Question

A buffer contains 0.4 moles of hydrocyanic acid and 0.7 moles of potassium cyanide. When 0.1 moles HCl is added to the buffer solution, the pH slightly____, the concentration of HCN___, and the concentration of CN-___ (increases/decreases). Please explain why.

Answer 1

A buffer contains 0.4 moles of hydrocyanic acid and 0.7 moles of potassium cyanide. When 0.1 moles HCl is added to the buffer solution, the pH slightly decreases, the concentration of HCN increases, and the concentration of CN^-decreases. (increases/decreases).

Using Henderson-Hessalbalach equation

pH = pKa + log { [salt] / [acid]

pKa of HCN = 9.31

So, pH = 9.31 + log (0.7 / 0.4)

= 9.31 + log (1.75)

= 9.31 + 0.24

= 9.55

When 0.1 moles of HCl is added to the buffer, it will react with 0.1 mole KCN and will produce 0.1 mole of HCN.

So, moles of KCN will decrease and moles of HCN will increase.

Final moles are

HCN = 0.4 moles + 0.1 mole = 0.5 mole

KCN = 0.7 moles - 0.1 mole = 0.6 mole

Let the volume = 1 L.

So,

[HCN] = 0.5 M

[KCN] = 0.6 M

Using Henderson-Hessalbalach equation

pH = pKa + log { [salt] / [acid]

pKa of HCN = 9.31

So, pH = 9.31 + log (0.6 / 0.5)

= 9.31 + log (1.2)

= 9.31 + 0.08

= 9.39