Consider the titration of 30.0 mL of 0.0200 M nitrous acid, HNO2, by adding 0.0500 M aqueous ammonia to it. Which statement is true? The pH at the equivalence point is . A) is greater than 7 B) is equal to 7 C) is less than 7 D) is less than 0.02 E) zero

C) is less than 7

Explanation:

The dissociation constant, Ka of nitrous acid, HNO2 = 4.5 * 10^-4

Ammonia, NH3, is a weak base with a Kb value of 1.8x10^-5

When a weak acid like HNO2 reacts with an equivalent amount of a weak base NH3, complete neutralization does not occur. The concentrations of the species in equilibrium with each other will depend on the equilibrium constant, K, for the reaction,

Since the dissociation constant, ka of nitrous acid is higher, at equivalence point the pH of the solution will be slightly acidic with a pH that is less than 7.

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