An aqueous solution is prepared by dissolving of HF (a weak acid with at ) and of NaF…

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

Problem

An aqueous solution is prepared by dissolving

0.225\,\text{mol}

of HF (a weak acid with

K_a = 6.6\times10^{-4}

25\,^{\circ}\text{C}

) and

0.125\,\text{mol}

of NaF in enough water to make

1.50\,\text{L}

of solution. Temperature is held constant at

25\,^{\circ}\text{C}

. (a) Write the chemical equilibrium that governs the acidity of the solution and define all symbols that will be used in your calculations. (b) Designate

w

as the molar change in HF concentration that occurs as the system reaches equilibrium. Derive algebraic expressions for the equilibrium concentrations of HF,

F^-

, and

H^+

in terms of

w

. (c) Using the equilibrium-constant expression, obtain a numerical value for

w

and compute the equilibrium concentration of every species, including

OH^-

. (d) Calculate the pH of the solution and comment on the effect of the initial

F^-

concentration on the degree of HF ionization.

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