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# Gas Laws watch

1. Hello, can anybody help me with the following please?
A flask of volume 9.0x10-4 m^3 contains air. A vacuum pump reduces the pressure in the flask to 150Pa at temperature 30K. For the air remaining in the flask calculate:
Its density
the number of molecules present, assuming the molar mass of air is 0.029Kg/mol
2. (Original post by stanners 27)
Hello, can anybody help me with the following please?
A flask of volume 9.0x10-4 m^3 contains air. A vacuum pump reduces the pressure in the flask to 150Pa at temperature 30K. For the air remaining in the flask calculate:
Its density
the number of molecules present, assuming the molar mass of air is 0.029Kg/mol
To find the density, you'll need the mass and the volume because density = mass/volume. You have the volume but not the mass. You'll need to find the mass using the equation pV=nRT. Substitute your values of p, V, T and R (which is 8.31) and you'll get n.
n = Mass/Molar mass
Mass = n x Molar mass.
This way you have a mass and a volume.

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