CHM 2046C Module 11 Chapter 17 Sample Exam Answers

Part J: pH of Salts 10 points

Predict whether the following salts will test

A. acidic, B. basic, C. N neutral, D. Not Enough Information to Predict

when dissolved in water. Then show the hydrolysis reaction(s), if any

__B-Basic__1. KCN(s) + HOH à K¹⁺(aq) + CN^1-(aq)

K¹⁺⁽aq) + HOH à No reaction (salt of a strong base)

CN^1-(aq) + HOH ß à HCN (*aq) + OH^1- (aq)

HCN is molecular un-ionized HCN because it is a weak acid, therefore the solution is BASIC

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_ N-Neutral_ 2. KNO₃(s) + HOH à K¹⁺(aq) + NO₃^1-(aq)

K¹⁺⁽aq) + HOH à No reaction (salt of a strong base)

NO₃^1-(aq) + HOH à No reaction (salt of a strong acid)

Therefore the solution is Neutral because neither ion hydrolyzes in water and the pH is determine by the ionization of pure water which 7.00.

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_A-Acidic_3. NH₄NO₃(s) + HOH à NH₄¹⁺(aq) + NO₃^1-(aq)

NH₄¹⁺⁽aq) + 2HOH ß à NH₄OH* (aq) + H₃O⁺¹ (aq)

* [ Ammonium hydroxide, NH₄OH (aq) is really NH₃ (aq) + HOH (l) ]

NO₃^1-(aq) + HOH à No reaction (salt of a strong acid)

Therefore the solution is acidic, since hydronium ions are formed from the hydrolysis of the ammonium ion forming a weak base ammonium hydroxide and nitrate ion does not hydrolyze because it comes from the salt of a strong acid.

_B-Basic_4. KC₂H₃O₂(s) + HOH àK¹⁺⁽aq) + C₂H₃O₂^1-(aq)

K¹⁺⁽aq) + HOH à No reaction (salt of a strong base)

C₂H₃O₂^1-(aq) + HOH à HC₂H₃O₂ (aq) + OH^1- (aq)

Therefore the solution is basic because the Potassium ion come from a salt of a strong acid, nitric acid), and the acetate ion hydrolyzes with water to form molecular acetic acid molecules (a weak acid) and producing excess hydroxide ions to the solution

__D-NE InFo__ 5. NH₄C₂H₃O₂ (s) + HOH à NH₄¹⁺⁽aq) + C₂H₃O₂^1-(aq)

NH₄¹⁺⁽aq) + 2HOH ß à NH₄OH* (aq) + H₃O⁺¹ (aq)

* [ Ammonium hydroxide, NH₄OH (aq) is really NH₃ (aq) + HOH (l) ]

C₂H₃O₂^1-(aq) + HOH à HC₂H₃O₂ (aq) + OH^1- (aq)

Since both ions hydrolyze in water because the salt comes from a weak and a weak base, there is not enough information to predict. The K_a of acetic acid and the K_b of ammonium hydroxide are both needed to make a prediction.

Bonus:

K_a of acidic acid is 1.75 x 10^-5

and the K_b of ammonium hydroxide is 1.78 x 10^-5,

Is the solution more acidic or more basic?

Explain your answer by showing equilibria expressions:.