# Calculations involving masses

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Calculate relative formula mass given relative atomic masses.
Relative formula mass (Mr) of a compound: sum of the relative atomic masses of the atoms in the numbers shown in the formula. In a balanced chemical equation: sum of Mr of reactants in quantities shown = sum of Mr of products in quantities shown
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Calculate the formulae of simple compounds from reacting masses.
1. divide mass by formula mass to find moles 2. divide by the smaller amount of moles to get a ratio 3.use the ratio to form the simplified empirical formula
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Deduce: the empirical formula of a compound from the formula of its molecule
If it is a common multiple then the empirical formula would be the simplest whole number ratio.
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Work out the the molecular formula of a compound when given the empirical formula and its relative molecular mass.
1.Find relative molecular mass of the empirical formula 2.Divide relative molecular mass of compound by that of the empirical formula
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Describe an experiment to determine the empirical formula of a simple compound such as magnesium oxide.
Heat magnesium to form magnesium oxide (oxygen in the air). mass of the magnesium oxide - mass of the empty crucible.  mass oxygen = mass magnesium oxide – mass magnesium. Divide masses by ram and simplify the ratio.
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What is the law of conservation of mass?
No atoms are lost or made during a chemical reaction so the mass of the products = mass of the reactants
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Explain the law of conservation of mass applied to a precipitation reaction in a closed flask.
Precipitate that forms is insoluble and is a solid, as all the reactants and products remain in the sealed reaction container then it is easy to show that the total mass is unchanged.
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Explain the law of conservation of mass applied to a non-enclosed system including a reaction in an open flask that takes in or gives out a gas
Does not hold for a reaction in an open flask that takes in or gives out a gas, since mass will change from what it was at the start of the reaction because gas can escape from the beaker.
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Calculate masses of reactants and products from balanced equations, given the mass of one substance
1. moles = mass ÷ molar mass. 2. Mass = moles x molar mass to find mass.
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Calculate the concentration of solutions in g dm-3.
Concentration of a solution can be measured in mass per given volume of solution. To calculate mass of solute in a given volume of a known concentration do mass = concentration x vol.
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Explain why, in a reaction, the mass of product formed is controlled by the mass of the reactant which is not in excess.
The reactant that is not in excess is called the limiting reactant. If a limiting reactant is used, there will be less product since there are less reactants.
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## Other cards in this set

### Card 2

#### