A buffer solution of a weak acid and its salt had a pH of 7.85 when the acid was 0.251 M and the anion was 1.10 M. How do you calculate the equilibrium dissociation constant (Ka)?

1 Answer
Mar 22, 2016

Answer:

You can do it like this:

Explanation:

We will call the acid #HA# which dissociates:

#HA_((aq))rightleftharpoonsH_((aq))^(+)+A_((aq))^-#

We know the #pH# is #7.95#.

#:.-log[H^+]=7.85#

From which #[H^+]=1.41xx10^(-8)"mol/l"#

The expression for #K_a# is:

#K_a=([H_((aq))^(+)][A_((aq))^-])/([HA_((aq))])#

These are equilibrium concentrations which the question has already provided us with.

#:.K_a=(1.41xx10^(-8)xx1.1)/(0.251)#

#K_a=6.18xx10^(-8)"mol/l"#