Determine the empirical formula for the compound of aluminum with carbon in which the mass fraction of aluminum is 75%

Decision:
1. Let’s designate: x is the molecular weight of the compound, a is the number of aluminum atoms in the compound, b is the number of carbon atoms in the compound.
2. The relative atomic mass of aluminum is 27, carbon is 12.
3. If the mass fraction of aluminum is 75%, then the mass fraction of carbon is 25%.
4. Let’s compose the following expressions:
0.75 = (27 * a) / x;
0.25 = (12 * b) / x;
5. Let’s express the molecular weight:
x = (27 * a) / 0.75 = 36a;
m = (12 * b) / 0.25 = 48b;
Equating expressions: 36a = 48b. Therefore: x = 4, y = 3. The result is a substance – aluminum carbide. Its formula is Al4C3.
Answer: The empirical formula is Al4C3.