When 3.4 g of an unknown substance burns in an atmosphere of oxygen, 5.4 g of water and 2.8 g

| February 27, 2021 | education

When 3.4 g of an unknown substance burns in an atmosphere of oxygen, 5.4 g of water and 2.8 g of nitrogen are formed, the density of which for hydrogen is 8.5. Determine the formula of the unknown substance.

The reaction equation is:
X + O2 → N2 ↑ + H2O.
Substance X contains atoms N, H and possibly O.
Let’s find the amount of C, H and O, for this we count the amounts of N2 and H2O:
ν (N2) = m (N2) / M (N2) = 2.8 / (14 * 2) = 0.1 mol.
ν (H2O) = m (H2O) / M (H2O) = 5.4 / 18 = 0.3 mol.
There are 2 N atoms in 1 N2 molecule (ratio 1: 2) ⇒ In 0.1 mol N2 0.2 mol N atoms.
In 1 H2O molecule there are 2 H atoms (ratio 1: 2) ⇒ In 0.3 mol H2O 0.6 mol of H atoms.
Let’s find the amount of O in the original substance:
m (substance) = m (C) + m (H) + m (O) ⇒ m (O) = m (substance) – m (C) – m (H).
m (N) = ν * M = 0.2 * 14 = 2.8 g.
m (H) = ν * M = 0.6 * 1 = 0.6 g.
m (O) = m (X) – m (C) – m (H).
m (O) = 3.4 – 2.8 – 0.6 = 0.
This means that there are no oxygen atoms in the substance.
The ratio of H and N atoms in the formula of a substance: n (N): n (H) = 0.2: 0.6 = 1: 3.
We get a simple formula N1H3.
D (X / H2) = M (X) / M (H2).
M (H2) = 1 * 2 = 2 g / mol.
M (X) = D (X / H2) * M (H2) = 8.5 * 2 = 17 g / mol
M (N1H3) = 14 * 1 + 3 * 1 + 16 * 14 = 17 g / mol.
The ratio M (X) / M (N1H3) = 17/17 = 1, the molecular formula X is the formula of the substance.
Answer: The formula for NH3 is ammonia.



One of the components of a person's success in our time is receiving modern high-quality education, mastering the knowledge, skills and abilities necessary for life in society. A person today needs to study almost all his life, mastering everything new and new, acquiring the necessary professional qualities.