J.R. S. answered • 01/30/19

Ph.D. University Professor with 10+ years Tutoring Experience

Is it only the math or the physics/physical concepts that you don't get?

In this problem, you have a constant pressure of 1.00 atm. This being the case, it is an isobaric system (constant pressure), and so you have the equation that work (W) = -pressure(P) x change in volume (∆V)

This is written as W = -P∆V and is known as pressure-volume work.

Now, understand the conventions that are used.

-If work is done ON the system, it has a positive sign (+)

-If work is done BY the system, it has a negative sign (-)

In this problem, the gas is being compressed, i.e. the volume decreases, and **so work is being done ON the system**. Thus, the sign of W is positive (+). This comes out automatically because of negative sign in front of P∆V,

So, we have W = -P∆V and substituting values, we have

W = -(1.00 atm)(1.5L - 1.25L) = -(1.00 atm)(0.25L) = -0.25 L-atm

Converting this to joules, we get -0.25 L-atm x 101.3 J/L-atm = **-25.3 J** (note the negative sign)

Hope this all helps a little.