Question

How many grams of solid potassium cyanide should be added to 1.00 L of a 0.239 M hydrocyanic acid solution to prepare a buffer with a pH of 10.174 ?

grams potassium cyanide=

Answer 1

Ka = 6.2*10^-10

pKa = - log (Ka)

= - log(6.2*10^-10)

= 9.208

use formula for buffer

pH = pKa + log ([KCN]/[HCN])

10.174 = 9.2076 + log ([KCN]/[HCN])

log ([KCN]/[HCN]) = 0.9664

[KCN]/0.239 = 9.2553

[KCN] = 2.212

volume , V = 1 L

use:

number of mol,

n = Molarity * Volume

= 2.212*1

= 2.212 mol

Molar mass of KCN,

MM = 1*MM(K) + 1*MM(C) + 1*MM(N)

= 1*39.1 + 1*12.01 + 1*14.01

= 65.12 g/mol

use:

mass of KCN,

m = number of mol * molar mass

= 2.212 mol * 65.12 g/mol

= 144 g

Answer: 144 g