Question

0.110 M in formic acid and 5.5×10−2 M in hypochlorous acid . How to find pH

Answer 1

Look up Ka values. Formic acid is about 10^-4 and HClO is about 10^-8 so formic acid is a much stronger acid. Also there is more of it. I would be tempted to ignore the HClO and calculate pH from HCOOH alone.

Answer 2

For weak acids in pure water ...
pH = -log[H^+]
[H^+] = SqrRt[(Ka)(Molar Conc of Acid)]

HClO:
[H^+]=(SqrRt[(10^-4)(0.055)])M
= 0.0024M
pH=-log(0.0024)=-(-2.63)=2.63

HCOOH:
[H^+]=(SqrRt[(10^-8)(0.110)])M
=3.32E-5M
pH=-log(3.32E-5)=-(-4.50)=4.50

Answer 3

Note: Dr Rebel has reversed the Ka values (as well as using my estimates instead of the tabular values for Ka) AND calculated as if the acids were alone and not in a mixture.