Which value of R? Question 105

[GraceChen2A]

Part a) asks to calculate the work that must be done against the atmosphere for the expansion of the gaseous products in the combustion of 1.00 mol C6H6 (l) at 25C and 1.00 bar. If you write out the equation, you get C6H6+15/2 O2 -> 6CO2 + 3H20. To calculate work I used the ideal gas law so that w=-PdeltaV=-deltaNRT. However, how do we know that the R constant is 8.3145 J/K mol instead of 0.08206 L atm/K mol?

[Neil DSilva 1L]

The units for work is joules. Using the 8.3145 J/K just allows some of the units to cancel and give us a final answer in joules. When deciding which value of R to use, just look to see what the question is asking and use the value that will allow you to use the least amount of steps for simplicity.

[martha-1I]

8.314 JK^-1mol^-1 because we want the final answer to be in units of Joules since the units of work is Joules or KJ. In general, we use 0.08206 L.atm.K^-1mol^-1 when using PV=nRT because the units will cancel out and leave us with whatever it is that we are looking for.