Question

A chemistry graduate student is given 500 ml of a 0.90 M hydrocyanic acid (HCN) solution. Hydrocyanic acid is a weak acid with K.-4.9 10 What mass of NaCN should the student dissolve in the HCN solution to turn it into a buffer with pH 8.987 You may assume that the volume of the solution doesn't change when the NaCN is dissolved in it. Be sure your answer has a unit symbol, and round it to 2 significant digits. box 5 ?

Answer 1

Ka = 4.9*10^-10

pKa = - log (Ka)

= - log(4.9*10^-10)

= 9.31

use formula for buffer

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

8.98 = 9.3098 + log ([NaCN]/[HCN])

log ([NaCN]/[HCN]) = -0.3298

[NaCN]/0.9 = 0.4679

[NaCN] = 0.4212

volume , V = 5*10^2 mL

= 0.5 L

use:

number of mol,

n = Molarity * Volume

= 0.4212*0.5

= 0.2106 mol

Molar mass of NaCN,

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

= 1*22.99 + 1*12.01 + 1*14.01

= 49.01 g/mol

use:

mass of NaCN,

m = number of mol * molar mass

= 0.2106 mol * 49.01 g/mol

= 10.32 g

Answer: 10. g