10. a 20.00 ml sample of 0.150 mol/l ammonia (nh3(aq)) is titrated to the equivalence point by 20.0 ml of a solution of 0.150 mol/l of the strong acid hydroiodic acid (hi (
a) write a balanced equation for the titration reaction.
b) what is the ph of the ammonia solution before the titration begins?
c) what is the ph at the equivalence point?

Explanation:

a) Balanced equation

The balanced chemical equation for the titration is

b) pH at start

For simplicity, let's use B as the symbol for NH₃.

The equation for the equilibrium is

(i) Calculate [OH]⁻

We can use an ICE table to do the calculation.

                      B + H₂O ⇌ BH⁺ + OH⁻

I/mol·L⁻¹:     0.150               0         0

C/mol·L⁻¹:       -x                 +x       +x

E/mol·L⁻¹:  0.150 - x            x          x

Check for negligibility:

(ii) Solve for x

(iii) Calculate the pH

(c) pH at equivalence point

(i) Calculate the moles of each species

                 B    +    HI   ⇌   BH⁺ + I⁻

I/mol:       3.00    3.00         0

C/mol:    -3.00   -3.00     +3.00

E/mol/:       0          0          3.00

(ii) Calculate the concentration of BH⁺

At the equivalence point we have a solution containing 3.00 mmol of NH₄I

Volume = 20.00 mL + 20.00 mL = 40.00 mL

(iii) Calculate the concentration of hydronium ion

We can use an ICE table to organize the calculations.

                      BH⁺+ H₂O ⇌ H₃O⁺ +  B

I/mol·L⁻¹:     0.0750                 0        0

C/mol·L⁻¹:        -x                     +x      +x

E/mol·L⁻¹:   0.0750 - x             x         x

(iv) Calculate the pH

The titration curve below shows the pH at the beginning and at the equivalence point of the titration.