PHYSIC"S CHEM QUESTIONS THREAD

[physics]

First of all. thank you vcenotes for helping me with VCE!  :smitten:

and now for the questions

Which of the following fuel cells has the highest density measured in kJkg^-1?
methanol
ethanol
1-propanol
2-propanol

i looked at the data booklet and wrote our ALL the equations and then did the ratios and then got the answer but is there any easier way. cause this question is time consuming as a MC

[Blakhitman]

What equations?

You just need to divide the delta H by the molar mass to get Kj/g and then multiply by 1000 to get in Kj/Kg

[physics]

omg u lost me there. you know the combustion equations because the ratio is sometime 2 to delta H but sometime 1 to delta H

[linahh123]

omg u lost me there. you know the combustion equations because the ratio is sometime 2 to delta H but sometime 1 to delta H

wow twin you're always studying (hehehe im the evil/dumb twin) =="

[physics]

omg u lost me there. you know the combustion equations because the ratio is sometime 2 to delta H but sometime 1 to delta H

wow twin you're always studying (hehehe im the evil/dumb twin) =="

im sure you are not! doncha do chem?

[Blakhitman]

omg u lost me there. you know the combustion equations because the ratio is sometime 2 to delta H but sometime 1 to delta H

I don't think it's asking that. It's asking about the energy released per Kg.

We'll do methanol delta H=725KJ/mol

change that to KJ/g by multiplying the denominator (mol) by molar mass of methanol so

we get   then to get to Kg multiply both top and bottom by 1000, to get

EDIT: O.k that may have not explained it properly. Think of it like this, the delta H in the booklet is given as Energy (KJ) per mol of methanol, we want to know how much energy per gram, so take 1 mol and multiply by Mr from n=m/Mr to get 32g in each mol of methanol. so 725 Kj per mol is the same as 725KJ per 32g or 22.656KJ per 1 g.

Then we want to change grams to Kg so multiply by 1000.

[Greggler]

find number of mole of each molecule by doing 1000/Molar mass.

Then multiple this mole value by the enthalpy in the data book.

[linahh123]

omg u lost me there. you know the combustion equations because the ratio is sometime 2 to delta H but sometime 1 to delta H

wow twin you're always studying (hehehe im the evil/dumb twin) =="

im sure you are not! doncha do chem?

yeah i do chem but ive only done 3 prac exams =S

[physics]

omg u lost me there. you know the combustion equations because the ratio is sometime 2 to delta H but sometime 1 to delta H

I don't think it's asking that. It's asking about the energy released per Kg.

We'll do methanol delta H=725KJ/mol

change that to KJ/g by multiplying the denominator (mol) by molar mass of methanol so

we get  then to get to Kg multiply both top and bottom by 1000, to get

EDIT: O.k that may have not explained it properly. Think of it like this, the delta H in the booklet is given as Energy (KJ) per mol of methanol, we want to know how much energy per gram, so take 1 mol and multiply by Mr from n=m/Mr to get 32g in each mol of methanol. so 725 Kj per mol is the same as 725KJ per 32g or 22.656KJ per 1 g.

Then we want to change grams to Kg so multiply by 1000.

i see it now.