# Alevel chemistry

Isooctane (2,2,4-trimethylpentane) is an important component of petrol used in
cars.
When isooctane is burned, the enthalpy change is −47.8 kJ g−1
Isooctane is a liquid at room temperature with a density of 0.692 g cm−3
Calculate the heat energy released, in kJ, when 1.00 dm3 of isooctane burns in
excess oxygen.
Here is my working https://ibb.co/gckG0y
The best way to solve this is to think about the units. For example if density if g PER cm^3 then multiplying it by the volume (cm3) gives grams (mass).

Even if you don't know the entire method to get the end result, work your way towards such quantities by looking at your units.
(edited 5 years ago)
Original post by splitter2017
Here is my working https://ibb.co/gckG0y
The best way to solve this is to think about the units. For example if density if g PER cm^3 then multiplying it by the volume (cm3) gives grams (mass).

Even if you don't know the entire method to get the end result, work your way towards such quantities by looking at your units.

I really dont understand density at all in chemistry as theres no mention at all in the workbook and i cant find it anywhere online, is it possible you could give some basic examples like the units and what they mean? Maybe a pattern which i could apply to other questions, Thank you so much for the solution!
Original post by splitter2017
Here is my working https://ibb.co/gckG0y
The best way to solve this is to think about the units. For example if density if g PER cm^3 then multiplying it by the volume (cm3) gives grams (mass).

Even if you don't know the entire method to get the end result, work your way towards such quantities by looking at your units.

so density is g per cm^3 and to get just g by itself you times by the volume which is in cm^3
Could you also explain how you used the mass and the enthalpy change to find the energy released, is this a formula ?
Here you are, I have made a summary of the equations that we use in chemistry including the density one.

I have some other summaries on my website if needed http://tdck.weebly.com/chemistry.html but I think the one I attached is what you are after
(edited 5 years ago)
Original post by sohaail23
so density is g per cm^3 and to get just g by itself you times by the volume which is in cm^3
Could you also explain how you used the mass and the enthalpy change to find the energy released, is this a formula ?

For the enthalpy change.

The units the question gave was kilo (x 1000) Joules PER gram (g) and so I multiplied this by the mass.
i.e. if there are 692g and we know the enthalpy change is -47800 J for every gram then multiply the two gives you the enthalpy change for all 692 g of the substance.

It can be helpful to draw a triangle like the ones shown in my summary sheet.
Original post by splitter2017
For the enthalpy change.

The units the question gave was kilo (x 1000) Joules PER gram (g) and so I multiplied this by the mass.
i.e. if there are 692g and we know the enthalpy change is -47800 J for every gram then multiply the two gives you the enthalpy change for all 692 g of the substance.

It can be helpful to draw a triangle like the ones shown in my summary sheet.

I dont know how ive gone through 2 years of chemistry without knowing those conversions ! Thank you for your help with everything i really appreciate it!
Original post by sohaail23
I dont know how ive gone through 2 years of chemistry without knowing those conversions ! Thank you for your help with everything i really appreciate it!

What spec is that question from :?
Original post by Nettled
What spec is that question from :?

Aqa, it was from the June 2017 As paper i did last year.
Original post by splitter2017
Here is my working https://ibb.co/gckG0y
The best way to solve this is to think about the units. For example if density if g PER cm^3 then multiplying it by the volume (cm3) gives grams (mass).

Even if you don't know the entire method to get the end result, work your way towards such quantities by looking at your units.

hiii this might sound dumb but i thought cm3 to dm3 was / 10000