10.03.04 Moles and mass in reactions

Syllabus

The masses of reactants and products can be calculated from balanced symbol equations.
Chemical equations can be interpreted in terms of moles. For example:

Mg + 2HCI --> MgCI₂ + H₂

shows that one mole of magnesium reacts with two moles of Hydrochloric acid to produce one mole of Magnesium Chloride and one mole of Hydrogen gas.

calculate the masses of substances shown in a balanced symbol equation
calculate the masses of reactants and products from the balanced symbol equation and the mass of a given reactant or product.

We know from the equation

C_(s) + O_2(g) --> CO_{2 (g)}

that 1 mole of Carbon reacts with 1 mole of Oxygen to make 1 mole of CO₂.

And we also know that the Molar Mass (mass of one mole) of Carbon = A_r (Carbon) - the Mass Number from the Periodic Table = 12

And that the Molar Mass (mass of one mole) of O₂

= 2 x A_r (O) = 2 x 16 = 32.

And that the Molar Mass (mass of one mole) of CO₂

= 1 x A_r (C) + 2 x A_r (O) = 12 + (2 x 16) = 44.

So, we can say 12g of Carbon will need exactly 32g of Oxygen to burn and will make 44g of Carbon Dioxide

But how will we know how much Oxygen we need if we want to burn more (or less) than 12 g of Carbon?

We must work out how many moles of Carbon we have to burn using the equation:

Moles = Mass ÷ Molar Mass

If we wish to burn 18 g of Carbon:

Moles of Carbon = Mass Carbon ÷ Molar Mass (C)

= 18 ÷ 12 = 1.5 moles of Carbon

1 mole of Carbon needs 1 mole of O₂ so 1.5 moles of Carbon will need 1.5 moles of O₂

Moles of O₂= Mass O₂÷ Molar Mass (O₂)

Mass of O₂needed= Moles of O₂neededX Molar Mass (O₂)

= 1.5 x 32 = 48 g

Burning 1.5 moles of Carbon will also make 1.5 moles of CO₂. How much is this?

Moles of CO₂ = Mass CO₂ ÷ Molar Mass (CO₂)

Mass of CO₂= Moles of CO₂X Molar Mass (CO₂) M_r (CO₂) = A_r (C) + 2A_r (O) = 12 + 32 = 44 g/mol

= 1.5 x 44 = 66 g

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