A chemist has found experimentally that a 0.015 $\frac{mol}{L}$ nitrous acid solution has an ionisation degree of 0.16. What is the value of $pK_a$ he deduces?
$HNO_2$: weak acid $K_a$ $=$ $\frac{\alpha^2c }{1-\alpha}$ = $\frac{0.16^20.015}{1-0.16}$ $pK_a$ $=$ $-log(\frac{0.16^20.015}{1-0.16})$ = 3.3
