Temperature and Kinetic Energy question

[9giddjl]

Homework Statement

At a certain temperature a sample of oxygen has a pressure of 2.0 x 10^6 Pa, and a density of 4.0 x 10^-3 g cm^-3. Find the average speed of the oxygen molecules of the gas.

Homework Equations

Ek= 3/2kT
Ek= 1/2mv^2

The Attempt at a Solution

I know that the mass of oxygen is 32 g per mole so with this i could find the mass of an individual molecule= 32 g/ 6.02 x 10^23 g
=5.3 x 10^-26 kg
I know P= 2.0 x 10^6 Pa

I just don't understand what to do with the density? can somebody please help me, any help would be much aprpeciated:)
Thankyou!

 

[CompuChip]

I think you should assume that the sample is an ideal gas.

 

[Count Iblis]

You can't compute the average speed form the average kinetic energy, as that would yield the the average of squared velocity and then taking the squre root does not give you the average of the absolute value (modulus) of the velocity.