OK, so Daniel and Tamara got slightly different answers. Why is that? And which one is correct?
They both use the ideal gas law: PV=nRT. And they isolated V:
V=nRT/P This is correct.
They both agree on the number of moles of gas, which is given in the problem:
n = the number of moles of the gas (mol) = 0.0100 mol
They give different values for they ideal gas constant, R:
Tamara says R = the universal gas constant = 0.0821 L·atm/K·mol
Daniel uses R = 8.314 J/(mol K)
They are the same value, just with different units. If you don't believe me, just convert them to the same units.
Where they disagree is in their defnintion of STP.
They both use a temperature of T=273.15K, but Tamara says the pressure is P=1 atm, and Daniel says it is P=100kPa. These are not the same value. 1 atm is equal to 101.325 kPa, not 100kPa.
The reason for this is that the definition of STP has been changed by IUPAC. STP, is defined by IUPAC as a pressure of 100 kPa which is equal to 0.9869 atm and a temperature of 273.15 K. It used to be 1atm, but IUPAC changed it to .9869 atm in 1982 (10/28/12 http://goldbook.iupac.org/).
So Daniel was correct. We must use 100kPa or .9869 atm as the pressure:
P =pressure of the gas = .9869 atm
Now all there is left to do is substutite your known values for P, n, R, and T into the ideal gas equation, and you can see if Daniel got the correct answer using the proper value of P.
Is Daniel's answer perfect?
Rememer, your solution can have only as many significant figures as your knowns have. Can Daniel know for sure that the volume is .2271 given that the known n=.0100 only has three significant figures? Be careful not to report too many significant figures
because it implies that you know the answer more precisely than you do.
Also, remember to report the answer in the units the question is asking for.
Helpful tip for all chemistry and physics problems:
It is very good how Tamara included the units in her calculations. The only way you can know for sure if you were using the right units is if they all cancel out by the end, and you end up with the units you are supposed to have. When you write it out,
it helps to draw a line and write all the numerators on top and the denominators on bottom. For example, I would write R like this:
That way, you can easily see what units cancel out.