# gcse chem- just confused about moles and avagdros constant n how it relates to C-12..

Watch this thread
Thread starter 2 years ago
#1
I'm just confused about moles and Avogadro's constant n how it relates to C-12 and where the volume=moles*24 equation comes from?

Thanks so much in advance... I have my chemistry mock very soon!!!
0
reply
2 years ago
#2
(Original post by vix.xvi)
I'm just confused about moles and Avogadro's constant n how it relates to C-12 and where the volume=moles*24 equation comes from?

Thanks so much in advance... I have my chemistry mock very soon!!!
Hello, moles is a tricky one at GCSE so I don't blame you about being confused!!

Moles is a unit of measurement, this wasn't made clear to me when I was learning it, it is the measurement for 'amount of substance'. Just like you would have 10mm in a cm, there are 6.02 x 10^23 particles in 1 mole. It is also important to point out that the measurement of a mole isn't specific to atoms, I use the word particle because the mole could measure atoms, protons, neutrons, electrons, molecules, etc.

Avogadro's constant is just the number 6.02 x 10^23.

From what I can recall, there isn't a link between C-12 and the mole but maybe that is something to do with a different exam board, I did aqa, maybe someone else may know.

The moles equation for a gas is volume= moles x 24dm^3 (or 24000cm^3). This is because at room temperature and pressure, 1 mole of a gas occupies 24dm^3. Therefore, using this idea, if you have 2 moles it will occupy 48dm^3, and 3 moles will occupy 72dm^3 etc. That is why you multiply the number of moles by 24 (or 24,000).

Hope this helps you, just do your best in your mock, I was never happy with my mocks but it pushed me to try even harder for the real thing and I came out with a grade I was really happy with. Let me know if there is anything I haven't been clear about or can answer for you.
1
reply
2 years ago
#3
The value of the Avogadro constant was chosen so that the mass of one mole of a chemical in grams, was equal to the relative mass of one molecule of that chemical in daltons. A dalton is defined as being 1/12 of the mass of a carbon-12 atom. Which is about the mass of a proton or neutron.

The actual equation to calculate the volume is PV=nRT

Rearranging it gives us V=nRT/P

When we substitute in 1 for n as the number of moles.
R is the gas constant, ~8.314 numerical value
T is the temperature in kelvins, 298K = 25 degrees Celsius
P is the pressure in Pascals, 1 atmosphere = 101325Pa

V = (1 x 8.314 x 298)/101325

We get 0.02445... m^3

Which is equal to 24.45dm^3
1
reply
Thread starter 2 years ago
#4
(Original post by Student01234)
Hello, moles is a tricky one at GCSE so I don't blame you about being confused!!

Moles is a unit of measurement, this wasn't made clear to me when I was learning it, it is the measurement for 'amount of substance'. Just like you would have 10mm in a cm, there are 6.02 x 10^23 particles in 1 mole. It is also important to point out that the measurement of a mole isn't specific to atoms, I use the word particle because the mole could measure atoms, protons, neutrons, electrons, molecules, etc.

Avogadro's constant is just the number 6.02 x 10^23.

From what I can recall, there isn't a link between C-12 and the mole but maybe that is something to do with a different exam board, I did aqa, maybe someone else may know.

The moles equation for a gas is volume= moles x 24dm^3 (or 24000cm^3). This is because at room temperature and pressure, 1 mole of a gas occupies 24dm^3. Therefore, using this idea, if you have 2 moles it will occupy 48dm^3, and 3 moles will occupy 72dm^3 etc. That is why you multiply the number of moles by 24 (or 24,000).

Hope this helps you, just do your best in your mock, I was never happy with my mocks but it pushed me to try even harder for the real thing and I came out with a grade I was really happy with. Let me know if there is anything I haven't been clear about or can answer for you.
Thank you soooo much!!!! I understand it now, thanks again! Hope you had a good day
Last edited by username3477548; 2 years ago
0
reply
2 years ago
#5
OK, Student01234 has answered a lot of this correctly. On the C-12 thing, it's how relative atomic mass is measured. Carbon-12 atoms are taken to have a mass of exactly 12 by definition, with atoms of other elements being measured relative to that. So hydrogen atoms have a mass very close to a twelfth of that (and you can take their mass as being 1 for calculations) but it's not exactly one.

