What Is the Molecular Formula of the Alkane in This Chemistry Problem?

[daphap]

Eurgh, for some reason I can't get my head around this chemistry homework!

"When 20cm^3 of this alkane were burnt in 200 cm^3 of oxygen (an excess), the residual gases occupied 150cm^3. On shaking with an aqueous potassium hydroxide, this volume decreased to 70 cm^3. (All at rtp and pressure). What is the molecular formula of the alkane."

Oh yeah, and I devised that the empirical formula of th alkane was C2H5.

I am aware I have to show my work to get help... but I really don't know where to start. If someone good guide me through a similar problem with alter numbers I would greaty appreciated. Thanks.

 

[SN1987a]

How did you devise the empirical formula of the alkane was C_2 H_5?

If you're sure this is the right empirical formula, then you're basically done.

You now have to know a bit about alkanes.

An alkane is a chain of single-bonded carbons with enough hydrogens attached to each carbon. Knowing that each carbon needs fours bonds to be "happy", you can find a relationship between the number of carbons in an alkane and the number of hydrogens.

You now use the empirical formula, which also gives you another relationship between the number of carbons and hydrogens present in your alkane. With those two formulas in hand, you can figure out precisely how many carbons and hydrogens a molecule of your alkane is made up of.