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# Chemistry - Combined Moles watch

1. 392 dm^3 of a gas is dissolved in 2.65 dm^3 of a solvent. What is the concentration of the resulting solution?
Temperature, pressure of the gas prior to dissolving in the solvent...?
3. (Original post by TutorsChemistry)
Temperature, pressure of the gas prior to dissolving in the solvent...?
No, but I am assuming that it is at room temperature since I am given that 1 mole of any gas takes up 24 dm^3 at standard condition. (25 Celsius and 1 bar). Sorry for not stating so earlier.
4. (Original post by Igorzycho)
No, but I am assuming that it is at room temperature since I am given that 1 mole of any gas takes up 24 dm^3 at standard condition. (25 Celsius and 1 bar). Sorry for not stating so earlier.
Using that information, how many moles of gas are present?

Once you have that number of moles, just divide by the volume. (Number of moles) / (Volume in dm3) = moles dm-3
5. (Original post by TutorsChemistry)
Using that information, how many moles of gas are present?

Once you have that number of moles, just divide by the volume. (Number of moles) / (Volume in dm3) = moles dm-3
I don't have that information given. I guess that I'll have to ask my chemistry teacher about this, as the question seems to be missing necessary information. Thanks for you're help though.

Out of curiosity, would you then add the two volumes in order to find the concentration (assuming you know the amount of moles)?
6. (Original post by Igorzycho)
I don't have that information given. I guess that I'll have to ask my chemistry teacher about this, as the question seems to be missing necessary information. Thanks for you're help though.

Out of curiosity, would you then add the two volumes in order to find the concentration (assuming you know the amount of moles)?
You can calculate the number of moles of gas from the information you already have.
7. (Original post by Maker)
You can calculate the number of moles of gas from the information you already have.
392/24 right?
8. (Original post by Igorzycho)
I don't have that information given. I guess that I'll have to ask my chemistry teacher about this, as the question seems to be missing necessary information. Thanks for you're help though.

Out of curiosity, would you then add the two volumes in order to find the concentration (assuming you know the amount of moles)?
No, you wouldn't add these together. When calculating the concentration you must know the number of moles of your solute (i.e. the gas).

You do have the information you need, you just haven't realised it:
You have a known volume of gas, 392 cm3, and have been told that 1 mole of gas occupies 24 dm3 at the (standard) conditions. From this you can work out the number of moles of gas.
Moles = (392 cm3) / (24 dm3)
= (392 cm3) / (24000 cm3)

work that number out, then divide by the solvent volume to find concentration of solution.
9. (Original post by TutorsChemistry)
No, you wouldn't add these together. When calculating the concentration you must know the number of moles of your solute (i.e. the gas).

You do have the information you need, you just haven't realised it:
You have a known volume of gas, 392 cm3, and have been told that 1 mole of gas occupies 24 dm3 at the (standard) conditions. From this you can work out the number of moles of gas.
Moles = (392 cm3) / (24 dm3)
= (392 cm3) / (24000 cm3)

work that number out, then divide by the solvent volume to find concentration of solution.
Spoiler:
Show

I feel that we're getting there guys...

So I divided 392/24 giving me 16.3 recurring (or 16.3 to 3 significant figures) a.k.a the no. of moles, but why do I then need to divide it be the volume of the solvent and not the gas?
10. (Original post by Igorzycho)
Spoiler:
Show

I feel that we're getting there guys...

So I divided 392/24 giving me 16.3 recurring (or 16.3 to 3 significant figures) a.k.a the no. of moles, but why do I then need to divide it be the volume of the solvent and not the gas?
We are getting there I hope!

But you must put the gas volumes in the same units before you do the calculation of moles - look again at my previous post
11. (Original post by TutorsChemistry)
We are getting there I hope!

But you must put the gas volumes in the same units before you do the calculation of moles - look again at my previous post
I've got 0.0416 recurring as the concentration for both the gas and the solvent:

GAS) 392/24 = 16.3 recurring, 16.3/392 = 0.0416 recurring
SOLVENT) and 2.65/24 = 0.110416 recurring, 0.110416/2.65 = 0.0416 recurring

So as the concentrations are the same, does that mean the concentration of the solution is also 0.0416 recurring mol dm^-3. (1/24 mol dm^-3)?
12. (Original post by Igorzycho)
I've got 0.0416 recurring as the concentration for both the gas and the solvent:

GAS) 392/24 = 16.3 recurring, 16.3/392 = 0.0416 recurring
SOLVENT) and 2.65/24 = 0.110416 recurring, 0.110416/2.65 = 0.0416 recurring

So as the concentrations are the same, does that mean the concentration of the solution is also 0.0416 recurring mol dm^-3. (1/24 mol dm^-3)?
To find the concentration of a solution its:
C=n/v
so for the solution you do 16.3/2.65 to get the concentration in dm^3
13. (Original post by LLearnt)
To find the concentration of a solution its:
C=n/v
so for the solution you do 16.3/2.65 to get the concentration in dm^3
I understand the equation but why do you divide the number of moles of the gas by the volume of the solvent?
14. (Original post by Igorzycho)
I understand the equation but why do you divide the number of moles of the gas by the volume of the solvent?
Because that volume of the gas has X number of moles which are dissolved into the solvent forming the resultant solution.
15. (Original post by Igorzycho)
I understand the equation but why do you divide the number of moles of the gas by the volume of the solvent?
Because concentration is the number of moles per unit of volume (hence the units of concentration).

You don't have the mole calculation correct yet, I'm afraid. You need 392/24000. (Forget you're 16.3333... that came from an incorrect use of the calculation).
16. (Original post by TutorsChemistry)
Because concentration is the number of moles per unit of volume (hence the units of concentration).

You don't have the mole calculation correct yet, I'm afraid. You need 392/24000. (Forget you're 16.3333... that came from an incorrect use of the calculation).

I really appreciate your help especially as you aren't giving me the explicit answer but I'm really confused. Surely you divide 392 dm^3/24 dm^3 to give you the number of moles?
17. (Original post by Igorzycho)
I really appreciate your help especially as you aren't giving me the explicit answer but I'm really confused. Surely you divide 392 dm^3/24 dm^3 to give you the number of moles?
My apologies, I misread the volume in your initial post. You are quite correct.
18. (Original post by TutorsChemistry)
My apologies, I misread the volume in your initial post. You are quite correct.
So is the final answer 0.0416 recurring mol dm^-3 (concentration of solution) or am I missing something?
19. No, you divide the moles of gas by the volume of SOLVENT (2.65 dm3), not the volume of gas.
20. (Original post by Igorzycho)
392 dm^3 of a gas is dissolved in 2.65 dm^3 of a solvent. What is the concentration of the resulting solution?
This question is not possible without knowing the volume of the solution formed.

The definition of concentration (molarity) is moles of solute per litre of SOLUTION.

Unless you are in the USA and working in Molality.

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