What is the formula for X?

Excess water and 0.144 g of aluminum compound X were reacted to form a gas. The gas burns completely in O2 to form H2O and 72cm3 of CO2 only. The volume of CO2 was measured at room temperature and pressure.
What is the formula for X?
A Al2C3
B Al3C4
C Al5C3
D Al4C3

What is the method to solve this type of questions?

1 Like


I didn’t find Al5C3, could someone tell me where i went wrong? or what the calcultaion here is? thank you!!

looking at the combustion of the gas:
CxHy + O2 ---- CO2 + H2O
the lowest values of x and y possible are:
C1H4 + 2O2 ---- CO2 + 2H2O
so methane is a possible gas formed in the first step

AlxCy + H2O ---- Al(OH)3 + CH4
when balancing the equation i get:
Al4Cl3 + 12H2O ---- 4Al(OH)3 + 3CH4



I got D as the correct answer too!

My method was:

  1. Calculating the moles of CO2 produced
  2. Find moles of C = moles of CO2
  3. Find mass of C: moles * 12 = 0.036g
  4. Same grams must be in original compound, so mass of compound - mass of C = mass of aluminium
  5. Find the relative proportion of Al to C by finding their moles (mass / RAM) = 4 : 3