344
Hydrogen-Ion Concentration and pH
The solution is acidic because there is 0 00500 0 00400 = 0 00100 mole more
acid than base The 0 00400 mole of NaCl that is formed has no effect on the pH
The 0 00100 mole of HC1 is completely dissociated and is in a total of 75 0 ml
of solution As a result, the concentration of H
+ is
The pH is 1 88, as determined directly from your calculator, or as follows with
a log table
pH = -log (1 33 x 10 -) = -log 1 33 - log (10
2 )
= -0 12 + 2 00 = 1 88
PROBLEM:
What is the pH after 25 0 ml of 0 200 M NaOH are added to 50 0 ml of
0 100 M HCP
SOLUTION:
As in the previous problem, first find the number of moles of acid and base
used
Moles of HC1 = (0 0500 liter) (0 100
= 0 00500 mole
liter/
(0
\
Moles of NaOH = (0 0250 liter) (o 200
= 0 00500 mole
V
liter/
Because the same number of moles of acid and base are used, the solution is
the same as that of pure water, with pH = 70 It must be emphasized that
this conclusion will be correct only if a strong acid reacts with a strong base
The pH of solutions of salts of weak acids or bases is considered in the next
chapter
PROBLEM:
Find the pH of the solution that results from the addition of 26 00 ml of 0 200 M
NaOH to 50 0 ml of 0 100 M HC1
SOLUTION:
As before,
Moles of HC1 = (0 0500 liter) ffl 100-™^ = 0 00500 mole
V
liter/
Moles of NdOH = (0 0260 liter) fo 200 0¥L!~\ = o 00520 mole
V
liter/
The amount of excess NaOH is 0 00520 mole - 0 00500 mole = 0 00020 mole
This excess of NaOH is present in 76 0 ml, and it will be completely dissociated Therefore,