Hello, Emily,
The idela gas law, PV=nRT offers the path for a solution, but we need to payt attention to the units. Pressure in kPa is the first clue that we need to be careful. Let's start with the gas constant R. Many English texts will use a value for R of 0.08206 L*atm*K−1⋅mol−1. It has units of atm, but we need pascals. We can either convert kPa to atm, of use a different gas constant that is expressed with kPa as one of it's units. To convert kPa to atm, we can use the conversion factor 1 atm/101.3kPa. That means we have 0.52 atm.
The value of R that has kPa in it's units is 8.314 kPa*L*K−1⋅mol−1.
We can use either approach. I've already converted kPa to atm, so I'll go with that, but both will have the same answer.
We also need to convert C to K, by adding 273.15 to the 25.0C. = 298.2K
Rearrange the ideal gas law to isolate the unknown, V:
V = nRT/P
V = (0.45 mole)(0.0821L⋅atm⋅K−1⋅mol−1)(298.2K)/(0.52atm)
Make sure the units all cancel except for liters, which is what we hope will be left.
V = (0.45 mol)(0.0821L⋅atm⋅K−1⋅mol−1)(298.2K)/(0.52atm)
V = 21.2 liters, to 3 sig fig.
Liters is the only remaining unit, and since 1 mole of any gas occupies 22.4 liters at STP, I'm not uncomfortable that our 0.45 mole occupies a similar volume, since the pressure is half that of standard pressure of 1 atm.
Bob
[Note that you can convert the two values of R by using the conversion factor for atm and kPa]