Question

Suppose a student failed to dry (remove the water from) the KHP before using it to standardize the NaOH solution. What effect would this have on the calculated molarity of the NaOH solution?

	A) The calculated molarity would be too high.
	B) The calculated molarity would be too low.
	C) It would have no effect on the calculated molarity.

Suppose a student failed to dry (remove the water from) the KHP before using it to standardize the NaOH solution. What effect would this have on the calculated equivalent mass of the unknown acid?

	A) It would have no effect on the calculated equivalent mass.
	B) The calculated equivalent mass would be too high.
	C) The calculated equivalent mass would be too low.

Suppose a student performing this experiment calibrated the pH meter using the pH 6 buffer instead of the pH 7 buffer. As a result, all the pH meter readings were too low. What effect would this procedural error have on the experimentally determined pKa of the unknown acid? A) The pKa would not be affected. B) The pKa would be too high. C) The pKa would be too low.

Answer 1

1) If a student failed to dry (remove the water from) the KHP before using it to standardize the NaOH solution. The resultant molarity calculated of the NaOH solution would be,

B) The calculated molarity would be too low.

1 mole of KHP reacts with 1 mole of NaOH. Since the actual concentration of KHP would be low by increasng the initial weight due to presence of water in it, the net molar concentration of NaOH would also be low.

2) If a student failed to dry (remove the water from) the KHP before using it to standardize the NaOH solution. The effect on the calculated equivalent mass of the unknown acid would be,

B) the calculated equivalent mass of acid would be too high

moles of base calculated would be low here, which automatically means moles of acid neutralized would be high as calculate. So the equivalent mass would also be high.

3) If a student performing this experiment calibrated the pH meter using the pH 6 buffer instead of the pH 7 buffer. As a result, all the pH meter readings were too low. The calculated pK_a of the unknown acid would thus turn out to be,

C) The pKa would be too low

at half equivalence point pH measured would be lower and this implies the pKa to be low too for the acid.