# Pressure And Temperature

Hi.

Can someone maybe give me an equation to determine the number of molecules per unit volume (N/V) when the pressure(P) and the temperature(T) is given. The pressure is in PA and the temperature is in Kelvin.

Thanx

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dav2008
Gold Member
The ideal gas law says that for an ideal gas: PV=NkT where P, V, and T are pressure (Pascals), volume(m-3), and temperature(K) respectively, N is the number of molecules, and k is Boltzmann's constant of 1.381 × 10−23 J/K

I know that one. But the volume is not given, so how can i get the N/V, the number of molecules per volume?

dav2008
Gold Member
Can you solve the ideal gas law for N/V?

Yeah, so I must determine the volume first before I can determine the number of molecules. Let me give you the question. Maybe you can point out what im misssing.

On the sunlit surface of Venus, the atmospheric pressure is 9.0 x 10 6Pa, and the temperature is 740K. On the earth's surface the atmospheric pressure is 1.0 x 10 5Pa, while the surface temp can reach 320K. These data imply that Venus has a "thicker" atmosphere at its surface than does the earth, which means that the number of molecules per unit volume (N/V) is greater on the surface of Venus than on Earth. Find the ratio (N/V)venus / (N/V)earth.

Thanx

dav2008
Gold Member
Why do you want to solve for the number of molecules?

You are trying to solve for N/V. Solve for N/V in the ideal gas law and see what it is dependent on.

Ok but isn't N the number of molecules?

Hootenanny
Staff Emeritus
-Hoot 