The Foundations of Chemistry

Even though the reaction occurs in

aqueous solution, this calculation is

similar to earlier examples because

we are given the amounts of pure

reactants.

3-4 Percent Yields from Chemical Reactions 99

Plan

Interpreting the balanced equation as usual, we have

NiCl 2 
2NaOH88nNi(OH) 2 
 2NaCl

1 mol 2 mol 1 mol 2 mol

129.6 g 2(40.0 g) 92.7 g 2(58.4 g)

We determine the number of moles of NiCl 2 and NaOH present. Then we find the number
of moles of each reactant required to react with the other reactant. These calculations iden-
tify the limiting reactant. We base the calculation on it.

Solution

__? mol NiCl 2 25.9 g NiCl 2 0.200 mol NiCl 2

__? mol NaOH10.0 g NaOH0.250 mol NaOH

We return to the balanced equation and calculate the number of moles of NaOH required to
react with 0.200 mol of NiCl 2.

__? mol NaOH0.200 mol NiCl 2 0.400 mol NaOH

But we have only 0.250 mol of NaOH, so NaOH is the limiting reactant.

g of NaOH 88n mol of NaOH 88n mol Ni(OH) 2 88n g of Ni(OH) 2

__? g Ni(OH) 2 10.0 g NaOH

 11.6 g Ni(OH) 2

You should now work Exercises 30 and 34.

92.7 g Ni(OH) 2



1 mol Ni(OH) 2

1 mol Ni(OH) 2



2 mol NaOH

1 mol NaOH



40.0 g NaOH

2 mol NaOH



1 mol NiCl 2

1 mol NaOH



40.0 g NaOH

1 mol NiCl 2



129.6 g NiCl 2

Problem-Solving Tip:How Can We Recognize a Limiting Reactant

Problem?

When we are given the amounts of two(or more) reactants, we should suspect that we

are dealing with a limiting reactant problem. It is very unlikely that exactlythe stoichio-

metric amounts of both reactants are present in a reaction mixture.

PERCENT YIELDS FROM CHEMICAL REACTIONS

The theoretical yieldfrom a chemical reaction is the yield calculated by assuming that
the reaction goes to completion. In practice we often do not obtain as much product from
a reaction mixture as is theoretically possible. This is true for several reasons. (1) Many
reactions do not go to completion; that is, the reactants are not completely converted to
products. (2) In some cases, a particular set of reactants undergoes two or more reactions

3-4

In the examples we have worked to

this point, the amounts of products

that we calculated were theoretical

yields.

A precipitate of solid Ni(OH) 2 forms

when colorless NaOH solution is

added to green NiCl 2 solution.