1. An aqueous solution contains 0.34 M sodium cyanide . One liter of this soluti

Question

1. An aqueous solution contains 0.34 M sodium cyanide.

One liter of this solution could be converted into a buffer by the addition of:

(Assume that the volume remains constant as each substance is added.)choose all that apply.

0.33 mol HCN

0.17 mol HI

0.17 mol Ca(OH)2

0.33 mol NaI

0.34 mol HI

2. How many grams of solid sodium fluoride should be added to 1.00 L of a 0.287 M hydrofluoric acid solution to prepare a buffer with a pH of 3.644 ?

grams sodium fluoride =

3. How many grams of solid potassium cyanide should be added to 2.00 L of a 0.186 M hydrocyanic acid solution to prepare a buffer with a pH of 8.488 ?

grams potassium cyanide

Explanation / Answer

1) NaCN is salt of weak acid (HCN) and strong base.

so 0.33 mol HCN is the answer.

2) PKa of HF = 3.17

1.00 L of a 0.287 M hydrofluoric acid = 1.00 * 0.287 = 0.287 mole.

PH = PKa + log [salt] / [acid]

3.644 = 3.17 + log [NaF] / 0.287

[NaF] = 0.8548 mole = 0.8548 * 41.988 = 35.9 gm NaF (answer)

3) Pka of HCN = 9.31

2.00 L of a 0.186 M hydrocyanic acid = 2.00 * 0.186 = 0.372 mole.

PH = PKa + log [salt] / [acid]

8.488 = 9.31 + log [KCN] / 0.372

[KCN] = 0.056 mole = 0.056 * 65.12 = 3.65 gm (answer)

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