Three monoprotic weak acids—labeled X, Y

CHEMISTRY · Multi-part · AP · Chemistry · Acids and Bases · Acids and Bases

Problem

Three monoprotic weak acids—labeled X, Y, and Z—have the following acid dissociation constants at

25\,\text{°C}

: • Acid X:

K_a = 1.0 \times 10^{-3}

• Acid Y:

K_a = 4.0 \times 10^{-6}

• Acid Z:

K_a = 7.0 \times 10^{-10}

Equal-volume

0.050\,\text{M}

aqueous solutions of the sodium salts

NaX

NaY

, and

NaZ

are prepared separately at

25\,\text{°C}

. The auto-ionization constant of water is

K_w = 1.0 \times 10^{-14}

. Answer all parts below. (1) Part 1: For each conjugate base (

X^-

Y^-

Z^-

) calculate the numerical value of its base dissociation constant,

K_b

, at

25\,\text{°C}

. (2) Part 2: Determine the pH of the separate

0.050\,\text{M}

solutions of

NaX

NaY

, and

NaZ

. (3) Part 3: Rank the three conjugate bases from most basic to least basic. Justify your ranking using the results from parts (a) and (b). (4) Part 4: Equal volumes of the

0.050\,\text{M}

NaX

and

0.050\,\text{M}

NaZ

solutions are mixed. Predict whether the resulting solution will be acidic, basic, or neutral at

25\,\text{°C}

, and support your prediction with quantitative reasoning.

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