When 7.5 g of the substance was burned, 5.6 liters of carbon dioxide and 4.5 g of water were formed.

When 7.5 g of the substance was burned, 5.6 liters of carbon dioxide and 4.5 g of water were formed. The hydrogen density of the substance is 15. What is this compound?

Given:
m (substance) = 7.5 g
V (CO2) = 5.6 l
m (H2O) = 4.5 g
D (H2) = 15
Find: the formula of a substance
Solution.
In combustion reactions, according to the law of conservation of mass, the same amount of hydrogen and carbon is formed as was in the original compound. Oxygen atoms in the composition of the products can be both from the burned-out compound and from the oxygen that has reacted, therefore, all calculations will be made for carbon and hydrogen.
n (CO2) = V / Vm = 5.6 / 22.4 mol = 0.25 mol.
n (C) = n (CO2) = 0.25 mol.
n (H2O) = m / M = 4.5 / 18 mol = 0.25 mol.
The amount of hydrogen will be twice the amount of water, since there are twice as many particles in two water atoms than in one H2O molecule.
n (H) = 2n (H2O) = 2 × 0.25 mol = 0.5 mol.
Now you need to find out if there was oxygen in the original substance. To do this, we find the masses of carbon and hydrogen, and then subtract them from the mass of the burnt substance.
m (H) = Mn = 0.5 × 1 g = 0.5 g.
m (C) = Mn = 12 × 0.25 g = 3 g.
m (O) = m (substances) – m (C) – m (H) = 7.5 – 3 – 0.5 g = 4 g.
n (O) = m / M = 4/16 mol = 0.25 mol.
Let’s write down the ratio of quantities and divide all numbers by the smallest (0.25).
n (C): n (H): n (O) = 0.25: 0.5: 0.25 = 1: 2: 1.
The simplest formula CH2O corresponds to the general formula for aldehydes and ketones, CnH2nO. Therefore, we will assume that this substance is formaldehyde HCHO.
You can check the assumption using the hydrogen density of the substance given in the condition.
D = M (substances) / M (H2).
M (substances) = D × M (H2) = 15 × 2 g / mol = 30 g / mol.
M (HCHO) = 30 g / mol. Therefore, our assumption is correct.