6NO +2H2O
4H++ NO3- 6NO +2H2O
3e + 4H++ NO3- 6NO +2H2O
X3
X2
+
2+
3Cu(s) +6e + 8H +2NO3 -Cu + 6e + 2NO + 4H2O
3Cu(s) + 8H+(aq) +2NO3- (aq) - Cu2+(aq) + 2NO(g) + 4H2O(l)
B (5 points) Balance the following redox reaction that occurs in basic conditions:
Cl2(g) 6 Cl-(aq) + Ocl-(aq)
This is a little tough because it is a disporportionation reaction:
Cl2-Cl
Cl2-OClCl2-2Cl
Cl2-2OCl2e- + Cl2 - 2Cl2H2O +Cl2-2OCl2H2O +Cl2-2OCl- +4H+
2H2O +Cl2- 2OCl- +4H+ +2eX1
X1
2e- + Cl2 +2H2O +Cl2-2Cl- +2OCl- +4H+ +2e4OH- + Cl2 +2H2O +Cl2-2Cl- +2OCl- +4H+ + 4OH4OH- + Cl2 +2H2O +Cl2-2Cl- +2OCl- +4H2O
2OH -(aq) + Cl2 (g) - Cl-(aq) + OCl-(aq) +H 2O(l)
6. (10 points) I have a vacuum pump that makes a vacuum of 10 mtorr
What is this pressure in atm?
10 mtorr X (1x10-3 torr/mtorr) X (1atm/760torr) = 1.32x10-5 atm
In mm Hg?
1 mm Hg = 1 torr!
10 mtorr X (1x10-3 torr/mtorr) X (1mmHg/1torr) = .01 mm Hg
In lbs/in2?
10 mtorr X (1x10-3 torr/mtorr) X (1 atm/760torr) X (14.7psi/atm) = 1.93x10-4 psi
4
7. (10 points) I have a 12L SCUBA tank filled with air at 30oC to a pressure of 156 atm.
What is the volume of air that I will get from this tank when I dive to 33 ft where the pressure
is 2 atm, and the temperature is 10oC?
PV=nRT
P,V,and T are all variables
PV/T = nR
P1V1/T1 = P 2V2/T2
156(12)/303 = X(2)/283
X = [156(12)283]/[303(2)]
X = 874.2 L
8. (10 points) Silicon tetrachloride (SiCl4) is used as a starting material for the production
of electronics grade silicon. What is the density of SiCl4 a 85oC and 750 torr.
Molar mass = density X (RT/P)
Density = (Molar Mass X P)/ RT
Molar mass = 28.09 +4(35.45) = 169.89g/mol
P = 750/760 = .987 atm
T = 85 + 273.15 = 358.15 K
X = (169.89 g/mol x .987 atm)/ (358.15K X .08206l atm/K mol)
= 5.70 g/L
5
9 (10 points) Consider a 1.0L container of neon gas at STP.
A. What will happen to the average velocity of the gas molecules when I increase the
temperature to 100oC?
Velocity correlates with temperature so T8, V8
B. What will happen to the number of gas molecules colliding with the surface of the
container when I increase the temperature to 100oC?
Molecules moving faster so they will hit the wall more frequently
C. What will happen to the average velocity of the gas molecules when I increase the
volume of the container to 5.0L?
Velocity does NOT correlate with Volume so velocity will not change
D. What will happen to the number of gas molecules colliding with the surface of the
container when I increase the volume of the container to 5.0L
The molecules will be spread out over a larger volume so they will hit the surface less
frequently
E. If I add O2 molecules at the same temperature will the average kinetic energy of the gas
molecules change?
Velocity correlates kinetic energy, which in turn correlates with T. T does not
change so KE does not change.
10. (10 points) An equation given in class for the behavior of a real gas is:
2
Pobs
nRT
 n
=
− a   What are each of the variables in this equation? How is this
V
V − nb
equation similar to or different than the ideal gas law? What changes have been made
from the ideal gas law and why?
Pobs= observed pressure
T = Temperature (in K)
R = Gas Constant = .08206latm/Kmol
V= Volume (in l)
n = number of moles of gas
b = volume of gas molecule
a= attraction (or repulsion) between gas molecules
Equation is similar to Ideal Gas law except for 2 changes. 1.) V has been changed to V-nb
to reflect the actual volume of a gas molecule, and 2.) -a(n/V)2 has been added to reflect
possible gas-gas attractions.