Substitute the given values, and solve for the unknown quantity.
[salt]
4.2 = 4.75 + log ------
[base]
[salt]
4.2 - 4.75 = log ------
[base]
[salt]
-0.6 = log ------
[base]
[salt]
antilog (-0.6) = --------
[base]
[salt]
0.3 = ------
[base]
Express the desired mol/L concentration of the buffer as an equality.
[salt] + [base] = 0.100 mol/L
Solve the equality for either [salt] or [base].
[salt] = 0.100 mol/L - [base]
Now, based on the substitution property, you are able to express the [salt] in
terms of [base], and in effect derive an equation in one variable instead of the two in the
[salt]
0.3 = ------
[base]
0.100 mol/L - [base]
0.3 = --------------------
[base]
0.3 [base] = 0.100 mol/L - [base]
MD0837
9-20