mfd hem 120 Exam March , 201a The Ka\'s for a few acids are given below.ee them

Question

mfd

hem 120 Exam March , 201a The Ka's for a few acids are given below.ee them as needed in the following questions 1-10 i1o 9-10 INO 5x10 5 Given the Ka's in the table above, which is the strongeat base in an aqueous solution? b.HCOO CN d. NO eCi,coo 16. What is the pi a solution containing 0 10M HCO,H and 0.025M HE a. 1.00 b. 0.90 0.13 d. 1.60 e. 1.12 17, a NaNO c. NH.F d KCN e. K 18. Predict the value of the equilibrium constant for this reaction in aqueous solustion a. Keq > 1, equilibrium lies to the left b. Keq

Explanation / Answer

(15)

The weaked the acid, the stronger is its conjugate base.

So the strongest base is CN-, because the acid HCN is the weakest due to smallest Ka value.

(16)

[H+] from HCOOH = (Ka*C)0.5 = (7.1*10-4*0.1)0.5 = 0.0084 M

[H+] from HCl = 0.025 M

So,

Total [H+] = 0.0084+0.025 = 0.0334

So,

pH = -log(0.0334) = 1.47

This is the actual answer. But if you ignore contribution from HCOOH, them pH = -log(0.025) = 1.6

(17)

NH4F will produce an acidic solution, because pKb of NH4+ is higher than the pKa of HCN.

Hope this helps !

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