SSLC, Chemistry Chapter 2 GAS LAWS AND MOLE CONCEPT notes

Volume of a Gas

Volume of a gas is the volume of the container which it occupies. We know the gas has no definite volume. If a gas in a container of volume 1 liter is transferred to a container of volume 10 liters, the gas takes 10 liter volume.

Pressure of a Gas

A gas in a closed container exerts a force in all direction inside the container. The pressure exerted by the gas can be calculated by dividing the total force exerted on the surface by surface area. That is force exerted per unit area is called pressure.

Temperature

Temperature is the average kinetic energy of molecules in a substance. When a substance is heated, the kinetic energy of molecules increases. As a result the average kinetic energy also increases and hence temperature.

Gas laws

Boyle’s law

At constant temperature, volume of a definite mass of gas is inversely proportional to its pressure. If P is the pressure and V the volume,

then P*V is a constant.

You might have observed that the size of the air bubbles rising from the bottom of an aquarium increases. Explain the reason?

Ans: As the bubbles rise up the pressure on the bubbles decreases and as a result the volume increases

Charles law

At constant pressure, volume of a definite mass of gas is directly proportional to temperature in Kelvin scale. If V is the volume and T the temperature,

then V/T is a constant.

Avogadro’s Law

At constant temperature and pressure the volume of a gas is directly proportional to the number of molecules.

Practice questions

A definite mass of gas at constant temperature is taken in a cylinder having an airtight piston. The following table shows the variation in pressure and volume. Complete the table.

Which gas law is used to fill the table?

A definite mass of gas at constant pressure is taken in a cylinder having an airtight piston. The following table shows the variation in temperature and volume. Complete the table.

Which gas law is used to fill the table?

Relative atomic mass

The atomic mass of elements are expressed by considering 1/12 mass of an atom of carbon-12 as one unit.

Atomic mass of an element is expressed in atomic mass unit (amu). One atomic mass unit is 1/12 mass of an atom of carbon-12.

For example: atomic mass of Oxygen is 16amu. This means mass of one atom of Oxygen is 16 times 1/12 mass of an atom of carbon-12.

When average atomic masses of elements are calculated taking in to account the different isotopes of elements, the atomic mass of elements may have fractional values. For practical purposes and calculations most of these values are taken as whole numbers.

Gram atomic mass

The mass of an element in grams equal to its atomic mass is called 1 Gram Atomic Mass(GAM). This may also be shortened as 1 Gram Atom.

for example: 1 GAM of Oxygen = 16g Oxygen

1 GAM of Carbon = 12g Carbon

1 GAM of Nitrogen = 14g Nitrogen

1 GAM of Sulphur = 32g Sulphur

1 GAM of Hydrogen = 1g Hydrogen

Avogadro number