[cloudy1290]

A 40 l sample of N2 gas containing SO2 gas as an impurity was bubbled through a 3% solution of H2O2. The SO2 was converted to H2SO4 : SO2 + H2O2 --> H2SO4
A 25 ml portion of 0.01 mol/l NaOH was added to the solution, and the excess base was back-titrated with 13.6ml of 0.01 mol/l HCL. Calculate the ppm of SO2 ( that is, ml SO2/10^6ml sample) if the density of SO2 is 2.85g/l. (Atomic weights; H=1.0, N=14.0, O=16.0, Na=23.0, S=32.0 and Cl = 35.5)

My solution :
from this equation NaOH + HCl ---> Nacl + H2O ,mol of NaOH = 13.6*10^-5

from H2SO4 + 2NaOH ----> Na2SO4 +2H2O
I                     25*10^-5
C     -5.7*10^-5              -11.4*10^-5
E                   13.6*10^-5

from SO2         + H2O2 ---> H2SO4
I    5.7*10^-5
C
E                                         5.7*10^-5

ppm = [(5.7*10^-5 *22.4)/40] *10^6

I got 31.92ppm but the answer is 32ppm. Did I make a mistake?

[mjc123]

I don't think so. You're pretty close. How many significant figures do you think are justified in your answer?

An interesting point is that the temperature and pressure are not specified in the question (at least as quoted). 22.4L is the molar gas volume at STP, i.e. 0°C and 1 atm, not at the more usual ambient temperature of 298K. Unless "40 L" means "40 L STP", i.e. the amount of nitrogen that would occupy 40 L at STP, whether or not it is actually at STP.
Actually you can work out that SO₂ has the quoted density at STP, not at 298 K and 1 atm, but it's a bit subtle if you're expected to do it that way.

[cloudy1290]

I don't think so. You're pretty close. How many significant figures do you think are justified in your answer?

An interesting point is that the temperature and pressure are not specified in the question (at least as quoted). 22.4L is the molar gas volume at STP, i.e. 0°C and 1 atm, not at the more usual ambient temperature of 298K. Unless "40 L" means "40 L STP", i.e. the amount of nitrogen that would occupy 40 L at STP, whether or not it is actually at STP.
Actually you can work out that SO₂ has the quoted density at STP, not at 298 K and 1 atm, but it's a bit subtle if you're expected to do it that way.

Sorry for late reply

I got it. I used density of SO2 to calculate and I got 32ppm