Finding concentration

Question

A 25.0 mL sample of acid rain (H2SO3) was analyzed by titration with a 0.105 M NaOH. The result showed that 14.5 mL of the base was used to neutralize the acid. Determine the concentration of sulfurous acid in the rain sample.

Answer 1

First, write a correctly balanced equation for the neutralization reaction taking place.

H₂SO₃ + 2NaOH ==> Na₂SO₃ + 2H₂O

Next, find mols NaOH used to neutralize the sample.

14.5 ml x 1 L / 1000 ml x 0.105 mol / L = 0.00152 mols NaOH used

Using the balanced equation, calculate mols H2SO3 present.

0.00152 mol NaOH x 1 mol H2SO3 / 2 mol NaOH = 0.000761 mols H2SO3

Finally, calculate the concentration of H2SO3 using moles H2SO3 / liters of solution

Liters of solution = 25.0 ml x 1 L / 1000 ml = 0.0250 L

moles = 0.000761

Concentration = 0.000761 mol / 0.0250 L = 0.0305 M (3 sig. figs.)