Question

At the same conditions of pressure and temperature, ammonia gas is less dense than air. Why is this true?

Short answer

Under the same pressure and temperature conditions, ammonia gas is less dense than air because it has a lower molecular mass (17 atomic mass units) compared to the average molecular mass of air (approximately 21.8 atomic mass units). Consequently, for a given volume, ammonia gas has more, lighter molecules, resulting in a lower overall mass and density.

Step-by-step solution

Understand the concept of density

Density is defined as the mass per unit volume of a substance, represented mathematically as: \(Density = \frac{Mass}{Volume}\) Density depends on factors such as temperature, pressure, and molecular mass.

Determine the molecular mass of ammonia and air

Ammonia gas (NH3) is composed of one nitrogen atom and three hydrogen atoms. To find the molecular mass of ammonia, we need to sum the atomic masses of its constituent atoms: Molecular mass of ammonia (NH3) = (1 Nitrogen atom x 14 atomic mass units) + (3 Hydrogen atoms x 1 atomic mass unit) = 14 + 3 = 17 atomic mass units Air is a mixture of gases primarily composed of nitrogen (N2, approximately 78%) and oxygen (O2, approximately 21%), with trace amounts of other gases. To find the average molecular mass of air, we can use a weighted average based on the percentages of nitrogen and oxygen. Molecular mass of nitrogen gas (N2) = 2 Nitrogen atoms x 14 atomic mass units = 28 atomic mass units Molecular mass of oxygen gas (O2) = 2 Oxygen atoms x 16 atomic mass units = 32 atomic mass units Average molecular mass of air ≈ (0.78 x 28) + (0.21 x 32) ≈ 21.8 atomic mass units

Compare the densities of ammonia and air

Under the same pressure and temperature conditions, if a gas has a lower molecular mass, it tends to have a lower density. Since the molecular mass of ammonia (17 atomic mass units) is less than that of air (approximately 21.8 atomic mass units), ammonia gas will be less dense than air under the same conditions of pressure and temperature. This is because for a given volume, there will be more, lighter molecules in the ammonia gas, leading to a lower overall mass of the substance. Due to its lower molecular mass, the ammonia molecules occupy more space, thus leading to a lower density compared to air.