Stoichiometry Question

[wildareal]

In order to determine the molecular formula of a compound known to contain only carbon and hydrogen, two experiments were carried out. In the first experiment, a 1.122 g sample of the compound was burnt in excess oxygen. When the gases evolved were passed through anhydrous CaCl2, its mass increased by 1.442 g. The remaining gases, when bubbled through a NaOH solution, increased its mass by 3.521 g. In the second experiment a 2.000 g sample of the compound was vaporized. The vapour occupied a volume of 797 mL at STP. Calculate the empirical formula and hence the molecular formula of the compound.

Thanks.

[herzy]

mmk don't have a calc on me, but i believe the H20 formed will attach to the anhydrous CaCl2, so the mass increase is due to water. divide this difference by 18, times by 2 and you'll have your mol of H. CO2 will be responsible for the mass increase (as it forms HCO3 acid), so again divide by 44 and you'll have your mol of C. get your masses of H and C, then add together. 1.122 - (m(H) + m(C)) gives m(O). from here you can get your empirical formula

you can get the total mol from the STP value, which will give you your molecular formula

[dptjandra]

Agree with herzy.  Looks like a play on Liebig's method for estimation of carbon and hydrogen to me.  First mass increase is due to H2O, second increase is due to CO2.  Did it lead to the right answer?

[herzy]

why thankyou dptjandra - nice to see you today too!

[dptjandra]

*like*

had seen your posts before but didn't realise it was you...the "i have a girlfriend in biomed" should really have given it away!

[herzy]

lols