# Number of moles equation in exhaust gas

1. Mar 26, 2014

### marellasunny

I've been given information that the exhaust gas contains only water and no HydroCarbons. So,the only hydrogen composition found in the exhaust gas is in the water.

Similarly,the hydrogen in the air coming-in is only found in the water molecules as the incoming air is pure and without any fuel.

My book hence states the equation relating the number of moles of water and number of moles of hydrogen:
∴ $$n_{H20 in air}=\frac{n_{H, air}}{2}$$

Shouldn't it be the reverse i.e 2*n_{H,air}?

2. Mar 26, 2014

### Staff: Mentor

There are two hydrogen atoms in each molecule of water, so the number of water molecules is half the number of hydrogen atoms.

Chet

3. Mar 27, 2014

### marellasunny

Also,I've been given the mass fraction of the hydrogen in the fuel(h). I now want to find the number of moles of hydrogen in the fuel n_{H,fuel}. I'm confused on which molar mass(/molecular weight) to use in the equation:

$$n_{H,fuel}=\frac{mass_{H,fuel}}{M_{fuel/H?}}$$

Must I use the molar mass of the fuel or the molar mass of hydrogen?

4. Mar 27, 2014

### Staff: Mentor

Are you saying you want to find the mole fraction of atomic hydrogen in the fuel, given the mass fraction of atomic hydrogen in the fuel?

Chet