Mole – EP – 2

Molar volume is the volume occupied by one mole of any gas at standard temperature and pressure, which equals 22.4 litres. A mole represents a specific count of particles, exactly 6.02 into 10 to the power of 23 atoms or molecules, and its weight in grams is the molar mass of that substance.

Standard temperature and pressure

STP refers to a set of fixed conditions used to measure gases. Standard temperature is 0 degrees Celsius, which is the point where water turns into ice. Standard pressure is 1 atmospheric pressure, which is the normal pressure the air exerts around us. At this standard temperature and pressure, exactly one mole of any gas occupies 22.4 litres of space.

Finding molar mass using molar volume

• Avogadro showed that equal volumes of different gases contain the same number of particles.
• Enclosing 22.4 litres of a gas at standard temperature and pressure gives exactly one mole of that gas.
• Measuring the weight of this 22.4 litres of gas gives the molar mass of that element.
• One mole of any gas measured this way contains exactly 6.02 into 10 to the power of 23 particles.

Mole ratios in chemical reactions

Chemical equations use numbers in front of formulas to show the exact mole ratios needed for a reaction. For example, two moles of hydrogen molecules combine with one mole of oxygen molecules to produce two moles of water. If the ratio is unbalanced, some molecules are left over without reacting and cannot form the full product. Knowing molar mass allows a scientist to weigh out exact mole amounts in grams to react substances completely.

Example of mole calculations

• One mole of a substance equals its molar mass in grams.
• If an element has a molar mass of 294 grams, one mole of it weighs exactly 294 grams.
• If another element has a molar mass of 0.5 grams, 25 moles of it weigh 12.5 grams.
• Combining one mole of the first substance with 25 moles of the second produces a compound with a formula ratio of 1 to 25.