# Homework Help: What am I doing wrong? chemistry problem

1. Sep 15, 2011

### Nikitin

1. The problem statement, all variables and given/known data
If one leads chloride gas over salty water with a density of 1,1kg/L containing 4700ppm Bromide (Br-) then the bromide shall react with the chloride gas. How much chloride gas does one need to produce 1kg of Br2?

2. Relevant equations
The reaction is 2Br- + Cl2 --> Br2 + 2Cl-.. not really relevant, I think
3. The attempt at a solution

I think we can safely assume that the average molar mass of the water is ~ 19grams maybe , and thus the molar density is (1,1kg/19gramspermole)L = ~ 59 moles/L

Br- concentration is 4,7 parts per thousand molecules. So concentration of Br- is 59*4,7*10^-3 moles/L = 0,2773moles/L.

We needed 1Kg of Br2/Br-, with the molar mass of Br being 80g/mole. 1kg/80g/mole = 12,5 moles.

12,5 moles/0,2773 moles= 45. We need 45L of salty water to produce 1Kg of Br2, in theory.

But the correct answer is 192L.. What am I doing wrong?

Last edited: Sep 15, 2011
2. Sep 15, 2011

### Nikitin

fk, i made quite the typo.. I said average density of the water is 19 grams when i meant average molar mass of the water molecules is roughly 19 grams.. facepalm

anyway can somebody help me?

3. Sep 15, 2011

### Staff: Mentor

Actually that's the only relevant thing here. You are asked about amount of chloride, not of water. At least that's what you wrote.

4. Sep 16, 2011

### Nikitin

goddamn it, I meant how much *water* I need. it was late yesterday

anyway apparantly 4700ppm can mean 4700 ppm of a kg, as well as 4700ppm of a mole. I didn't know that, so the problem is solved lol

5. Sep 16, 2011

### Staff: Mentor

Ppm can by anything - weight/weight, volume/volume, molecules/molecules (to name only those making sense). W/w is the most common one.