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.Using Table 10.1, identify the unknown acid, HA.Hydrofluoric acid               HF                          7.2X10-4  Acetic acid                         CH3COOH             1.8X10-5Hypobromouss acid          HIO                        2X10-9Hydrocyanic acid               HBrO                     6.2X10-10 Hypoiodous acid                HIO                       2X10-11A. HIO, hypoiodous acidB. HF, hydrofluoric acidC. HCN, hydrocyanic acidD. CH3COOH, acetic acidE. HBrO, hypobromous acid

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-.

Step 6. Identify the unknown acid.

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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.

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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