# Problem: 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?A. 5.13X10-6B. 2.00X10-19C. 1.95X10-9D. 2.00X10-10E. 5.01X103

###### FREE Expert Solution

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-.

82% (212 ratings) ###### Problem Details

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?

A. 5.13X10-6

B. 2.00X10-19

C. 1.95X10-9

D. 2.00X10-10

E. 5.01X103