Question

A chemistry graduate student is given 250.mL of a 0.10M chlorous acid HClO2 solution. Chlorous acid is a weak acid with Ka=1.1x10^−2.

What mass of NaClO2 should the student dissolve in the HClO2 solution to turn it into a buffer with pH =2.40?

You may assume that the volume of the solution doesn't change when the NaClO2 is dissolved in it.

Be sue your answer has a unit symbol, and round it to 2 significant digits.

Answer 1

Calculate moles of HClO ₂

We have, [ HClO ₂ ] = No. of moles of HClO ₂ / volume of solution in L

No. of moles of HClO ₂ = [ HClO ₂ ] volume of solution in L

No. of moles of HClO ₂ = 0.10 mol / L 0.250 L = 0.025 mol

We know that, pH of buffer solution is calculated by using Henderson's equation.

pH = pKa + log [ Salt ] / [ Acid ]

We have , pH = 2.40 , pKa = - log Ka = - log ( 1.1 10 ^-02 ) , [ Salt ] = moles of NaClO ₂ , [ Acid ] = moles of HClO ₂

2.40 = - log ( 1.1 10 ^-02 ) + log ( moles of NaClO ₂ / moles of HClO ₂ )

2.40 = 1.96 + log ( moles of NaClO ₂ / 0.025 )

log ( moles of NaClO ₂ / 0.025 ) = 2.40 - 1.96

log ( moles of NaClO ₂ / 0.025 ) = 0.44

Taking anti log on both sides, we get ( moles of NaClO ₂ / 0.025 ) = 10 ^0.44

( moles of NaClO ₂ / 0.025 ) = 2.75

moles of NaClO ₂ = 2.75 ( 0.025 ) = 0.06875 mol

We have, no. of moles = Mass / molar mass

Mass = no. of moles molar mass

Molar mass of NaClO ₂ = 22.99 + 35.45 + ( 2 16.00 ) = 90.44 g / mol

Therefore, Mass of NaClO ₂ = 0.06875 mol 90.44 g /mol = 6.2 g

ANSWER : Mass of NaClO ₂ : 6.2 g