Significant figures for logarithms This is only encountered for pH. 

1. Calculate the logarithm of the number. The rules for expressing significant figures with logarithms will only rarely be encountered in chemistry. The main context for this rule will be the calculation of pH, during in acidbase chemistry. 

Lets see some examples. Calculate the pH of a 0.0452 M HCl. 

A solution has a [H_{3}O^{+} ] = 0.003189 M. What is the pH of the solution? => The [H_{3}O^{+} ] = 0.003189 

A solution has a [H_{3}O^{+} ] = 0.00032 M. What is the pH of the solution? => The [H_{3}O^{+} ] = 0.00032 