A chemist has found experimentally that a 0.049 $\frac{mol}{L}$ propanoic acid solution has an ionisation degree of 0.017. What is the value of $pK_a$ he deduces?
$CH_3CH_2COOH$: weak acid $K_a$ $=$ $\frac{\alpha^2c }{1-\alpha}$ = $\frac{0.017^20.049}{1-0.017}$ $pK_a$ $=$ $-log(\frac{0.017^20.049}{1-0.017})$ = 4.87
