[TheShehanigan]

Hello,

Out of my First Law of Thermodynamics problem set for practice, I am unable to solve one of the problems given to me. I'd like some direction on where to proceed, since I am a bit lost with the problem. It reads:

A 15.0 g piece of Mg(s) is deposited in a container with HCl(aq). Calculate the work done by the system as a consequence of the reaction, if P_atm = 1.1 atm and T = 298.15 K. Presume ideal behavior.

The reaction I calculated was: Mg(s) + 2 HCl(aq) --> H₂(g) + MgCl₂(aq)

Thus far, we've covered (and which are stuff I may be able to relate)

dW = -Pext dV (ext in this case, since the process is highly irreversible)
dU = dQ + dW
H = U + PV

I am assuming that the container will be open to the environment, and that thus the external pressure will be constant, making the process isobaric. However, I have doubts on how to calculate the initial volume of the system, or if my reasoning is correct at all.

I really don't want answers on how to do it, I just need tugs in the right direction to begin thinking on how to solve the problem.

Thanks.

[Yggdrasil]

Because solids, liquids, and solutions are much more dense than gasses, you can generally assume that their volume is negligible compared to the volume of any gas in the reaction.

[TheShehanigan]

Hence from said logic (which is actually a good approximation, don't know why I forgot it), I can calculate from the given quantity of Mg(s) and assuming it will be my limiting reactant the amount of H₂(g) produced by the reaction, and assume the initial volume will be 0, and calculate the final volume of the gas. Then I can simply calculate from the isobaric process, using P_ext given, the work by the reaction.

This actually sounds about right. I'll proceed to do that.