NAME:
CHE 121 SUMMER QUARTER 2008
MIDTERM 2
Constants:
Avogadro’s Number; NA = 6.022 x 1023 particles per mole
Universal gas constant; R = 0.0821 L atm K-1mol-1
Kelvin = oC + 273
760 torr = 1 atm
1. Indicate how many electrons the following atoms will gain or lose when
they form ions.
a. K
b. S
(5 marks)
(a) lose 1 electron
(b) gain 2 electrons
2. Sodium sulfide is a binary ionic compound
a. What is the formula weight of sodium sulfide in gmol-1 ?
b. In 1 mole of sodium sulfide how many moles of sodium ions are
there?
(5 marks)
(a) FW = 2x22.90 + 32.065 = 77.865 gmol-1
(b) 2 moles of sodium ions
3. The phosphate anion has the formula PO43a. Draw the Lewis structure for the phosphate anion.
b. Use VSEPR theory to determine the shape of the phosphate
anion.
(6 marks)
(b) tetrahedral
4. Identify the bond in the following compounds as either ionic, polar
covalent or nonpolar covalent.
a. NaBr
b. H2
c. HCl
(5 marks)
(a) ionic
(b) nonpolar covalent
(c) polar covalent
5. Name the following binary covalent compounds.
a. N2O5
b. CBr4
(5 marks)
(a) dinitrogen pentoxide
(b) carbon tetrabromide
6. Identify the following statements as either true or false.
a. Dipolar forces occur between polar molecules True
b. Dipolar forces occur between nonpolar molecules False
c. Hydrogen bonding is the result of attractive forces between
molecules on which hydrogen atoms are covalently bonded to
weakly electronegative elements. False
(5 marks)
7. Hydrogen gas (H2(g)) reacts with oxygen gas (O2(g)) to produce water
(H2O(l)).
a. Write a balanced equation for the production of H2O by this
reaction.
b. What kind of reaction is this?
c. Is the reaction a redox reaction?
d. If the reaction is a redox reaction what are the oxidant and
reactant?
(a)
(8 marks)
2H2 + O2  2H2O
(b) combination
(c) yes
(d) oxidant is oxygen, reductant is hydrogen
8. Consider the following reaction:
3C(s) + 2Fe2O3(s)  4Fe(s) + 3CO2(g)
a. What kind of reaction is this?
b. Is the reaction a redox reaction?
c. If the reaction is a redox reaction what are the oxidant and
reactant?
(a) single replacement
(b) yes
(c) reductant is carbon, oxidant is iron
(6 marks)
9. Consider the following reaction:
NaOH(aq) + HCl(aq)  NaCl(aq) + H2O(l)
a. What kind of reaction is this?
b. Write the total ionic equation for this reaction
c. Write the net ionic equation for this reaction
d. Identify the spectator ions in this equation
e. Is this reaction a redox reaction?
(10 marks)
(a) double replacement, metathesis reaction
(b) Na+(aq) + OH-(aq) + H+(aq) + Cl-(aq)  Na+(aq) + Cl-(aq) + H2O(l)
(c) OH-(aq) + H+(aq)  H2O(l)
(d) Na+ and Cl(e) No
10. Identify the following statements as true or false.
a. Exothermic reactions liberate heat. True
b. Endothermic reactions liberate heat. False
c. Exothermic reactions result in the surroundings becoming cooler.
False
d. Endothermic reactions result in the surroundings becoming hotter.
False
(4 marks)
11. Consider the following reaction:
CH4(g) + 2O2(g)  CO2(g) + 2H2O(l)
a. How many moles of O2 gas will be required to react completely
with 1.94 moles of CH4(g)?
b. How many moles of CH4(g) are required to produce 12.56 g of
CO2(g)?
(4 marks)
(a) 2 x 1.94 = 3.88 moles of O2
(b) n = m/M = 12.56/(12.011 + 15.999x2) = 0.2853 moles of CO2 requires
0.2853 moles of CH4
12. Identify the following statements as true or false.
a. Solids have high density True
b. Solids have large thermal expansion False
c. Gases have large thermal expansion True
d. Liquids have indefinite shape True
e. Gases are easily compressed True
(5 marks)
13. A sample of helium gas (He) is contained in a cylinder with a fixed
volume of 5.00 L at a temperature of 25oC and a pressure of 0.951
atmospheres. What pressure would the helium be at if the cylinder was
heated to 500C.
(8 marks)
Pi = 0.951 atm
Vi = 5.00 L
Ti = 273 + 25 = 298 K
Pf = ???
Vf = 5.00 L
Tf = 50 + 273 = 323 K
PiVi/Ti = PfVf/Tf
PiViTf/(TiVf) = Pf
(0.951 x 5.00 x 323)/(298x5.00L) = 0.951x323/298 = 1.03 atm
14. A gas is compressed from a volume of 2V to a volume of V while
maintaining a constant temperature. If the initial pressure was P what
would the final pressure be?
Pi = P
Vi = 2V
Ti = Tf
Pf = ???
Vf = V
PiVi = PfVf
Pf =PiVi/Vf
Pf = Px2V/V = 2P
(8 marks)
15. A gas at a pressure of 1 atm and a temperature of 298K in a 2L
container is compressed until the volume of the container is 1L and it is
at a pressure of 2 atm. What is the temperature of the compressed
gas?
(8 marks)
Pi = 1 atm
Vi = 2 L
Ti = 298 K
Pf = 2 atm
Vf = 1 L
Tf = ?
PiVi/Ti = PfVf/Tf
Ti/PiVi = Tf/PfVf
PfVfTi/PiVi = Tf
Tf = 2 x 1 x 298 / 1 x 2 = 298 K
16. Calculate the volume of 0.413 moles of H2 gas at a temperature of
20oC and a pressure of 1200 torr.
P = 1200 torr x 1 atm/760 torr = 1.58 atm
T = 273 + 20 = 293 K
n = 0.412 moles
R = 0.0821 mol L K-1atm
PV = nRT
V = nRT/P = 0.412 x 0.0821x293/1.58 = 6.28 L
(8 marks)