A mole of a substance will have a mass of the R.A.M. number in grams. So a mole of Carbon 12 weighs 12 grams. Does that help?
1
reply
Thread starter 2 years ago
#6
(Original post by Student01234)
Let me know if there is anything I haven't been clear about or can answer for you.
Would you be able to help me out with something else too? I'm just confused about exothermic and endothermic reactions because an exothermic reaction needs more heat energy to 'make the bonds than break the bonds', what does that mean? Ty!! xx
0
reply
2 years ago
#7
(Original post by vix.xvi)
Would you be able to help me out with something else too? I'm just confused about exothermic and endothermic reactions because an exothermic reaction needs more heat energy to 'make the bonds than break the bonds', what does that mean? Ty!! xx
In exothermic and endothermic reactions, energy is required to make bonds, and energy is released when bonds are broken.
In an exothermic reaction, the combined energy released when the bonds of the reactants which are broken is greater than the energy used to make the bonds of the products. This means you have spare energy left over when you do the reaction, which is released into the environment.

To put it simply:
Exothermic reaction: energy released when the bonds are broken is greater then the energy required to make the bonds later on, so meaning there is an excess of energy.

Endothermic reaction: energy released when the bonds are broken is less than the energy required to make the bonds later on, meaning there is a lack of energy to make the bonds of the products of the reactants.
Last edited by Icy Wolf; 2 years ago
1
reply
Thread starter 2 years ago
#8
(Original post by biran4454)
In exothermic and endothermic reactions, energy is required to make bonds, and energy is released when bonds are broken.
In an exothermic reaction, the combined energy released when the bonds of the reactants which are broken is greater than the energy used to make the bonds of the products. This means you have spare energy left over when you do the reaction, which is released into the environment.

To put it simply:
Exothermic reaction: energy released when the bonds are broken is greater then the energy required to make the bonds later on, so meaning there is an excess of energy.

Endothermic reaction: energy released when the bonds are broken is less than the energy required to make the bonds later on, meaning there is a lack of energy to make the bonds of the products of the reactants.
ohhh tysm!!!!!
Last edited by username3477548; 2 years ago
0
reply
Thread starter 2 years ago
#9
(Original post by Trumbles)
OK, Student01234 has answered a lot of this correctly. On the C-12 thing, it's how relative atomic mass is measured. Carbon-12 atoms are taken to have a mass of exactly 12 by definition, with atoms of other elements being measured relative to that. So hydrogen atoms have a mass very close to a twelfth of that (and you can take their mass as being 1 for calculations) but it's not exactly one.

A mole of a substance will have a mass of the R.A.M. number in grams. So a mole of Carbon 12 weighs 12 grams. Does that help?
ohhh I get it. So, for example, oxygen has a mass of 16g? Tysm for your explanation!! x
0
reply
2 years ago
#10
(Original post by vix.xvi)
ohhh tysm!!!!!
You're welcome!
0
reply
2 years ago
#11
(Original post by vix.xvi)
Would you be able to help me out with something else too? I'm just confused about exothermic and endothermic reactions because an exothermic reaction needs more heat energy to 'make the bonds than break the bonds', what does that mean? Ty!! xx
Biran4454 has answered that nicely. I would like to add, my teacher told me a way to remember the difference between exothermic and endothermic reactions: Bendo Pendo, Mexo Nexo. 'Bendo' is breaking bonds is endothermic, 'Pendo' tells you that the energy change in an endothermic reaction is positive, 'Mexo' is making bonds is exothermic, and 'Nexo' tells you that the energy change in an exothermic reaction is negative. It's a little quirky rhyme which works for me, so might help you too. As before, please ask if there is anything else!!
1
reply
2 years ago
#12
(Original post by vix.xvi)
ohhh I get it. So, for example, oxygen has a mass of 16g? Tysm for your explanation!! x
You're welcome – and that's right: a mole of oxygen has a mass of 16 grams (that's assuming it's a mole of oxygen atoms, a mole of O2 molecules would have a mass of 32g (16g x 2) ).
0
reply
2 years ago
#13
(Original post by Student01234)
I would like to add, my teacher told me a way to remember the difference between exothermic and endothermic reactions: Bendo Pendo, Mexo Nexo.
That's an interesting way to remember it.
I just remember that 'exo'means 'out', like 'exoskeleton' and 'exoplanet', and so I know that in an exothermic reaction, the energy is going out of the reaction.
1
reply
X

Write a reply...
Reply
new posts
Back
to top
Latest
My Feed

### Oops, nobody has postedin the last few hours.

Why not re-start the conversation?

see more

### See more of what you like onThe Student Room

You can personalise what you see on TSR. Tell us a little about yourself to get started.

### Poll

Join the discussion

#### Have you done work experience at school/college?

Yes (121)
41.72%
Not yet, but I will soon (53)
18.28%
No (116)
40%

View All
Latest
My Feed

### Oops, nobody has postedin the last few hours.

Why not re-start the conversation?

### See more of what you like onThe Student Room

You can personalise what you see on TSR. Tell us a little about yourself to get started.