[blondyashy]

Hello

I'm doing an independant learning 12U chem course and don't understand one of the examples in the book.  I don't understand how they got the answer and  why some of the values are placed where they are.  I've been out of school for 4 years and are sturggling to say the least.  Any help in explaining how this works would be appreciated.

The question is:  How many moles of oxygen gas are present if the volume of the gas is 274 mL at a pressure of 120 kPa and a temperature of 75 degrees C?

V= 274 mL - 0.274 L  n=?  R= 8.31  kpa x L/mol x K  T= 75 dgC  P=120 kPa
PV = nRT

therefore n = PV/RT

than it shows that n = 120 kPa x 0.274 L/ 8.31 kpa x L x mol^-1 x K^-1 x 348 K

the answer is 0.0114 mol

Where I'm confused is where the mol^-1 x K^-1 comes from, why are those values there and how do they all work with the equation?  None of them cancel out besides the L and kPa and what values are supposed to go into the mol^-1 x K^-1 to get the answer?

Thanks

[Borek]

R= 8.31  kpa x L/mol x K

They are here - in the gas constant. R has units kpa x L/(mol x K) or kpa x L x mol^-1 x K^-1 (it may also have different units, but it will have different value then; as long as you use same units for expressing your data that's OK).

[blondyashy]

So the values that I use for the n = pv/rt will plug into the k^-1 and what happens to the mol^-1?  I'm sorry, I'm having a very difficult time grasping this for some reason...
Thanks

[Borek]

Most of these units simply cancel out. Try to write whole epxression using line to see whats below and whats above.