Chemistry questions

Original post by otrivine

are my answers correct or wrong to the following questions please

1) How many peaks will be found in low resolution proton NMR spectrum of propan-1-ol

Is the answer 3

There are four different hydrogen environments = 4 peaks

2) Given that detaH combustion of C8H18 = -5430Kj mol-1 , the amount of energy released when 5.70g of octane are completely burnt in oxygen at 298K and 1.0atm is ?

I got 108,600 KJ

work out the moles of octane and multiply by the energy per mole ...

Reply 2

Original post by charco

There are four different hydrogen environments = 4 peaks

:frown:

work out the moles of octane and multiply by the energy per mole ...

Okay but wait the first one why 4 , we are not adding any D2O?

second one? I calculated the number of moles which gave 0.05 and then did the energy -5430 divided by 0.05 = -108600

Reply 3

Original post by otrivine

Okay but wait the first one why 4 , we are not adding any D2O?

second one? I calculated the number of moles which gave 0.05 and then did the energy -5430 divided by 0.05 = -108600

CH3CH2CH2OH

You must multiply, not divide, by the moles

Reply 4

Original post by charco

CH3CH2CH2OH

You must multiply, not divide, by the moles

Okay but when do we know if we have to multiply or divide?

Reply 5

Original post by otrivine

Okay but when do we know if we have to multiply or divide?

It's just common sense!

If you produce x kJ when you burn one mole, will you produce more, or less, when you burn fewer than 1 moles?

Reply 6

Original post by charco

It's just common sense!

If you produce x kJ when you burn one mole, will you produce more, or less, when you burn fewer than 1 moles?

Yes but how do we know if we have to divide so you are saying if its less than 1 mole we divide and if bigger than 1 moles we multiply ?

Reply 7

Original post by charco

It's just common sense!

If you produce x kJ when you burn one mole, will you produce more, or less, when you burn fewer than 1 moles?

Also, in titrations where you need to get the mean point, I need to select the volumes that I want in which the values are close together but by how much difference should it be?

Reply 8

Original post by otrivine

Yes but how do we know if we have to divide so you are saying if its less than 1 mole we divide and if bigger than 1 moles we multiply ?

That is NOT what I'm saying.

If you have 2000 kJ released and you have only burned 0.1 moles would you expect to have more or less energy produced by burning MORE moles?

It's common sense!

Reply 9

Original post by charco

That is NOT what I'm saying.

If you have 2000 kJ released and you have only burned 0.1 moles would you expect to have more or less energy produced by burning MORE moles?

It's common sense!

obviously more right because more moles means more energy

Reply 10

Original post by otrivine

obviously more right because more moles means more energy

So you are given the energy released by burning 1 mole ...

Then you are asked for the energy released by burning MUCH LESS.

Is it going to be larger or smaller?

Reply 11

Original post by charco

So you are given the energy released by burning 1 mole ...

Then you are asked for the energy released by burning MUCH LESS.

Is it going to be larger or smaller?

smaller but I still dont get when we have to multiply or divide :confused:

Reply 12

Original post by otrivine

smaller but I still dont get when we have to multiply or divide :confused:

Just use common sense ...

You divide by the number of moles when the ANSWER you want is PER MOLE ...

You multiply by moles when you are given the value per mole and the ANSWER required is the value for a DIFFERENT number of moles.

Reply 13

Original post by charco

thanks

makes sense now

Reply 14

Original post by charco

That is NOT what I'm saying.

If you have 2000 kJ released and you have only burned 0.1 moles would you expect to have more or less energy produced by burning MORE moles?

It's common sense!

Also, in titrations where you need to get the mean point, I need to select the volumes that I want in which the values are close together but by how much difference should it be?

Reply 15