# Homework Help: Hydrocarbons analysis

1. Aug 8, 2010

### Grhymn

1. The problem statement, all variables and given/known data
10mLCxHy reacts with excess O2
After reacting and cooling until 298K the volume GAS = 55mL. Then NaOH is added and then the final volume of GAS = 35mL. Give the emperical formular of the hydrocarbon used in this reaction.

2. Relevant equations

-CxHy + (X+Y/4)O2 -> X CO2 + Y/2 H2O

-CO2+ 2NaOH -> Na2CO3 + H20

3. The attempt at a solution
10ml CxHY in the beginning and 55ml CO2 at the end => 1/X=10/55
55ml/X=2*VolumeH20/Y

2. Aug 8, 2010

### Staff: Mentor

Why do you think there was 55 mL of CO2?

Last edited by a moderator: Aug 13, 2013
3. Aug 8, 2010

### Grhymn

Because at the end of the reaction after cooling there is 55mL gas, H20 is liquid then and I think you shouldn't take acount of the O2 in the 55mL

4. Aug 8, 2010

### Staff: Mentor

So what was the gas adsorbed by the NaOH?

Last edited by a moderator: Aug 13, 2013
5. Aug 8, 2010

### Grhymn

You are right, I meant the amount of CO2 after the first reaction.

6. Aug 8, 2010

### Staff: Mentor

I think you are still misunderstanding the question. What gases were present in 55 mL, what gas is present in the remaining 35 mL?

Last edited by a moderator: Aug 13, 2013
7. Aug 8, 2010

### Grhymn

I think the answer is CO2 for both questions

8. Aug 8, 2010

### Staff: Mentor

So why part of the CO2 was adsorbed and part not?

Last edited by a moderator: Aug 13, 2013
9. Aug 8, 2010

### Grhymn

20 mL absorbed of the CO2 created by the first reacting absorbed and 35 mL remaining after the second reaction

10. Aug 8, 2010

### Staff: Mentor

What second reaction? There was just a one reaction producing carbon dioxide, and that was combustion.

Last edited by a moderator: Aug 13, 2013
11. Aug 8, 2010

### Grhymn

NaOH reacts with CO2 (I guess)

-CO2+ 2NaOH -> Na2CO3 + H20

12. Aug 8, 2010

### Staff: Mentor

So how come some of CO2 was left?

Last edited by a moderator: Aug 13, 2013
13. Aug 9, 2010

### Grhymn

Because there wasn't enough NaOH, or because there is a dynamic equilibrium. I am not sure what kind of reaction this is (<=> or ->)

14. Aug 9, 2010

### Staff: Mentor

No. All CO2 was absorbed. What other gas was present in the mixture? Reread the question.

Last edited by a moderator: Aug 13, 2013
15. Aug 9, 2010

### Grhymn

I see, so the gass at the end is the excess O2, this would mean that the volume CO2 after the first reaction = 20mL

Molfraction=volumefraction for ideal gasses so if I am not mistaken :

20mL/X=2*VolumeH20/Y=10mL/1=VolumeO2/(X+Y/4)

But I still don't get the formular out of this.

16. Aug 9, 2010

### Staff: Mentor

From ideal gas ratio of volumes is identical to ratio of moles, isnt't it? 10 mL of gas gave 20 mL of carbon dixode, how many carbon atoms per mole?

Edit: not sure what to do with hydrogen, as far as I can tell there is no enough information. Or I have a senior moment :grumpy:

Last edited by a moderator: Aug 13, 2013
17. Aug 9, 2010

### Grhymn

20mL/X=10mL so 2

18. Aug 9, 2010

### Staff: Mentor

Correct. And as far as I can tell that's all that can be said.

Unless I am missing something.

Have you posted whole question?

Last edited by a moderator: Aug 13, 2013
19. Aug 9, 2010

### Grhymn

I also think there is missing something, but that's the whole question.

Thank you for the help anyway