F6 Al Chem question

2007-11-01 2:35 am

12 cm3 of a gaseous hydrocarbon CxHy were sparked with excess oxygen and a contraction of 24 cm3 was observed.
A further contraction of 24 cm3 occurred on absorption with sodium hydroxide solution.
If all volumes were measured under room conditions, what is the molecular formula of the hydrocarbon?
( Hint: CxHy(g) + (x + y/4) O2(g) → xCO2(g) + y/2 H2O(l)
Use the relation mole ratio of gas = volume ratio )
呢條問題有兩次氣體體積收縮，第一次收縮我諗極都諗唔明係咩黎。

回答 (1)

魏王將張遼

2007-11-01 5:50 am

✔ 最佳答案

First of all, we may focus on the second contraction which is due to the absorption of CO2 by NaOH.
From the given, we can see that 12 cm3 of the hydrocarbon produced 24 cm3 of CO2 and by the Avogadro's law, we can see that 1 mole of the hydrocarbon produced 2 moles of CO2.
Therefore x = 2.
Back to the first contraction, by observing the mole ratio of gaseous reactants and product from the given equation, we have:
1 + (x + y/4) - x = 2
Explanation: Since gaseous reactants have a total volume of 24 cm3 more than gaseous product, also every 12 cm3 of gaseous substance correspond to a mole ratio of 1.
So we have:
1 + y/4 = 2
y/4 = 1
y = 4
Therefore, the molecular forumla of the hydrocarbon is C2H4.

參考: My chemical knowledge

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