OCR Chemistry B (Salters) Thursday 10th January Discussion

The question gave us the energy density per mole, which was something like -5400 kJ mol^-1
We are asked to find out the kJ per kilogram.
We were given the Ar of the molecule, something like 112.

So I found the amount of moles per kilogram .... 1000 / 112. Multiplied the moles by the value above (5400).
Obviously these are rough estimates of what was given, I can't remember the exact values.
But I would have got a value of about 47321, but below <47500.
Then it asked for 2 sig fig. in the answer.
The answer isn't a negative value as itis energy per kilogram and not an enthalpy change.

3 reps for next page :d

Please please please! I'm stressing so much Don't think I got the A I needed x I've repped x

i see my mistake... DAMN

I really dont see, how the bond angles question for bond angle B was 109, Bond angle A had 120, it had 3 areas of density, but again the bond angle B also had the same and even less electrons.
Can somebody please explain where i went wrong?

I have a feeling your answer may not be true, the first question we worked out the energy transferred to water, they gave us the volume of water ... I think they had us assume that 200cm^3 of water is equal to 200grams.



Yes, definitely 2 significant figures and not decimal places.. I had to read it over 3 times just to make sure I wasn't being stupid

The question gave us the energy density per mole, which was something like -5400 kJ mol^-1
We are asked to find out the kJ per kilogram.
We were given the Ar of the molecule, something like 112.

So I found the amount of moles per kilogram .... 1000 / 112. Multiplied the moles by the value above (5400).
Obviously these are rough estimates of what was given, I can't remember the exact values.
But I would have got a value of about 47321, but below <47500.
Then it asked for 2 sig fig. in the answer.
The answer isn't a negative value as itis energy per kilogram and not an enthalpy change.

I still dont see where i went wrong?

I did it all right and got 47,000, but I put negative D: How many marks will I lose?

It came up on a previous paper, i dont think you lose any marks from what i can remember, i stupidly put my answer in standard form, would i still get full marks or 2 out of 3? -4.7x104

nope you lose non, on a previous paper it says ignore (-) sign. On a very similar question

2 as you'll lose the signifant figures mark.

Were we meant to take into account hydrogens as bonds? Like bond angle a only has three bonds so were we meant to assume there was hydrogen on there aswell? Because skeletal doesn't show hydrogen bonds does it? Arghh! Majorly confused haha x