Post by: physics on October 24, 2010, 03:44:18 pm
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
Post by: Blakhitman on October 24, 2010, 04:00:08 pm
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
Post by: physics on October 24, 2010, 04:01:58 pm
Post by: linahh123 on October 24, 2010, 04:04:54 pm
omg u lost me there. you know the combustion equations because the ratio is sometime 2 to delta H but sometime 1 to delta Hwow twin you're always studying (hehehe im the evil/dumb twin) =="
Post by: physics on October 24, 2010, 04:14:01 pm
im sure you are not! doncha do chem?omg u lost me there. you know the combustion equations because the ratio is sometime 2 to delta H but sometime 1 to delta Hwow twin you're always studying (hehehe im the evil/dumb twin) =="
Post by: Blakhitman on October 24, 2010, 04:18:31 pm
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
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.
Post by: Greggler on October 24, 2010, 04:26:59 pm
Then multiple this mole value by the enthalpy in the data book.
Post by: linahh123 on October 24, 2010, 04:27:44 pm
yeah i do chem but ive only done 3 prac exams =Sim sure you are not! doncha do chem?omg u lost me there. you know the combustion equations because the ratio is sometime 2 to delta H but sometime 1 to delta Hwow twin you're always studying (hehehe im the evil/dumb twin) =="
Post by: physics on October 24, 2010, 07:07:17 pm
i see it now.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 getthen 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.