3.2 - Amount Of Substance In Relation To Masses 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 × 10²³.

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

mass = M_r × moles

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

• mass = M_r × moles

• M_r = 100

• 100 × 20 = 2000 g

Calculate the amount of carbon dioxide in moles in 0.32 g of carbon dioxide. Relative atomic masses (A_r): carbon = 12 || oxygen = 16

• moles = mass ÷ M_r

• 0.32 ÷ 44 = 0.007

Nitrogen and hydrogen form ammonia shown by the following equation:

• N_{2 (g)} + 3H_{2 (g)} ⇌ 2NH_{3 (g)}

Calculate the mass of nitrogen needed to form 6.8 tons of ammonia. Relative atomic masses (A_r): H = 1 || N = 14

• 6800000 ÷ 17 = 400000 moles of ammonia

• The ratio of nitrogen to ammonia is 1:2

• 200000 × 28 = 5600000 g = 5.6 tons

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

• In a chemical reaction involving two reactants, it’s common to use an excess of one of the reactants to ensure that all of the other reactant is used.

• The reactant that’s completely used up is called the limiting reactant because it limits the amount of products.

Hydrogen peroxide decomposes in water and oxygen. How many grams of oxygen gas will be given off from 40.8 g of hydrogen peroxide?

• 2H₂O_{2 (l)} → 2H₂O + O_{2 (g)} || M of HO = 34

• Number of moles in 40.8 g: 40.8 ÷ 34 = 1.2 moles

• Ratio in the balanced equation of H₂O₂ : O₂ = 2:1

• Therefore number of moles of = 0.6 moles

• Mass of oxygen = 0.6 × 32 (M_r of O₂) = 19.2