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-.
Step 6. Identify the unknown acid.
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.
Using Table 10.1, identify the unknown acid, HA.
Hydrofluoric acid HF 7.2X10-4
Acetic acid CH3COOH 1.8X10-5
Hypobromouss acid HIO 2X10-9
Hydrocyanic acid HBrO 6.2X10-10
Hypoiodous acid HIO 2X10-11
A. HIO, hypoiodous acid
B. HF, hydrofluoric acid
C. HCN, hydrocyanic acid
D. CH3COOH, acetic acid
E. HBrO, hypobromous acid
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What scientific concept do you need to know in order to solve this problem?
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