How do I solve this problem on concentration,pH, and pOH?

Question

Calculate the Ka for a 0.100M HF solution if the [H+] concentration is 8.13×10^-3M.

 

What is the [H+] in a solution of phenol (C6H5OH) if the Ka is 1.02×10^-10?

 

Calculate the pH and pOH of the solutions above

Answer 1

Ka = [H+][F-]/[HF] = (8.13x10-3)²/0.1

Solve for Ka. 

 

Ka = [H+][A-]/HA

1.02x10-10 = (x)(x)/[HA]

This cannot be solved without knowing the [C6H5OH]

 

pH of first solution is -log 8.13x10-³ = =2.09