The ph of 0.015 m hno2 (nitrous acid) aqueous solution was measured to be 2.63. what is the value of pka of nitrous acid?

Nitrous acid dissociates as follows:
HNO₂(s) ⇄ H⁺(aq) + NO₂⁻(aq)            
According to the equation, an acid constant has the following form:
Ka = [H⁺] × [NO₂⁻ ] / [HNO₂] 
From pH, we can calculate the concentration of H⁺ and NO₂⁻:
[H⁺] = 10^-pH = 10^-2.63 = 0.00234 M = [NO₂⁻]
Now, the acid constant can be calculated:
Ka = 0.00234 x 0.00234 / 0.015  = 3.66 x 10⁻⁴
And finally, 
pKa = -log Ka = 3.44