I'm so useless in chemistry?

[Luwayla]

If 50cm^3 of methane is burned with 200cm^3 of oxygen, what volume of gas remains when cooled to 20 degrees celsius.

CH4 + 2O2 -----> CO2 + 2H20

A 50 cm^3
B 150 cm^3
C 200 cm^3
D 250 cm^3

[ziggle]

You're not useless. It is all just practice and effort.

A mole of any gas at a given temperature and pressure will occupy the same volume. So 50 cm³ of CO2 has the same number of moles as 50 cm^3 of CH4.

The reaction is limited by the CH4. You burn two moles of O2 for every mole of CH4. You do not have enough CH4 to burn all the O2. 50 cm^3 of CH4 will burn up 100 cm^3 of O2.

From your equation, for each mole of CH4 you burn you will get one mole of CO2. So the volume of CH4 must equal the volume of CO2. So the answer is 50 cm³+100 cm² of O2, which is 150 cm³.

The answer is (B).

[gladys]

1 mole of CH4 reacts with 2 moles of O2 to produce 1 mole of CO2 and 2 moles of water.

Now for gases at the same temperature, 1 mole occupies the same volume so this is quite easy.

50cm^3 of CH4 will react with 100 cm^2 of the oxygen to produce 50cm^2 of CO2. The water produced will be a liquid at 20C so you can ignore it.

After the reaction we will have 50cm^3 of CO2 plus what O2 is left over that was not needed for the reaction.

We started with 200cm^3 of O2 and used 100 so there is 100cm^3 left. We have produced 50cm^3 of CO2

Total volume of gas left = 100 + 50 = 150cm^3

so the answer is B