We are being asked to calculate the Ka of the unknown acid, HA if a 0.010 M solution of the sodium salt of the conjugate base, Na+A–, was found to have a pH of 10.35 at 25 °C.
NaA is a salt and it will break up in the solution:
NaA(aq) → Na+(aq) + A-(aq)
concentraion of NaA = concentration of A-
A- is the conjugate base of the unknown acid HA:
HA(aq) + H2O(l) → A-(aq) + H3O+(aq)
(weak acid) (conjugate base)
We're going to calculate Ka of HA using the following steps:
Step 1. Create an ICE chart for the equilibrium reaction of the conjugate base A-.
Step 2. Write the Kb expression for A-.
Step 3. Calculate the pOH of and OH- concentration from the pH.
Step 4. Calculate Kb.
Step 5. Calculate Ka of HA from Kb of A-.
During the analysis of an unknown acid, HA, a 0.010 M solution of the sodium salt of the conjugate base, Na+A–, was found to have a pH of 10.35 at 25 °C.
What is the Ka of the unknown acid, HA?
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What scientific concept do you need to know in order to solve this problem?
Our tutors have indicated that to solve this problem you will need to apply the Weak Acids concept. If you need more Weak Acids practice, you can also practice Weak Acids practice problems.