Why is Acetic acid (pKa = 4.76) stronger than carbonic acid (pKa = 6.36)?
Well actually the $\mathrm{pK}_\mathrm{a1}$ for carbonic acid at 25 °C is 3.6. The pka = 6.36 is also including the equilibrium with dissolved $\ce{CO2}$ in aqueous solution.