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.