William W. answered • 04/26/20

BS in Mechanical Engineering with 35 years experience

The key to these problems is the manipulation of the Ideal Gas Law, typically written as PV = nRT.

In Problem 1, Temp and Pressure are constant, as is "R" the ideal Gas Constant. So, manipulating the Ideal Gas Law, we can make it:

The things on the right are constant. That means they will not change between time 1 and time 2. So, we can say:

or V_{1}/n_{1} = V_{2}/n_{2} or n_{2} = V_{2}n_{1}/V_{1} so n2 = 5.25•2.45/7.62 = 1.69 moles

The other problems are all similar in that you can manipulate PV = nRT to utilize it at time 1 and time 2 with certain elements constant. The critical thing is to ensure temperature is ALWAYS in Kelvin. The other elements need to be in matching units (example: if pressure is in atm at time 1, use atm at time 2)