Parts per million?

[christotaku]

A 40l sample of

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N2

gas containing

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SO2

gas as an impurity was bubbled through a 3% solution of

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H2O2

. The

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SO2

was converted to

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H2SO4

:

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SO2 + H2O2 = H2SO4

A 25ml portion of 0.0100mol/l

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NaOH

was added to the solution, and the excess base was back-titrated with 13.6ml of 0.0100mol/l

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HCl

. Calculate the parts per million of

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SO2

(that is ml

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SO2

/10^6ml sample) if the density of

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SO2

is 2.85g/l (Atomic weights: H=1, N=14, O=16, Na=23, S=32, Cl=35.5).

Please explain. I have never solved chemical problems of this kind.

[expert]

Are you still struggling with this problem?

Here is your reaction:
H₂SO₄ + NaOH + HCl = Na₂SO₄ + NaCl + H₂O

25ml portion of 0.0100mol/l NaOH = 0.25 mmol
13.6ml of 0.0100mol/l HCl = 0.136 mmol

Excess of NaOH = 0.25 - 0.136 = 0.114 mmol
that is neutralized by H₂SO₄:

2NaOH + H₂SO₄ = Na₂SO₄ +2H₂O

Amount of the sulfuric acid formed = 0.114/2 = 0.057 mmol
Consequently, the same amount of SO₂ absorbed in the reaction
SO₂ + H₂O₂ = H₂SO₄

Now this is important rule: 1 mol of every gas has volume 22.4 L
https://en.wikipedia.org/wiki/Molar_volume

0.057 mmol of SO₂ will have volume 0.057*10-3 mol * 22.4 L/mol = 1.2768*10-3 L

A 40l sample of N₂ gas containing 1.2768*10-3 L
SO₂ gas =3.192*10-5 = 31.92 ppm

Please double check my calculations