Stoichiometry ‹ 99
Since the 45.20 was rounded up, the answer is slightly high; therefore, pick the closest
answer that is slightly lower.
- B—The question states that in this reaction, all silver compounds are insoluble, which
means that Ag+ is not a possible product. Silver carbonate is insoluble, and its formula
should be written as Ag 2 CO 3. Hydrochloric acid is a strong acid, so it should be writ-
ten as separate H+ and Cl- ions. Silver chloride, AgCl, is insoluble, and carbonic acid,
H 2 CO 3 , quickly decomposes to CO 2 and H 2 O. - C—V:
2.39
50.94 = 0.0469; O:
1.00
16.0
1.00/16.0 = 0.0625
Divide by the smaller ratio (0.0469):
0.0469
0.0469 = 1 and0.0625
0.0469 = 1.33
It is necessary to have a whole-number ratio. The value 1.33 is too far from a whole
number to round; therefore, it is necessary to multiply by the smallest value leading to
a whole number. Multiplying both by 3 gives 3 V and 4 O.
- D—The doubling of the volume will result in halving the concentrations (Mg^2 + =
0. 050 M, NO 3 - = 0.10 M, K+ = 0.050 M, and OH- = 0.050 M). The reaction is
Mg(NO 3 ) 2 (aq) + 2 KOH(aq) → Mg(OH) 2 (s) + 2 KNO 3 (aq)
After the reaction, some of the magnesium remains (the remainder precipitated), the
potassium does not change (soluble), and there are two nitrate ions (soluble) per mag-
nesium nitrate.
- C—The calculation is
(6 mol H O)
18 g
mol HO
(250 g NaSO6HO)100 %
2
2
24 2
⋅
× = 43%
The numerator gives the mass of the water molecules present, and the denominator is
from the molar mass of the formula. The percentages for the other compounds are 11%
(A), 20% (B), and 50% (D). As in other calculations, rounding will simplify the work.
- B—Calculate the moles of acid to compare to the moles of Cu:
()
10.0 mL
12 mol
1,000 mL
= 0.12 mole.The acid is the limiting reactant, because 0.30 mole of copper requires 0.80 mole
of acid. Use the limiting reactant to calculate the moles of NO formed:
()
0.12 mol acid2 mol NO
8 mol acid = 0.030 mole- C—The balanced chemical equation is 2 Au 2 O 3 → 4 Au + 3 O 2
()
221 g AuO1 mol AuO
442 g AuO3 mol O(^23) 2 mol AuO
23
23
2
23
= 0.750 mole O 2
Note the 2:1 relationship between the formula mass and the mass of reactant.
- A—The molecular equation is HNO 2 + NaOH → NaNO 2 + H 2 O.
Nitrous acid is a weak acid; as such, it should remain as HNO 2. Sodium hydroxide is
a strong base, so it will separate into Na+ and OH- ions. Any sodium compound that
might form is soluble and will yield Na+ ions. The sodium ions are spectator ions and
are left out of the net ionic equation.