計molecular formula ....唔該＝〕

2008-06-26 5:23 am

20cm^3 of a gaseous hydrocarbon and 150cm^3 oxygen (which was in excess ) were exploded in a closed vessel . After cooling , 110cm^3 of gases remained . After passing the resulting gaseous mixture through concentrated sodium hydroxide solution , the volume of the residual gas became 50cm^3 . Determine the molecular formula of the hydrocarbon .

我係本F.6既書搵到的...
雖然係F.4既數....but 我計唔到...我太廢了>.<
希望大家幫幫手=]

回答 (1)

Uncle Michael

2008-06-26 8:09 am

✔ 最佳答案

Let CxHy be the molecular formula of the hydrocarbon.

CxHy (x + y/4)O2 → xCO2 + (y/2)H2O
Avogadro's law: Under the same temperature and pressure, the number of moles of a gas is directly proportional to its volume. Therefore:

Vol. of CxHy used = 20 cm3
Vol. of O2 used = 20(x y/4) cm3
Vol. of CO2 formed = 20x cm3
Vol. of H2O formed ≈ 0 cm3 (liquid)

Data from the question:
Vol. of CxHy used = 20 cm3
Vol. of O2 added = 150 cm3
Vol. of CO2 formed and O2 unreacted= 110 cm3
Vol. of O2 unreacted = 50 cm3

Consider the volume of carbon dioxide formed:
20x = 110 - 50
Hence, x = 3

Consider the volume of oxygen used (reacted):
20(x + y/4) = 150 - 50
20(3 + y/4) = 100
Hence, y = 8

Ans: The molecular formula = C3H8
=

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