Question 60 of 69intermediate🔧 ApplyNumerical3 marks

A 0.02M solution of pyridinium hydrochloride has pH = 3.44. Calculate the ionization constant of pyridine.

Correct Answer

The ionization constant of pyridine is 1.52×10−91.52 \times 10^{-9}.

Exercise: EXERCISES | Q: 6.62 | (Page 46)
For More Understanding

Explanation

Pyridinium hydrochloride is a salt of a weak base (pyridine) and a strong acid (HCl). In aqueous solution, the pyridinium ion hydrolyzes to produce H3O+H_3O^+ ions, making the solution acidic. Using the pH, we first find the H+H^+ concentration. Then, applying the hydrolysis relation for a salt of a weak base and strong acid, [H+]=Kw×CKb[H^+] = \sqrt{\frac{K_w \times C}{K_b}}, we can calculate the ionization constant (KbK_b) of the weak base pyridine.

Solution Steps

  1. Step 1: Calculate the hydrogen ion concentration from the given pH.

    pH=3.44pH = 3.44

    [H+]=antilog(−pH)=antilog(−3.44)=3.63×10−4[H^+] = \text{antilog}(-pH) = \text{antilog}(-3.44) = 3.63 \times 10^{-4} M

  2. Step 2: Write the hydrolysis reaction and the relation for [H+][H^+]. Pyridinium hydrochloride (C5H5NH+Cl−C_5H_5NH^+Cl^-) is a salt of a weak base and a strong acid. The cation hydrolyzes as:

    C5H5NH++H2O⇌C5H5N+H3O+C_5H_5NH^+ + H_2O \rightleftharpoons C_5H_5N + H_3O^+

    For this salt, [H+]=Kw×CKb[H^+] = \sqrt{\frac{K_w \times C}{K_b}}

  3. Step 3: Rearrange the formula to solve for KbK_b.

    Kb=Kw×C[H+]2K_b = \frac{K_w \times C}{[H^+]^2}
  4. Step 4: Substitute the known values into the equation. Given: C=0.02C = 0.02 M, Kw=1×10−14K_w = 1 \times 10^{-14}, [H+]=3.63×10−4[H^+] = 3.63 \times 10^{-4} M

    Kb=1×10−14×0.02(3.63×10−4)2K_b = \frac{1 \times 10^{-14} \times 0.02}{(3.63 \times 10^{-4})^2}
  5. Step 5: Calculate the final value of KbK_b.

    Kb=2×10−1613.17×10−8=1.52×10−9K_b = \frac{2 \times 10^{-16}}{13.17 \times 10^{-8}} = 1.52 \times 10^{-9}