A saturated solution of H2S ([H2S] = 0.10 M) is buffered at pH 3.00. With K_a1 = 9.1x10^-8 and K_a2 = 1.2x10^-13, find [S^2-] and report it in units of 10^-15 M, to two decimal places.

JEE Advanced levelNumerical answerDifficulty 3 of 3ia034

A saturated solution of $H X_{2} S$ ( $[H X_{2} S] = 0.10$ M) is buffered at pH 3.00. With $K_{a 1} = 9.1 \times 1 0^{- 8}$ and $K_{a 2} = 1.2 \times 1 0^{- 13}$ , find $[S X^{2 -}]$ and report it in units of $1 0^{- 15}$ M, to two decimal places.

Numerical answer type, answer in x 10^-15 M, tolerance 0.02.

[S X^{2 -}] = K_{a 1} K_{a 2} [H X_{2} S] / [H X^{+}]^{2} = (9.1 \times 1 0^{- 8}) (1.2 \times 1 0^{- 13}) (0.10) / (1 0^{- 3})^{2} = 1.092 e - 15

= 1.09 \times 1 0^{- 15}