[Shabbar]

Hello.
My teacher recently gave us this question and I'm having a bit of a hard time figuring it out. I've tried approaching him twice to no avail. I'd like some help in (iii) and (iv).

Question:

2 scientists collected samples of a flammable gas, A, which was found to be a hydrocarbon.

When 10cm³ of A was mixed at room temperature with 50cm³ of Oxygen (excess), it exploded. 40cm³ of gas remained.
When the 40cm³ of gas was shaken with an excess of aqueous KOH, 30cm³ of gas still remained.

i) What is the identity of the 30cm³ of gas that remained at the end of the experiment?
- Here, we both got oxygen so there's no problem.

ii) The combustion of A produced a gas that reacted with the KOH. What is the identity of the gas?
- Here, we both go CO₂ so again, no problem.

iii) What volume of the gas you've identified in (ii) above was produced by the combustion of A?
- The answer is 10cm³, supposedly. I'm not sure how he got it, however, I believe it has something to do with the general equation:

C_xH_y + (x + y/4)O₂ --> XCO₂ + y/2 H₂O

If 40cm3 of the gas (oxygen) remained then that means that 10cm3 was used up. As to how we would use this, I'm completely blank. Also, this is the first time I've seen this general equation.

(iv) What volume of O₂ was used up in the combustion of A?
- The answer he got is 20cm³ but I got 10cm³. This is because according to the question, 10cm³ were used for the combustion, and 10cm³ were used for the reaction with KOH. Which one is correct here?

Thanks.

[AWK]

KOH reacts only with carbon dioxide.
10 cm³ carbon dioxide means 10 cm³ methane (only possibility; ethane - 2 carbon atoms = 20 cm³ CO₂).
Write down the reaction of combustion CH₄ and calculate correctly the volume of oxygen needed.

[Vidya]

All gases used and formed are at the same temperature and pressure so  you can use volume ratio as mole ratio .
Total volume of O2 you started is 50mL and out of this 30 mL left which is Oxygen ..so how much has reacted ? 20mL
Gases formed are CO2 and water and both are soluble in KOH
Ratio in which A is reacting with O2 is 1:2
In 40 ml of gas formed -30mL is O2 and
It means volume  CO2 and H2O  formed is only 10 mL