chem q

https://imgur.com/mOkybwy

i have no idea how to go about this q

Avogadro's law. The volume of gas is proportional to the number of moles.

You are told that there is 10cm³ of the hydrocarbon, therefore 10x cm3 of carbon dioxide must be formed.

Carbon dioxide is absorbed by KOH(aq) and disappears. You now know the volume of carbon dioxide and hence 'x'.

Everything is measured at RTP so what is the state of water?

So, if the volume contracts by 20cm³ on reaction and you now the inital volume of the hydrocarbon and the volume of carbon dioxide you can now work out the total volume of oxygen.

im a bit confused because surely if KOH is added and the carbon dioxide goes away, then you are left with 0 volume of co2??

If you shake 40cm³ of carbon dioxide with KOH(aq) the result is zero volume, correct. So if you measure the reduction in volume you know how much carbon dioxide there WAS.

so 40cm3 so theres a 1:4 ratio for the hydrocarbon and carbon dioxide respectively. im just finding trouble with finding oxygen volu

total initial volume is made up of hydrocarbon + oxygen (let = A) = 10 + A
total final volume is made up of excess oxygen(let = B) and carbon dioxide = 40 + B
------------------------------------------------------------------------------------------------- subtract gives 20 as the initial volume is 20cm³ more than the final volume
(10 + A) - (40 + B) = 20

collect terms

(A-B) = 50

and (A-B) is the oxygen burned, this allows you to find the water formed and hence the formula of the hydrocarbon.

wait how did u get 20

read the question

yeah that makes sense, but now that i look back at the question, its puzzled me again because if youve started with 10cm3 of the hydrocarbon and it expands by 20, that means youve made 30cm3 of co2? and then this co2 reacts with koh to make gases that are worth 70cm3 of volume. so surely the amount of co2 is 30???

It contracts by 20cm³