# Chemistry C2 3.4 Equations and calculations.

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What do balanced symbol equations tell us?
The number of moles of substances involved in a chemical reaction.
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What can we use balanced symbol equations to calculate?
The masses of reactants and products in a chemical reaction.
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What do chemical equations show us?
The reactants and products of a reaction.
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When a chemical equation is balanced, what does it show?
The amounts of atoms, molecules or ions in the reaction, e.g. 2Mg+O_2→2MgO shows that two atoms of magnesium react with one molecule of oxygen to form two magnesium ions and two oxide ions.
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If we work in moles on the equation from the previous card, what does the equation tell us?
Two moles of magnesium atoms react with one mole of oxygen molecules to produce two moles of magnesium oxide. Meaning that 48g of magnesium react with 32g of oxygen to give 80g of magnesium oxide (A of Mg=24, A of O=16).
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Alternatively, what if we work in relative masses from this equation?
(2xA of Mg)+(2xA of O)=(2xM of MgO). Converting this to g it becomes 2x24g of Mg+2x16g of O=2x40g of MgO or 48g of Mg+32g of O=80g of MgO (which is the same as when we used moles.
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How can we work out the mass of magnesium oxide produced by 5g of magnesium given previous question calculations?
Using ratios, 1g of Mg will produce 80÷48g of MgO, so 5g of magnesium will produce 5x80÷48g of MgO=8.33g of MgO.
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What if moles are used to do the calculation from the previous question?
1 mole of Mg produces 1 mole of MgO. 5g of Mg=5÷24 of Mg and so it will produce 5÷24 mole of MgO. The mass of 5÷24 mole of MgO=5÷24x40g=8.33g of MgO.
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Calculate the mass of calcium oxide that can be made from 10g of calcium carbonate in the reaction: CaCO_3→CaO+CO_2 (A of Ca=40, A of O=16, A ofC=12).
CaCO_3=100, CaO=56, one mole CaCO_3 gives one mole of CaO or 100g CaCO_3 gives 56g CaO, 10g CaCO_3 gives (10÷100)x56=5.6g.
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**YOU CAN WORK IN MOLES OR USE RELATIVE MASSES WHEN DOING CALCULATIONS BUT WILL FAIL IF YOU DO NOT GIVE THE CORRECT UNITS IN YOUR ANSWER**
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## Other cards in this set

### Card 2

#### Front

What can we use balanced symbol equations to calculate?

#### Back

The masses of reactants and products in a chemical reaction.

### Card 3

#### Front

What do chemical equations show us?

### Card 4

#### Front

When a chemical equation is balanced, what does it show?

### Card 5

#### Front

If we work in moles on the equation from the previous card, what does the equation tell us?