Here we are being asked to find the mass of Helium that would be needed to pressurize a tank of a specific volume, to a specific pressure, at a specific temperature.
In other words, how much helium will we have at this volume, pressure, and temperature?
Whenever you are asked for some property of a gas, mental alarm bells should start ringing that you will probably need to use a Gas Law. There are many gas laws, but the most important one (in this chemist's opinion anyway) is the Ideal Gas Law. The ideal gas law is pV = nRT
p is Pressure
V is Volume
n is Number of moles
R is the Ideal Gas Constant
T is the temperature
(You may think those letters are obvious, but it can be confusing when it's just alphabet soup.) Notice we have all of these letters given to us in the question except one: Number of moles. That is what we want. (We can relate moles to kg using the molar mass.)
Before we plug everything in, take a look at the units... We are given L, atm, and °C. You should recall the ideal gas constant is 0.08206 (L · atm)/(mol · K). The only unit that does not match what we have is the temperature. (The units have to match!) So let's convert °C to Kelvin (with proper sig figs).
65°C + 273.15 = 338K
Now we plug it all in...
pV = nRT
(227atm)(60L) = n(0.08206 L · atm/mol · K)(338K)
13,620 L · atm = n(27.7 L · atm/mol)
492 mol = n
500 mol = n
Notice we round the final answer to the right number of sig figs, which in this case is just one, because exactly as written 60L only has 1 sig fig. The final step is to find the mass (in kg) of 500 moles of Helium. Referring to the periodic table, the molar mass of He is 4.00g/mol. Also remember there are 1000g in 1kg.
500 mol He x (4.00g/1 mol) x (1 kg/1000 g) = 2 kg He
This will require 2 kg of Helium.
