[Addi]

I'm having some difficulty with this problem:

"A 30,00L air sample was passed through an absorption tower containing a solution of Cd2+, where H2S was retained as CdS.
The mixture was acidified and treated with 10,00mL of 0,01070M I2.
After the reaction S2- + I2 → S(s) + 2I- was completed, the excess iodine was titrated with 12,85mL of 0,01344M thiosulfate.
Calculate the concentration of H2S in ppm; use 1,20g/L for the density of the gas stream."

When I first looked at it, it seemed like a pretty straightforward stoichiometry problem. I know the volume and the moles and knowing the moles I can figure out the mg. What throws me off is the density. I know that that's definitely the part I'm screwing up, but I can't figure out how it fits.

Any help would be greatly appreciated.

[Borek]

What is ppm concentration?

[Addi]

ppm describes is the amount of substance in a different substance in parts per million. In this case I would be using ppm = mg/L I think. 

[Borek]

ppm - just like percentage - needs to be specified. It can be w/w, w/v, v/v or atoms/atoms. Obviosuly you are asked to calculate ppm w/w.