Untitled - Kelly Walsh High School

74 CHEMISTRY FOR THE UTTERLY CONFUSED
Quick Tip
The calculation will begin as in the first example in this section:
(0.200 mol HC2H3O2 ) a 1 mol Ca(OH) 2
b
2 mol HC2H3O2 This will give us the moles of calcium hydroxide instead of the grams, thus we
need to add another step. We have moles and we need grams, the molar mass
relates these two quantities. We determine the molar mass by using the formula
and the atomic weights found in a table such as the periodic table.
Anytime the problem deals with the moles and the mass of a substance, you will
very, very likely need to know the molar mass (molecular weight) of the substance
to complete the problem.
The molar mass of calcium hydroxide is 74.10 g/mol. We can add the molar mass
to our calculation to convert our moles to grams:
(0.200 mol HC2H3O2 ) a 1 mol Ca(OH) 2
ba
2 mol HC2H3O2 74.10 g Ca(OH) 2
b
1 mol Ca(OH) 2
7.41 g Ca(OH) 2
This calculation leaves us with the appropriate number of significant figures and
the correct units.
Now let’s work one of the more typical titration problems. What is the concentration
of a calcium hydroxide solution if 0.0250 L of a calcium hydroxide solution
were necessary to titrate 0.0400 L of a 0.100 M acetic acid solution? We have
more information, but this does not change our first step of transferring this
information to the balanced chemical equation. You should recognize that by
“concentration,” the problem means molarity.
2 HC 2H 3O 2(aq) Ca(OH) 2(aq) l Ca(C 2H 3O 2) 2(aq) 2 H 2O(l)
0.0400 L ? M
0.100 M 0.0250 L