# Changing molar mass at diff temp and pressures - help

1. Apr 17, 2005

### rocketboy

Hi,

If I have a 50mL volume of hydrogen gas at standard temperature and pressure, and then increae the pressure, the volume is going to drop. Let's say that I know there is 1.5 grams of hydrogen gas in there. So if the volume drops, the mass of the hydrogen gas stays the same, but the number of moles of hydrogen will change, meaning that the molar mass is going to change. Is this correct? If not where did I go wrong and why?

If I am correct than that means molar mass of a substance is dependant on its surroundings....so are the molar masses in the periodic table at STP?

It's really important I know this for the evaluation of my lab report.
Thanks everybody,
-Jon

2. Apr 17, 2005

### dextercioby

You couldn't be any wronger.The molar mass is the mass of 1 mole of substance = the mass of $N_{A}\simeq 6.023\cdot 10^{23} atoms/molecules$.This mass (essentially the mass of the atoms/molecules) is constant with temperature & pressure,if u neglect

a) Einstein's formula $E=mc^{2}$
b) Radiation loss /gain of atoms.
c) Nuclear reactions...

Daniel.

3. Apr 17, 2005

### rocketboy

lol, that makes complete sense now, thanks.