Question

The concentration of SO₂ in air is determined by bubbling a sample of air through a trap containing H₂O₂. Oxidation of SO₂ by H₂O₂ results in the formation of H₂SO₄, which is then determined by titrating with NaOH.

H₂O₂ + SO₂ → H₂SO₄
H₂SO₄ + 2 NaOH → 2 H₂O + Na₂SO₄

In a typical analysis, a sample of air was passed through the peroxide trap at a rate of 3.1 L/min for 60 min and required 10.58 mL of 0.0253 M NaOH to reach the phenolphthalein end point. Calculate the mL/L SO₂ in the sample of air. The density of SO₂ at the temperature of the air sample is 2.86 mg/mL.
mL/L SO₂

Answer 1

for the reaction H2SO4+2NaOH--------->2H2O+ Na2SO4

2 mole of NaOH consumes 1 mole of H2SO4

moles of NaOH= molarity* volume in L= 0.0253*10.58/1000 =0.000268

moles of H2SO4= (1/2)* moles of NaOH= 0.000268/2= 0.000134

from reactio - 1, SO2+ H2O2------>H2SO4

1 mole of H2SO4 requires 1 mole of SO2

hence moles of SO2=0.000134 mol

mass of SO2= moles* molar mass of SO2= 0.000134*64 gm =0.0086 gm= 8.6 mg

density of SO2 = 2.86mg/ml, volume of SO2= mass/ density= 8.6/2.86 ml =3 ml

flow rate of air = 3.1 L/min time air is sent = 60 min, volume of air sent = 3.1*60 L= 186 L

hence ml of SO2/L= 3 /186 ml/L=0.016 ml/L