1. 10cm3 of a gaseous hydrocarbon was exploded with an excess of
oxygen. There was a contraction of 30cm3. A
further contraction of 30cm3 took place when the residual gases
were subjected to aq. NaOH. All volumes were measured at r.t.p.
Calculate the molecular formula of the hydrocarbon.
Is the first contraction due to O2 being reacted away? Anyway, I
know the no. of C atoms is 3, but I can't find the no. of H atoms,
got 0 H atoms. LOL. Is the 30cm3 in the 2nd line wrongly stated.
The answer given is C3H8.
2. 10cm3 of a hydrocarbon, C4Hy, were allowed to react with an
excess of O2 at 150deg cel and 1 atm. There was an expansion of
15cm3. Deduce the value of y.
What is the expansion due to?
Sorry, haven't studied the part on eudiometry but I do have a
rough idea what it's about. Just can't solve some of the slightly
'twisted' questions for now. Thanks once again.
These questions are vague and badly iterated (it's common to
encounter such vague, badly iterated questions from schools, and
occasionally even from Cambridge), because they do not
unambiguously state whether the contraction/expansion refers to in
comparsion with reactant gases or product/residual gases.
For instance, "You combust 10 cm3 of hydrocarbon with 10 cm3 of
O2. When the residual gases are cooled to rtp, a contraction of 5
cm3 was observed."
Does the qn mean 5cm3 contraction compared to product/residual
gases? (ie. 5cm3 then refers to only water vapour)
Or
Does the qn mean 5cm3 contraction compared to reactant gases?
(ie. 5cm3 then refers to the difference between the total volume of
reactant O2 + CxHy gases at rtp and the total volume of
product/residual gases CO2 + H2O + unreacted O2, at rtp)
If the question is ambiguous and you interpret it in the former,
you get one answer; if you interpret it in the latter, you get
another answer.
An exam smart student, will give both answers, BUT WITH
QUALIFICATIONS. In other words, explain to the examiner that the
quesiton is ambiguous, show clear working for each case /
interpretation, and how you arrived at the two alternative
answers.
You will score full credit / marks, if you give both answers. If
you only give one answer, you have about 50% chance that your
interpretation is the one expected by the examiner.
Of course, if you do not qualify/explain your alternative
answers, you will get zero marks, because the examiner thinks
you're trying to cheat by giving more than one answer.
So be exam smart, and secure your marks.
(Audi, it's admirable that
you're studying up on your own ahead of your school topics.
But it's not feasible to expect everything to be
explanied / taught clearly to you over an internet forum. If you
still do not understand how to do these questions, or topics
ahead of your school, then either wait for your school to
teach you, or/and attend private tuition
to keep ahead of your school.)