Stoichiometry Question

[Wizard]

This is a question I got for Holiday Homework. I suspect that it is from Neap, so if anyone has the book already, providing an answer to this question would be much appreciated It is question 1.1.21 in the Stoichiometry and Gravimetric Analysis Section (1.1)

Here is the question:

A 1.60g sample of an organic compound was burnt in excess Oxygen. 0.96g of water and 2.40g of Carbon Dioxide were produced. In a separate experiment, a 2.58g of the organic compound was vapourised. The vapour occupied 1.15L at 130 degrees Celcius (403 Kelvin) and 125 KPa.

Determine the Molecular formula of the compound. I got ethene (C₂H₄), but probably did something wrong.     

Thanks

[unknown id]

Hmm, I got C₂H₄O₂ after a fair bit of calculation

I can see how you derived C₂H₄ but remember that the molar mass of the compound has to be ~60 g/mol through the equation PV = nRT. The molar mass of ethane is 28 g/mol. Therefore, that leaves 32 g/mol (60 - 28) to be taken by another element present in the compound (most likely, oxygen). This means that the molecular formula of the compund should have 2 oxygen atoms, thus leading to the formula C₂H₄O₂.

 

[Collin Li]

Organic compounds burnt in excess oxygen: combustion

If only water and carbon dioxide is produced, then the organic compound is restricted to only containing C, H and O atoms.



(You must have assumed , i.e.: there is no oxygen)

* 2.40g of carbon dioxide implies there is of carbon that must have come from the organic compound (couldn't have come from the oxygen).

* 0.96g of water implies there is of hydrogen that must have come from the organic compound (also couldn't have come from the oxygen).

* This leaves us with of oxygen in the organic compound.


Now: , , and

So:

Hence, the empirical formula is

The second piece of information tells us that:

And since

Since , then the molecular formula must be two times the empirical formula, as 60 is twice as large as 30.

Therefore, the organic compound is as unknown id calculated.

[Wizard]

lol, I thought it was a hydrocarbon, so I left out the O from the formula. Thanks!

[Collin Li]

Ah.

Hydrocarbons: C, H
Carbohydrates: C, H, O
Proteins: C, H, N, O (and possibly others)
Organic compounds: anything, but must contain C

In VCE, you only need to know the results of molecular combustion involving molecules with C, H, O, though. (For curiosity, all the N atoms will become nitrogen dioxide, if I remember correctly).

[Wizard]

Another Stoich Question

The mass of Iron obtained by the complete reduction of 1kg of haematite(Fe₂O₃) if the ore is 90% pure will be:
a) 350 g
b) 700 g
c) 777 g
d) 389 g

I got 629.5 g. But the answers say C. Could someone tell me if I am right or wrong? Thanks

[Collin Li]

of pure

This means there is:

Yeah, you're right.