# (simple?) question on number of molecules in an area

1. Apr 15, 2015

### rwooduk

1. The problem statement, all variables and given/known data

2. Relevant equations

3. The attempt at a solution

I'm probably being really stupid here but how can the number of molecules equal

If we integrate we get a volume multiplied by density, how can that equal a number of particles?

2. Apr 15, 2015

### BvU

$\rho$ is not a mass density (kg/m3) but a particle density (number of particles/m3).

3. Apr 15, 2015

### rwooduk

Of course! many thanks for clearing this up.