Question

What concentration of Formic acid, HCOOH in M is required to give a solution with a pH of 3.0? (Ka=(HCOOH)=1.77x10^-4

Answer 1

write an expression for the dissociation of the acid,

for example: \(HA \rightarrow H^++A^-\)

\(K_a=\dfrac{[H^+][A^-]}{[HA]}\)

If you know the pH, then you know \([H^+]\), because \(pH=-log[[H^+]\).
So plug the values you now and solve for the concentration of acid [HA].

HINT: because each molecule of acid that dissociates yields equal amounts of \(H^+\) and \(A^-\), their concentrations are equal. \((i.e. ~[H^+]=[A^-])\)