Answer to Question #184342 in General Chemistry for indri

Question

A diprotic acid H2A has the following ionization constants: Ka1 = 1.1 x10 ^ -3 and Ka2 = 2.5 x 10 ^ -6. to have a buffer solution with a pH of 5.80, which combination do you choose: NaHA / H2A or NaA / NaHA

Answer 1

Whenever you need to make the decision of which acid to use the general rule of thumb is to use the one with the pKa value close to the desire pH value of the buffer solution. So you want the pKa value to be close to 5.8

You are given the Ka values so to get pKa you do -log(pKa)

$-log(1.1×10^{-3} )= 2.95$

$-log(2.5×10^{-6} ) = 5.6$

Clearly the second one is closer to 5.8 so you chose that one. So you would chose Na2A/NaHA