I've been looking at how Kaplan calculates density, and i'm kind of confused. They give you 2 ways to calculate density. The first is: density=m/v=P(MM)/RT. The second way is a little more confusing where they find: V2=V1*P1/P2*T2/T1, using STP for all the initial conditions and then use V2 to find density using: d=m/V2. For all their questions, they use the second equation to solve for density. Is there any reason why they choose to use this rather than using the first equation, which seems a lot easier? Are there any situations where using the second equation is better?