Question

What are the measurable properties of gases? 

OR 

Explain the following measurable properties of gases in detail: Mass, volume, pressure, temperature and diffusion.

Answer 1

Measurable properties of gases are as follows: 

i. Mass:

• The mass (m) of a gas sample is measure of the quantity of matter it contains.
• It can be measured experimentally.
• The SI unit of mass is kilogram (kg). 1 kg = 10³ g.
• The mass of a gas is related to the number of moles (n) by the expression:

n = \(\frac{mass\,in \,grams}{molar \,mass\,in\, grams}\) = \(\frac{m}M\)

ii. Volume:

• Volume (V) of a sample of gas is the amount of space it occupies.
• It is expressed in terms of different units like Litres (L), millilitres (mL), cubic centimetre (cm³), cubic metre (m³) or decimetre cube (dm³).
• The SI Unit of volume is cubic metre (m³).
• Most commonly used unit to measure the volume of the gas is decimetre cube or litre.

iii. Pressure:

• Pressure (P) is defined as force per unit area.

Pressure = \(\frac{Force}{area}\) = \(\frac{f}{a}\)

• Pressure of gas is measured with ‘manometer’ and atmospheric pressure is measured by ‘barometer’.
• The SI unit of pressure is pascal (Pa) or Newton per metre square (N m^-2).

iv. Temperature:

• It is the property of an object that determines direction in which energy will flow when that object is in contact with another object.
• In scientific measurements, temperature (T) is measured either on the Celsius scale (°C) or the Kelvin scale (K).
• The SI unit of temperature of a gas is Kelvin (K).
• The Celsius and Kelvin scales are related by the expression: T(K) = t °C + 273.15

v. Density: 

Density (d) of a substance is the mass per unit volume.

d = \(\frac{m}V\)

∴ The SI unit of density is kg m^-3 .

In the case of gases, relative density is measured with respect to hydrogen gas and is called vapour density.

∴ Vapour density = \(\frac{Molar \,mass}{2}\)

vi. Diffusion:

a. Diffusion is the process of mixing two or more gases to form a homogeneous mixture.

b. The volume of gas diffused per unit time is the rate of diffusion of that gas.

Rate of diffusion of a gas = \(\frac{volume\,of\,a\,gas\,diffusion}{Time\,required\,for\,diffusion}\)

c. SI Unit for rate of diffusion is dm³ s^-1 or cm³ s^-1 .