# Chemistry a level enthalpy

In q=mc🔺T how do i know when to use the volume of the water as the mass in the equation and when to use the actual change in mass of the compound
Original post by Arii256
In q=mc🔺T how do i know when to use the volume of the water as the mass in the equation and when to use the actual change in mass of the compound

m in the equation Q = mcΔT is always the mass of water (which is equivalent to the volume of water, as the density of water is roughly 1 g/cm^3).

The change in the mass of the compound is used in the enthalpy of combustion calculations, so that you can find the moles of substance burnt.

It is also useful to note that the magnitude of the enthalpy change is given by Q/n, where Q is the thermal energy released or taken in and n is the number of moles of the reacting substance.
(edited 6 months ago)
Original post by TypicalNerd

m in the equation Q = mcΔT is always the mass of water (which is equivalent to the volume of water, as the density of water is roughly 1 g/cm^3).

The change in the mass of the compound is used in the enthalpy of combustion calculations, so that you can find the moles of substance burnt.

It is also useful to note that the magnitude of the enthalpy change is given by Q/n, where Q is the thermal energy released or taken in and n is the number of moles of the reacting substance.

Hey thanks! But in this question that I just did it was asking me to calculate the enthalpy change of combustion but when using the q=mcat equation i used the vol of the water as the mass and got the correct answer, however I remember doing a similar question a week ago where I used the mass of the compound in the equation and got the answer right
Original post by Arii256
Hey thanks! But in this question that I just did it was asking me to calculate the enthalpy change of combustion but when using the q=mcat equation i used the vol of the water as the mass and got the correct answer, however I remember doing a similar question a week ago where I used the mass of the compound in the equation and got the answer right

Interesting.

It could be that it was a slightly different equation in that question, as there is similar equation that works for a known mass of the compound burned (but this would generally be given to you, with all the meanings of the symbols in the equation explained as it is not taught at A level).

I’d need to see the question though to tell you for certain.
Original post by TypicalNerd

Interesting.

It could be that it was a slightly different equation in that question, as there is similar equation that works for a known mass of the compound burned (but this would generally be given to you, with all the meanings of the symbols in the equation explained as it is not taught at A level).

I’d need to see the question though to tell you for certain.

It wont let me upload pictures on here i’ll try finding the questions online and linking them on this thread
Original post by Arii256
It wont let me upload pictures on here i’ll try finding the questions online and linking them on this thread

Fair enough. I think I know a question that may be similar (though it is for the AQA board), so I’ll see if I can go and find it.

Edit: It’s Q5 on this paper: https://filestore.aqa.org.uk/sample-papers-and-mark-schemes/2020/november/AQA-74053-QP-NOV20.PDF
(edited 6 months ago)
Original post by TypicalNerd

Fair enough. I think I know a question that may be similar (though it is for the AQA board), so I’ll see if I can go and find it.

Edit: It’s Q5 on this paper: https://filestore.aqa.org.uk/sample-papers-and-mark-schemes/2020/november/AQA-74053-QP-NOV20.PDF

Found them! This is the one where I didn’t use the volume of the water