3.2 - Amount Of Substance In Relation To Masses Of Pure Substances

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What is Avogadro’s constant?

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1

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.

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2

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

mass = Mr × moles

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3

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

  • mass = Mr × moles

  • Mr = 100

  • 100 × 20 = 2000 g

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4

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

  • moles = mass ÷ Mr

  • 0.32 ÷ 44 = 0.007

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5

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 tons of ammonia. Relative atomic masses (Ar): 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

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6

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.

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7

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

  • 2H2O2 (l) → 2H2O + O2 (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 (Mr of O2) = 19.2

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