A numerical on states of matter

Question

At 27 degree centigrade a cylinder of 20 litres capacity contains three gases HE O2 N2 their masses are 0.502g 0.250g 1g respectively if all these gases behave ideally calculate partial pressures of each gas as well as total pressure..

Answer 1

Moles He = 0.502 g x 1 mol/4 g = 0.1255 moles

Moles O2 = 0.250 g x1 mol/32 g = 0.00681 moles

Moles N2 = 1 g x 1 mol/28g = 0.0357 moles

total moles = 0.168 moles

Total pressure PV = nRT and P = nRT/V

P = (0.168)(0.0821)(300)/20 = 0.207 atm = TOTAL PRESSURE

Partial pressure is determined from mole fraction of each gas 

mole fraction He = 0.1255/0.168 = 0.747

partial pressure He = 0.747 x 0.207 atm = 0.155 atm

mole fraction O2 = 0.0068/0.168 = 0.0404

partial pressure O2 = 0.0404 x 0.207 atm = 0.0084 atm

mole fraction N2 = 0.0357/0.168 = 0.212

partial pressure N2 = 0.212 x 0.207 atm = 0.044 atm