# Chemistry alevel aqa amount of substance question

Q4.
What is the mole fraction of 1.0 g of a compound of relative molecular mass 100.0 dissolved in 30.0 g of a solvent of relative molecular mass 50.0?
A 6.0 × 10−3
B 1.6 × 10−2
C 1.7 × 10−2
D 3.0 × 10−2
Which compound needs the greatest amount of oxygen for the complete combustion of 1 mol of the compound?
A ethanal
B ethanol
C ethane-1,2-diol D methanol
(edited 1 year ago)
the mole fraction answer is B 1.6 x 10^-2. the mole fraction is the number of moles of the substance you're interested in divided by the total moles of all the substances. moles = mass/ mr
so moles of the compound is 1/100= 0.01
moles of solvent is 30/50= 0.6
total moles 0.01 + 0.6= 0.61
mole fraction is 0.01/0.61= 0.016... = 1.6 x 10^-2

the second answer is B ethanol as it has the highest ratio of carbon atoms to oxygen atoms within it's compound so it requires more oxygen to combine with those carbon atoms and can't utilise the oxygen within it's structure as much
the mole fraction answer is B 1.6 x 10^-2. the mole fraction is the number of moles of the substance you're interested in divided by the total moles of all the substances. moles = mass/ mr
so moles of the compound is 1/100= 0.01
moles of solvent is 30/50= 0.6
total moles 0.01 + 0.6= 0.61
mole fraction is 0.01/0.61= 0.016... = 1.6 x 10^-2
the second answer is B ethanol as it has the highest ratio of carbon atoms to oxygen atoms within it's compound so it requires more oxygen to combine with those carbon atoms and can't utilise the oxygen within it's structure as much

but for the second Q isn't the ratio for ethanal and ethanol the same?
Original post by coconut butter
but for the second Q isn't the ratio for ethanal and ethanol the same?

The hydrogen also needs oxygen for combustion
Original post by coconut butter
but for the second Q isn't the ratio for ethanal and ethanol the same?

For complete combustion, we need to account for the conversion of carbon to carbon dioxide (CO₂) and hydrogen to water (H₂O). Each carbon atom requires two oxygen atom hence the formation of CO₂ from C, and each hydrogen atom requires half an oxygen atom (since oxygen combines with hydrogen to form water in a 2:1 ratio).