[RenaHay]

In an experiment, 10.0cm³ of a gaseous hydrocarbon was exploded with an excess of oxygen, and a contraction of 35.0cm³ occurred. The resultant volume was passed through aqueous sodium hydroxide, and the volume was reduced by another 40.0cm³. Assuming that all volumes were measured at room temperature and pressure, determine the formula of the hydrocarbon.
Answer: (C₄H₁₀)

I only managed to do half, and then I got stuck. Can someone explain what is the contraction about, and what happened? My attempts are as follows:

C_xH_y+ [X+(Y/4)]O₂ XCO₂+(Y/2)H₂O

η_CxHy / η CO₂ = V_CxHy / V_CO2

1/ X = 10/40
X = 4

I'm stuck, I don't known how to find the number of Hydrogen atoms in the hydrocarbon :/

[Borek]

Two gases reacted - unknown and oxygen. After the mixture was cooled down produced water condensed, and the volume was lower than the initial volume of both gases. See how it relates to the reaction equation.