Consider 50 mL of a solution that is 0.25 M in both H_2PO^1-_4 and HPO^2- _4. (a

Question

Consider 50 mL of a solution that is 0.25 M in both H_2PO^1-_4 and HPO^2- _4. (a) What is the pH of this solution? (Use the table of Acid Dissociation Constants. Enter your answer to two decimal places.) (b) What would be the effect of adding 5 mL of 1.0 M HCl? The pH would decrease dramatically The ph would increase dramatically. The pH would decrease slightly. The pH would increase slightly. The pH would not change at all. (c) What would be the effect of adding 2 mL of saturated NaOH (50 mass %, density 1.5 g/mL)? The pH would increase dramatically. The pH would increase slightly. The pH would decrease dramatically. The pH would not change at all. The pH would decrease slightly.

Explanation / Answer

(a) since [acid] = [salt]

pH = pKa = 2.12 for H2PO4-

(b) Addition of HCl causes the decrease in the concentration of salt and increase in the concentration of acid.

pH will slightly decrease.

pH = pKa + Log[salt]/[acid]

(c) Addition of base decreases the concentration of acid but increase the concentration of salt.

So, pH will slightly increase.

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