## TrojanPoem one year ago Question : Help please ! 0.4122 grams of an organic compound was burnt totally , so the weight of the containers of absorbing water vapor and carbon dioxide were increased by 0.3618 grams , 0.762 grams respectively. 1- Find the percentage of carbon and hydrogen in the compound. 2- Is there any other element in the organic compound ? , Giving a reason.

1. phi

How far did you get ?

2. Michele_Laino

from your data, we can see that the ratio: water vapor and carbon dioxide =0.3618/ 0.762= 1/2 (approximated value)

3. TrojanPoem

What confuses me is that Organic compound -(burnt)-> nH2o + mCo2 so the sum of their masses must be equal or more to the compound itself but it is more.

4. TrojanPoem

I meant equal or less if equal -> Only H , C if less H, C, other elements

5. phi

I assume the O comes from the air, and the organic compound does not contain O ?

6. TrojanPoem

Maybe and maybe it was C2H5OH or something else with O

7. TrojanPoem

But I think you are right Phi, the O comes from the air as 0.3618 + 0.762 (not equal to) 0.4122

8. TrojanPoem

if so the mass of (O) = 0.3618 + 0.762 - 0.4122 = 0.7116 grams

9. phi

0.4122 grams of an organic compound was burnt totally I would like to know the weight of any solid ash. but you have (CH other stuff) + O= H2O + CO2 (+ ash?) so the total weight of H2O + CO2 - compound is the weight of added O ?

10. TrojanPoem

Yeah.

11. TrojanPoem

12. phi

my original thought was 2/18 of the H2O represents the H in the compound and 12/44 of the CO2 is the amount of C

13. TrojanPoem

|dw:1435431964444:dw|

14. TrojanPoem

Like 0.21 grams

15. TrojanPoem

so oxygen in Co2 = 0.762 - 0.21 = 0.552

16. TrojanPoem

Hmm phi am I mistaken ?

17. phi

sounds right. But the question is Find the percentage of carbon and hydrogen in the compound.

18. phi

How do they measure percentage? By weight? or fractions of atoms?

19. TrojanPoem

Mass of Carbon ( H only) / Mass of compound (C,H)

20. TrojanPoem

Like normal percentages in maths

21. phi

so %C is 0.21 grams / 0.4122

22. TrojanPoem

Yea , = 50.94 % ( 0.21/0.4122 times 100)

23. phi

now do the H

24. TrojanPoem

|dw:1435432795512:dw|

25. TrojanPoem

wait it's 0.292 so the compound contains other elements

26. phi

27. phi

H + C add up to about 70% of the weight of the compound 30% must be something else.

28. TrojanPoem

So maybe it's oxygen. The reason is : There is other elements because the sum of mass of (H) + mass of (C) is less than the mass of the compound so there is other elements and their mass is Compound - mass (h+c) =

29. phi

30. TrojanPoem

Like ?

31. phi

I'm not a chemist.

32. TrojanPoem

I mean is the reasoning enough ( in your opinion ) ?

33. phi

we know how many grams of H and C were in the original and are now in the CO2 and H2O. If the compound "burned completely", that must be all its C and H (burned completely means every C or H combined with O from the air) We know the C and H account for 70% of the mass of the compound. that leaves 30% of the compound being other elements, including O or whatever.

34. TrojanPoem

Thanks for the help @phi.