A base-dissociation constant of has been reported for the weak base Q

CHEMISTRY · Multi-part · A Level · Chemistry · Acids and bases · Acids and bases

Problem

A base-dissociation constant of

3.5 \times 10^{-3}

has been reported for the weak base Q. A chemist prepares an aqueous solution in which the initial concentration of Q is

2.0 \times 10^{-4}\,\text{M}

25\,^\circ\text{C}

. (a) Calculate

[\text{OH}^{-}]

, pOH, and pH for the solution using the assumption that the equilibrium concentration of Q remains equal to

2.0 \times 10^{-4}\,\text{M}

. (b) Determine the exact

[\text{OH}^{-}]

, pOH, and pH without relying on the assumption made in part (a). (c) Calculate the percent error in

[\text{OH}^{-}]

introduced by the assumption. (d) Derive the minimum initial concentration of Q (express your answer in

\text{mol}\,\text{L}^{-1}

) that would limit the percent error in

[\text{OH}^{-}]

to 5\% or less at the same temperature, and explain the chemical significance of your result.