Homework assignment – Chapters 5 and 6
1. Write and balance equations in “molecular” form for each of the following. Use the symbols (g), (l), (s) and (aq) as
appropriate.
a. solid lead(II) sufide reacts with aqueous hydrochloric acid (HCl) to form solid lead(II) chloride and dihyrogen
sulfide gas.
PbS(s) + 2HCl(aq) = PbCl2(s) + H2S(g)
b. Solid magnesium reacts with aqueous copper(I) nitrate to form aqueous magnesium nitrate and solid copper
Mg(s) + Cu(NO3)2(aq) = Mg(NO3)2(aq) + Cu(s)
c. dinitrogen pentoxide is changed into nitrogen dioxide and oxygen (all are gases)
2N2O5(g) = 4NO2(g) + O2(g)
d. Aluminum metal reacts with chlorine to form aluminum chloride.
2Al(s) + 3Cl2(g) = 2AlCl3(s)
2. Classify each of the reactions above as: a) redox or non-redox and b) combination, decomposition, single replacement
or double displacement.
a.
b.
c.
d.
Non-redox, double displacement
Redox, single replacement
Redox, decomposition
Redox, combination
3. Write and balance an equation in “molecular” form for the following. Use the symbols (g), (l), (s) and (aq) as
appropriate. Silver nitrate solution and calcium chloride solution form solid silver chloride and calcium nitrate solution.
2AgNO3(aq) + CaCl2(aq) = 2AgCl(s) + Ca(NO3)2(aq)
What are the spectator ions in this reaction? NO3- and Ca2+
Write both total ionic and net ionic equations.
Ionic : 2Ag+(aq) + 2NO3-(aq) + Ca2+(aq) + 2Cl-(aq) = 2AgCl(s) + Ca2+(aq) + 2NO3-(aq)
Net ionic: 2Ag+(aq) + 2Cl-(aq) = 2AgCl(s)
4. Reaction 1.d. above is known to be an exothermic reaction. Rewrite the equation in 1.d. to indicate this
characteristic.
2Al(s) + 3Cl2(g) = 2AlCl3(s) + heat
5. Given the equation in 1.d. above, perform the following calculations.
a. How many moles of AlCl3 can be formed if 0.25 moles of chlorine gas are consumed?
? mol AlCl3 = 0.25 mol Cl2 X 2 mol AlCl3/3 mol Cl2 = 0.17 mol AlCl3
b. If one begins with 4.0 grams of Al, how many grams of AlCl3 can be formed?
? g AlCl3 = 4.0 g Al X 1 mol Al/26.7 g Al X 2 mol AlCl3/2 mol Al X 133 g AlCl3/mol AlCl3 = 20 g AlCl3
c. If only 6.5 grams of AlCl3 are formed, what is the % yield?
% yield = 6.5 g/20 g X100 = 33%
6. A diver has a lung capacity of 6.3 L and inhales air at the surface where the pressure is 1.0 Atm. If the diver descends
to 25 m, where the pressure is 3.5 atm, what is the capacity of the lungs be at this depth? Assume a constant
temperature. P1V1/T1 = P2V2/T2. T drops out of the equation. P1 = 1.0 Atm. V1 = 6.3 L. P2 = 3.5 Atm.
V2 = 1.8 L
7. A container with 0.87 mol of gas has a volume of 334 mL. What would the volume be if an additional 0.22 mol of gas
is added? Assume a constant pressure and temperature.
Since V is directly proportional to the number of atoms (mol) of gas, V1/n1 = V2/n2 V1 = 334 mL. n1 = 0.87 mol.
n2 = 0.87 + 0.22 = 1.09 mol. V2 = 420 mL
8. A gas sample has a pressure of 500 Torr, volume of 450 mL and temperature of 20 oC. What would the pressure be if
the volume changed to 300 mL and the temperature increase to 25oC?
P1V1/T1 = P2V2/T2. T1 = 20 +273 = 293K. P1 = 500 Torr. V1 = 450 mL. T2 = 25 +273 = 298K. V2 =300 mL.
Solve for P2. P2 = 760 Torr.
9. How many grams of hydrogen are contained in 1.78 L at 40oC and 748 Torr? Use 0.0821 L Atm/molK for R. Hint: first
calculate n then convert to grams. Also don’t forget to convert Torr to Atm.
Use PV = nRT. P = 748 Torr X 1 Atm/760 Torr = 0.984 Atm. T = 40 + 273 = 313 K. Then calculate n.
n = 0.0682 mol. ? g H2 = 0.0682 mol H2 X 2.02 g H2/mol H2 = 0.14 g H2
10. A gas sample weighs 0.56 g, has a volume of 498 mL, temp of 24 oC and pressure of 750 mm Hg. Find the molecular
weight of the gas and guess its identity. Hint: first use the Ideal Gas Law equation to find the mol of gas, then use the
mass of the gas to find its formula weight.
Use PV = nRT. P = 750 Torr X 1 Atm/760 Torr = 0.987 Atm. T = 24 + 273 = 297 K. V = 0.498 L. Then calculate n.
n = 0.0202 mol. Since fomula weight is g/mol, 0.56 g/0.0202 mol = 28 g/mol. Closest to N2.
11. Rank the following from most to least in terms of an ideal gas.
H2O(g), Kr(g), H2(g), He
H2, He, Kr, H2O
12. Rank the following from fastest to slowest in effusion/diffusion rate.
H2O(g), Kr(g), H2(g), He
H2, He, H2O, Kr
13. Describe each of the following processes as exothermic or endothermic.
a. water(l) evaporates - endothermic
b. water(l) freezes - exothermic
c. paper burns - exothermic
d. a chemical “cold pack” becomes cold - endothermic