A chemist has found experimentally that a 0.049 $M$ propionic acid solution has a $pH$ = 3.09. Find the value of the $pK_a$ that he deduced from there !
$CH_3CH_2COOH$: weak acid Given $y$ $=$ $[H_3O^+]$ $=$ $10^{-pH}$ $=$ $10^{-3.09}$ The equation $y^2$ $+$ $K_a\;y$ $-$ $K_a\;c$ $=$ $0$ produces: $K_a$ $=$ $\frac{y^2}{c-y}$ $pK_a$ $=$ $-log(\frac{y^2}{c-y})$ = 4.87
