MCAT General Chemistry Question 150: Answer and Explanation

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Question: 150

15. How many liters of 2 M Ba(OH)₂ are needed to titrate a 4 L solution of 6 M H₃PO₄?

A. 1.33 L
B. 12 L
C. 18 L
D. 56 L

Correct Answer: C

Explanation:

Use the equivalence point equation: N_aV_a = N_bV_b. Ba(OH)₂ can dissociate to give two hydroxide ions, so its normality is 2 M × 2 = 4 N. H₃PO₄ can dissociate to give three hydronium ions, so its normality is 6 M × 3 = 18 N. Plugging into the equation, we get (18 N)(4 L) = (4 N)(V_b). Therefore, V_b is 18 L.

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