[rm]

i dont know what im doing wrong
i keep getting wrong numbers
any help is appreciated

what would the mass of air be inside a hot-air balloon that has a volume of 5.25x10^4L. The temp of the air inside is 30 degrees celsius and the air pressure is 95.5 kpa. Assume that the composition of the air is 80% N2 and 20% O2 to determine the molar mass of air.

so what i did was percentage composition
so 80g of N2/atomic mass of nitrogen
20g of O2/atomic mass of oxygen
to find the amount in moles
then i got the molar mass of air to be 72g (that may been my first mistake)

then i used the ideal gas law to get the number of moles then converted the number of moles into mass

[tamim83]

The molar mass would be the sum of the percent compositions (in decimal form) times the molar masses of O2 and N2 (remember that these are diatomics) 

Then use the ideal gas law.  An easy way to do this is to substitute m/MW (mass over molecular weight) for n in PV=nRT and solve for m. 

[rm]

k thanks a bunch