Untitled - Kelly Walsh High School

72 CHEMISTRY FOR THE UTTERLY CONFUSED
All possible titration problems will simply be adding steps before and/or after
one of these two mole ratio terms. The problem will begin with the substance
with the most information, and proceed through one of these mole ratios to the
substance with less information given.
Let’s begin with this titration question: How many moles of calcium hydroxide
are necessary to titrate 0.250 mol of acetic acid? We have information about the
acetic acid, and we are seeking information about the calcium hydroxide. We
will begin the problem with the acetic acid since we know more about it, and we
will end the problem with the calcium hydroxide since we do not know anything
about this compound. One way to remember what you need to do is to copy the
given information and the question to the balanced chemical equation, as
shown here:
2 HC2H3O2(aq) Ca(OH) 2(aq) l Ca(C2H3O2) 2(aq) 2 H2O(l) 0.250 mol ? mol
When written this way, you begin with the number and work towards the question
mark. We begin with the acetic acid and use the mole ratio with acetic acid
in the denominator:
(0.250 mol HC2H3O2 ) a 1 mol Ca(OH) 2
b 0.125 mol Ca(OH) 2
2 mol HC2H3O2 This calculation leaves us with the appropriate number of significant figures.
Note that the units of HC 2H 3O 2 have cancelled, leaving the desired units of
mol Ca(OH) 2.
How would the problem be different if the titration question asked is: How
many moles of calcium hydroxide are necessary to titrate 0.0500 L of a 0.100 M
acetic acid solution? We can begin by adding our information to the balanced
chemical equation:
2 HC 2H 3O 2(aq) Ca(OH) 2(aq) l Ca(C 2H 3O 2) 2(aq) 2 H 2O(l)
0.0500 L ? mol
0.100 M
As in the preceding example, we will begin with the acetic acid and go through
the same mole ratio to get to the calcium hydroxide. The difference in this problem
is that we are beginning with liters and molarity instead of moles. We will
need to add a step to our calculation to change liters and molarity to moles. Part