# How many gas molecules

1. Apr 28, 2010

### noname1

An excellent laboratory vacuum has a pressure of 4.00x10^-18 atm, or 4.04x10^-13 Pa. How many gas molecules are there per cubic centimeter in such a vacuum at 297 K?

n/v = p/rt

n/v = 4.04x10^-13 / (8.31x297) = 1.66x10^-16 mol/m³

1.66x10^-16 mol/m³ * (1/100cm)³ = 1.66x10^-22 mol/cm³

but something is wrong, please verify and she what i am doing wrong

2. Apr 28, 2010

### rock.freak667

1 mole contains how many molecules?

3. Apr 28, 2010

### noname1

6.022x10^23

Last edited: Apr 28, 2010
4. Apr 28, 2010

### rock.freak667

And you have 1.66x10-22 moles, so how many molecules do you have?

Actually now that I see it, you are finding per cm3.

R has the units J/mol.K

J = 1 Pa m3, so you'll need to change your units for R for it to work out properly.

5. Apr 28, 2010

### noname1

ok i see it now i forgot to multiply by (6.022x10^23/mol)

i only got 1 more try at answering this question :(

(1.66x10^-16 mol/m³) * (m/100cm)³ * (6.022x10^23/mol) = 99.97 molecules/cm³

am i correct now?