A chemist has found experimentally that a 0.1 $\frac{mol}{L}$ ammonium ion solution has an ionisation degree of 0.000076. What is the value of $pK_a$ he deduces?
$NH_4^+$: weak acid $K_a$ $=$ $\frac{\alpha^2c }{1-\alpha}$ = $\frac{0.000076^20.1}{1-0.000076}$ $pK_a$ $=$ $-log(\frac{0.000076^20.1}{1-0.000076})$ = 9.2
