In the combustion process using excess oxygen, each mole of methane results to 1 mole of co2 while ethane produces 2 moles of Co2. Under same conditions, these can be translated to volume. Hence the total volume absorbed is 10 cm3 + 20 cm3 = 30 cm3.
Water has hydrogen bonds between Hydrogen atoms (that are slightly positive in molecules of water ) and Oxygen atoms (that are slightly negative in molecules of water), so it is necessary more energy to break them down and move water molecules from liquid state to gas.
Answer is: elements are always combined in the same proportion by mass.
Law of multiple proportions or Dalton's Law said that the ratios of the masses of the second element which combine with a fixed mass of the first element will be ratios of small whole numbers.
For example, nitrogen(I) oxide N₂O; m(N) : m(O) = 2·14 : 16 = 7 : 4.
Another example, water (H₂O) is made of two hydrogen atoms and one oxygen atom:
m(H) : m(O) = 2·1 : 16 = 1: 8.
