For 4.0x10^-8 M HCl at 298 K (K_w = 1.0x10^-14), which statements are correct?

[H+] = 4.0x10^-8 M because HCl is strong.

Ionisation of water contributes a negligible part of [H+].

JEE Advanced levelOne or more correct optionsDifficulty 3 of 3ie041

For $4.0 \times 1 0^{- 8} M$ $HCl$ at 298 K ( $K_{w} = 1.0 \times 1 0^{- 14}$ ), which statements are correct?

[H X^{+}]^{2} - 4 \times 1 0^{- 8} [H X^{+}] - 1 0^{- 14} = 0

gives

[H X^{+}] = 1.22 \times 1 0^{- 7} M

, so

pH = 6.91

(true). Water supplies

[OH X^{-}] = K_{w} / [H X^{+}] = 8.2 \times 1 0^{- 8} M

H X^{+}

, about twice the acid's contribution, so options 3 and 4 are false.