25.0 mL of 0.100 M methylamine, CH3NH2 (K_b = 4.4x 10^-4), is titrated with 0.100 M HCl. Use [OH-] = sqrtK_bC for the initial solution and Henderson's equation in the buffer region. Which statements are correct?

The pH at the equivalence point is 5.97.

Methyl red (colour change range pH 4.4-6.2) is a suitable indicator.

The pH after 12.5 mL of HCl is 3.36.

The pH at the equivalence point is 5.97.

Methyl red (colour change range pH 4.4-6.2) is a suitable indicator.

JEE Advanced levelOne or more correct optionsDifficulty 3 of 3ia145

$25.0 mL$ of $0.100 M$ methylamine, $CH X_{3} NH X_{2}$ ( $K_{b} = 4.4 \times 1 0^{- 4}$ ), is titrated with $0.100 M$ HCl. Use $[OH X^{-}] = K_{b} C$ for the initial solution and Henderson's equation in the buffer region. Which statements are correct?

Initial:

[OH X^{-}] = 4.4 \times 1 0^{- 4} \times 0.100 = 6.633 e - 03 M

pH = 11.82

. Half-way:

pOH = p K_{b}

, so

pH = 14 - 3.36 = 10.64

(not

3.36

). Equivalence:

0.0500 M

CH X_{3} NH X_{3} X^{+}

K_{a} = K_{w} / K_{b} = 2.3 \times 1 0^{- 11}

[H X^{+}] = K_{a} C

pH = 5.97

, which lies inside the range of methyl red.