A 1.00g sample of D was burned in an excess of oxygen, and the
gases that were produced were first passed through a U-tube
containing phosphorus pentoxide ( to absorb the water vapour) and
then bubbled through concentrated NaOH(aq) (to absorb carbon
dioxide). The phosphorus pentoxide U-tube increased in mass by
1.55g, and the NaOH(aq) bottle increased in mass by 3.03g
(i) Calculate the number of moles of water, and of carbon
dioxide produced. ( Ans: 0.086mol)
(ii) Use these data to calculate the H : C ratio in alkane D,
and hence suggest its molecular formula.(2:5, C4H10)
For part (i), I got the no. of mole of water to be 0.08611 mol
and carbon dioxide to be 0.06886 mol. Is the answer wrong?
& How do I get the answer to part (ii)?
Thanks in advance!
Your answer is correct, mole of water 0.08611 mol and
carbon dioxide 0.06886 mol gives you a 2:5 for C:H ratio.
As for the molecular formula, the question is erroneous in
giving the sample mass as 1g.
Notice that you have two unknown variables : number of
moles of hydrocabon present; and n, where molecular formula =
[empirical formula]n.
Knowing only the sample mass of the molecular
formula without being given EITHER the molar mass of
the molecular formula OR number of moles present, is
insufficient.
Knowing only the empirical formula and the molar mass of
the empirical formula, without being given EITHER the
molecular formula OR the molar mass of the molecular formula,
is insufficient.
The question could be solved if either the molar mass of the
molecular formula is given, or the number of moles present given.
Since neither is given, the question is incomplete and flawed.
Furthermore, the answer suggested for the molecular formula, C4H10,
does not fit with the data provided by the question, that
the sample mass of 1g.
The question is flawed, and have wasted your time (and everyone
else's, who attempted the question).
The only other possible way to obtain C4H10, in
disregarding the deceitful red herring of 1g sample mass, is to
simply consider that the only alkane that you can construct with an
empirical formula of C2H5, is C4H10. Any other multiple (eg. n = 3,
4, 5, etc) will not allow you to obtain a alkane.