Substitute the given information, and solve for the unknown quantity.
[salt]
5.50 = 4.76 + log ------
[acid]
[salt]
5.50 -- 4.76 = log ------
[acid]
[salt]
0.74 = log ------
acid]
[salt]
antilog 0.74 = ------
[acid]
[salt]
5.5 mol/L
5.5 = ------ = ------------
[acid]
1 mol/L
You have now established the proper ratio of salt and acid that will yield the
desired pH. However, the problem states that a molar concentration of 0.100 mol/L for
the buffer is desired. This mol/L concentration is the sum of the mol/L concentrations of
the salt and the acid. The mol/L concentration of the buffer, based on your calculations
so far, is 6.5 mol/L. The following method is one of several that you may employ to
determine the concentrations necessary to yield the proper mol/L concentration of the
buffer while maintaining the proper ratio.
Express the desired mol/L concentration of the buffer as an equality.
[salt] + [acid] = 0.100 mol/L
Solve the equality for either [salt] or [acid].
[salt] = 0.100 mol/L - [acid]
Now based on the substitution property, you are able to express the [salt] in
terms of [acid], and in effect, derive an equation in one variable instead of the two, in the
[salt]
5.5 = ------
[acid]
0.100 mol/L - [acid]
5.5 = --------------------
[acid]
MD0837
9-18