as promised: a cylinder of volume 2x10^-3 m^3 contains a gas at a pressure of 1.50MNm^-2 and at a temperature of 300K. calculate number of moles. i think i'm ok with this bit: n=(pV)/(RT) =(1.5x10^6)x(2x10^-3) all divided by 8.31x300 gives 1.2033694... calculate number of molecules. number of molecules=number of moles x avocado's constant =1.2033694 x 6.023x10^23 =7.224283x10^23 calculate the actual mass of the gas if the molar mass is 0.032kg actual mass = number of moles x molecular mass =32 x 1.2033694 =38.5078 now calculate the mass of one molecule of the gas. erk! which equation do i use? i can't use the half-m-c-squared-bar ones cos i don't know speeds or anything! help is appreciated. i have yet another one! watch this space.