Question 1150357
You simply want to find negative logarithm of the hydronium ion (in moles per liter).   You could use a handheld calculator for that.


{{{pH=-log((8.7*10^(-5)))}}}, and then round to one or two decimal places; not further, since only two significant figures were given.


{{{-(log((8.7))+log((10^(-5))))}}}

{{{-log((8.7))+5*log((10))}}}

{{{-0.93952+5*1}}}

{{{5-0.93952}}}

{{{pH=4.1}}}



{{{or}}}{{{maybe}}}{{{4.06}}}