If you have a gas at a pressure of 16.5 atm, volume of 123.9 mL, and a temperature of 11°C, how many moles do you have?
If you assume that the gas behaves like an ideal gas, you can use the expression Pv=NRT where P is pressure, v is volume, N is number of moles, R is the gas constant and T is temperature.
As we have units of atm for pressure, we can use the gas constant value expressed in litre atm per degree K, which is 0.08206. We also need to convert the volume into litres, and the temperature into Kelvins to ensure we are working with consistent units.
Now simply rearrange for N: N = Pv/RT = (16.5 x 0.1239) / (0.08206 x 284) = 2.044 / 23.305 = 0.0877 moles.