Moles + Mass of Pure Substances

What is Avogadro's constant?

The number of atoms, molecules or ions in a mole of a given substance. The value of the constant is 6.02 x 10²³.

What is the formula that links mass, molecular mass and moles together?

Mass = Mr x Moles

What is the mass of 20 moles of calcium carbonate, CaCO₃?

Mass = Mr x Moles. Mr of CaCO₃ = 100. Mass = 100 x 20 = 2000 g.

Calculate the amount of carbon dioxide in moles in 0.32 g of carbon dioxide. (Ar: C = 12, O = 16)

Moles = Mass / Mr. Mr of CO₂ = 44. Moles = 0.32 / 44 = 0.007 moles.

Nitrogen and hydrogen form ammonia shown by the following equation:
N₂ (g) + 3H₂ (g) ⇌ 2NH₃ (g).
Calculate the mass of nitrogen needed to form 6.8 tonnes of ammonia. (Ar: H = 1; N = 14)

Step 1 - Work out the number of number of moles of ammonia (Mr of ammonia = 17) 6800000 / 17 = 400000 moles of ammonia

Step 2 - Use the balanced equation and number of moles of ammonia to work out the number of moles of nitrogen. The ratio of nitrogen to ammonia is 1:2 Therefore the number of moles of nitrogen is 400000/2 = 200000

Step 3 - Work out the mass of nitrogen (Mr of N₂ is 28) 200000 x 28 = 5600000 g = 5.6 tonnes.

State what we mean by a limiting reactant in a chemical reaction.

The reactant that is completely used up in a reaction, which limits the amount of product that can be formed.

Hydrogen peroxide decomposes to form water and oxygen (2H₂O₂ (l) ⟶ 2H₂O (l) + O₂ (g)). How many grams of oxygen gas will be given off from 40.8 g of hydrogen peroxide?

Step 1: Write the balanced equation 2H₂O₂ (l) → 2H₂O + O₂ (g) Mr of H₂O₂ = 34

Step 2: Number of moles in 40.8 g: 40.8/34 = 1.2 moles Ratio in the balanced equation of H₂O₂: O₂ = 2:1

Step 3: Therefore number of moles of O₂ = 0.6 moles

Step 4: Mass of oxygen = 0.6 x 32 (Mr of O₂) = 19.2