Untitled - Kelly Walsh High School

38 CHEMISTRY FOR THE UTTERLY CONFUSED
In the problem above, the amount of product calculated based upon the limiting
reactant concept is the maximum amount of product that will form from the
specified amounts of reactants. This maximum amount of product is the theoretical
yield. However, rarely is the amount that is actually formed (the actual
yield) the same as the theoretical yield. Normally it is less. There are many reasons
for this, but the principal one is that most reactions do not go to completion;
they establish an equilibrium system (see Chapter 14 for a discussion on
chemical equilibrium). For whatever reason, not as much product as expected
is formed. We can judge the efficiency of the reaction by calculating the percent
yield. The percent yield (% yield) is the actual yield divided by the theoretical
yield and the resultant multiplied by 100 in order to generate a percentage:
%yield
In the previous problem, we calculated that 71.5 g Fe 2O 3 could be formed.
Suppose that after the reaction we found that only 62.3 g Fe 2O 3 formed. Calculate
the percent yield.
%yield
3-5 Percent Composition and
Empirical Formulas
If we know the formula of a compound, it is a simple task to determine the percent
composition of each element present. For example, suppose you wanted
the percentage carbon and hydrogen in methane, CH 4. First, calculate the
molecular mass of methane:
Substituting the molar masses:
actual yield
100 %
theoretical yield
62.3 g
100 % 87.1%
71.5 g
1 mol CH 4 1 mol C 4 mol H
1 mol CH 4 1(12.01 g/mol) 4(1.008 g/mol) 16.042 g/mol
(This is an intermediate calculation—don’t worry about significant figures yet)
%carbon [mass C/mass CH4] 100%
[12.01 g/mol/16.042 g/mol] 100% 74.87 % C
%hydrogen [mass H/mass CH4] 100%
[4(1.008g/mol)/16.042 g/mol] 100% 25.13 % H