Question

A gas is allowed to expand at constant temperature from a volume of 2.00 L to 11.20 L against an external pressure of 1.500 atm. If the gas loses 256 J of heat to the surroundings, what are the values of q, w, and ΔU? Pay careful attention to units.

Question 8 options:

	+256 J, -1398 J, -1142 J
	-256 J, -419 J, -547 J
	-256 J, +838 J, +1094 J
	+256 J, +1398 J, +1654 J
	-256 J, -1398 J, -1654 J

Answer 1

This is a very simple question to answer IF you pay attention to the signs of the chemical quantities being asked about. Note that the gas is allowed to expand and the gas "loses" heat. This implies:

1). Heat is lost by the system, therefore the sign of q will be -ve.

2). Work is done BY the system (gas expanding is work BY the system), hence the sign of w is -ve

and

3). We know that by first law of Thermodynamics, .

Let us determine the numerical asnwers

Its very clear that q = -256J.

To find the value of w, we use the formulae which gives us w = . Note that 1 lit-atm equals 101.33 Joules of energy. Hence the total work done is -1398 J.

Now the value of is given by the equation, .

We get .

Note that this does NOT correspond with the given options.

Please note that this answer has been calculated according to the Standard (IUPAC) sign conventions being used. None of the options match, however if one was to answer the question, the closest option would be option e as 2 of the 3 values given are correct.

But if a different sign convention (say the signs of q and w are changed) were to be used, option a would be the best choice.