We use the Ideal Gas Law:

#n=(PV)/(RT)# #=# #(2.0*atmxx27.5*L)/(0.0821*L*atm*K^-1*mol^-1xx320*K)# #=# #??" moles"#

The choice of gas constant (#R#) is always problematic in theses sorts of problems. Chemists usually deal with atmospheres and litres. In other problems, sometimes you deal with pascals, and #""^@C#, and even (in the States) cubic feet and cubic inches, you may have to use different #R# values. Because, #atm# and #L# are sensible units, most of the time we use #R=0.0821*L*atm*K^-1*mol^-1#.

If you go thru and cancel the units in the expression above, you'll find that we get an answer in #"moles"#. This is a good thing because we wanted an answer in #"moles"#, so dimensional analysis is an extra way to check our calculation.