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

original expression.

[salt]

0.3 = ------

[base]

0.100 mol/L - [base]

0.3 = --------------------

[base]

Simplify the expression by clearing the fraction.

0.3 [base] = 0.100 mol/L - [base]

MD0837

9-20

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