Page 158 - Theory and Problems of BEGINNING CHEMISTRY

CHAP. 10] STOICHIOMETRY 147

EXAMPLE 10.11. How many moles of lithium nitride, Li 3 N, can be prepared by the reaction of 0.600 mol Li and 0.100
mol N 2 ?
Ans. 6Li + N 2 −→ 2Li 3 N
Since the ratio of moles of lithium present to moles of nitrogen present is 6 : 1, just as is required for the balanced
equation, either reactant may be used.
2 mol Li 3 N 2 mol Li 3 N

0.600 mol Li = 0.200 mol Li 3 N or 0.100 mol N 2 = 0.200 mol Li 3 N
6 mol Li 1 mol N 2
(The first sentence of this answer should be stated on an examination.)

10.4. CALCULATIONS BASED ON NET IONIC EQUATIONS
The net ionic equation (Chap. 9), like all balanced chemical equations, gives the ratio of moles of each
substance to moles of each of the others. It does not immediately yield information about the mass of the entire
salt, however. (One cannot weight out only Ba 2+ ions.) Therefore, when masses of reactants are required, the
specific compound used must be included in the calculation.

EXAMPLE 10.12. (a) How many moles of silver ion are required to make 5.00 g AgCl? (b) What mass of silver nitrate
is required to prepare 5.00 g AgCl?
Ans. Ag + Cl −→ AgCl
−
+
(a) The molar mass of AgCl, the sum of the atomic masses of Ag and Cl, is 108 + 35 = 143 g/mol. In 5.00 g of
AgCl, which is to be prepared, there is
1 mol AgCl

5.00 g AgCl = 0.0350 mol AgCl
143 g AgCl
+
Hence, from the balanced chemical equation, 0.0350 mol of Ag is required:
+
1 mol Ag
0.0350 mol AgCl = 0.0350 mol Ag +
1 mol AgCl

1 mol AgNO 170 g AgNO
(b)0.0350 mol Ag + 3 3 = 5.95 g AgNO 3
1 mol Ag + 1 mol AgNO 3
Hence, when treated with enough chloride ion, 5.95 g of AgNO 3 will produce 5.00 g of AgCl.
EXAMPLE 10.13. What is the maximum mass of BaSO 4 that can be produced when a solution containing 4.35 g of
Na 2 SO 4 is added to another solution containing an excess of Ba ?
2+
Ans. Ba 2+ + SO 4 2− −→ BaSO 4
2− 233 g BaSO

4
1 mol Na 2 SO 4 1 mol SO 4 1 mol BaSO 4
4.35gNa SO 4 2− = 7.14 g BaSO 4
2
142gNa SO 4 1 mol Na 2 SO 4 1 mol SO 4 1 mol BaSO 4
2
10.5. HEAT CAPACITY AND HEAT OF REACTION
Heat is a reactant or product in most chemical reactions. Before we consider including heat in a balanced
chemical equation, first we must learn how to measure heat. When heat is added to a system, in the absence of
a chemical reaction the system may warm up, or a change of phase may occur. In this section change of phase
will not be considered.
Temperature and heat are not the same. Temperature is a measure of the intensity of the heat in a system.
Consider the following experiment: Hold a lit candle under a pot of water with 0.5 in. of water in the bottom.
Hold an identical candle, also lit, under an identical pot full of water for the same length of time. To which
sample of water is more heat added? Which sample of water gets hotter?
The same quantity of heat is added to each pot, since identical candles were used for the same lengths of
time. However, the water in the pot with less water in it is heated to a higher temperature. The greater quantity
of water would require more heat to get it to the same higher temperature.