For any gas, the ideal gas law states the PV = nRT, where P= gas pressure in atmospheres, V = gas volume in liters, n= number of moles of gas present, T = gas temperature in degrees Kelvin, and R is the ideal gas constant, which is equal to 0.0821 Liter * atmospheres/moles * degrees K.
Hence your problem, set up mathematically, should look like this:
2.0 atm * 4.50 L = n * 0.0821 L atm/K * 300 K
Do the multiplication on both sides of the equation to get:
9.0 = 24.63 n
Divide both sides by 24.63:
0.365 = n so you know you have 0.365 moles of HF gas. Now, we have to convert that to grams.
Since the atomic mass of F is 19 and the atomic mass of H is 1, a single molecule of HF weighs 20 atomic mass units. This also means that one mole of HF weighs 20 grams. You have 0.365 moles of HF.
0.365 moles * 20 grams/mole = 7.3 grams.