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# A2 thermal physics watch

1. ok i got an exam next week, and i got not get my head around molar mass at all. i know it is simple but i just can't understand for some reason, he is and example of one of the questions, any help would be much appreciated

E.g A balloon is filled with helium gas of molar mass 4.0x10^-3 at 17 degrees c. At a pressure of 1.0x10^5 Pa calculate the number of moles of gas in the balloon?

Thanks
2. pV = nRT

edit - convert temp to kelvin---> temp in kelvin = temp degrees + 273

edit - oh no wait, moles would be just molar mass * avagadros constant

so 4.0*10^-3 * 6.02*10^23 i believe
3. (Original post by betto2468)
ok i got an exam next week, and i got not get my head around molar mass at all. i know it is simple but i just can't understand for some reason, he is and example of one of the questions, any help would be much appreciated

E.g A balloon is filled with helium gas of molar mass 4.0x10^-3 at 17 degrees c. At a pressure of 1.0x10^5 Pa calculate the number of moles of gas in the balloon?

Thanks
Actually i don't think we even use avagadros number. Since it's helium can we not the fact that it has atomic mass of 4u? 1 u = 1.66*10^-27 Kg. Therefore it consists of 2 protons and 2 neutrons. Get the mass of each, add them together. Then multiply that by the value given for u. Then divided the molar mass by this mass cause the masses will cancel out and you're left with the moles.

Proton mass = 1.00728 u, neutron mass = 1.00867 u

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