Calculate the pH of 0.10 M NH,CN. (Kb for NH,-1.8 × 10 5; Ka for HCN = 6.2 × 10

Question

Calculate the pH of 0.10 M NH,CN. (Kb for NH,-1.8 × 10 5; Ka for HCN = 6.2 × 10 10) A) 5.79 B) 8.21 C) 8.87 D) 5.13 E) 9.23 102. PTS: 1 DIF: difficult TOP: 7.8 ANS: E KEY: general chemistry | acids and bases | solutions of a weak acid or base | acid-base properties of salt solutions | pH of a salt solution 103. Arrange the following 0.10 M solutions from lowest to highest pH: NaF, NaC2H302, Ka for CsH5NHCI, KOH, HCN. (Ka for HCN is 6.2 × 10 10, Ka for HF is 7.2 × 10 4: HC2H3O2 is 1.8 × 10 5; and Kb for C5H5N is 1.7 × 10 9) A) CsHsNHCl, HCN, NaF, NaC2H302, KOH B) NaF, NaC2HzO2, HCN, C5H;NHCl, KOH C) KOH, NaC2H;02, NaF, HCN, CjH5NHCI D) HCN, CsHsNHCl, NaF, NaC2H302, KOH E) none of these PTS: 1 DIF: difficult TOP: 7.8 ANS: A KEY: general chemistry | acids and bases | acid and base strength | relative strengths of acids and bases

Explanation / Answer

The NH4CN is a salt formed by HCN and NH4OH, these are weak acid and base

You can calculate the ph is this by applying the equation

PH = 1 /2 (PKw + PKa - PKb)

Ka = 6.2x10-10

PKa = -log(6.2x10-10 ) = 9.207

similarly PKb = 4.744

PH = 1/2 * (14 + 9.207 - 4.74) = 1/2 * (18.46) = 9.23

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