The $\mathrm{C}-\mathrm{H}$ bond in acetone, $\left(\mathrm{CH}_{3}\right)_{2} \mathrm{C}=\mathrm{O}$, has a $\mathrm{pK}_{\mathrm{a}}$ of $19.2 .$ Draw two resonance structures for its conjugate base. Then, explain why acetone is much more acidic than propane, $\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{3}\left(\mathrm{pK}_{\mathrm{a}}=50\right)$.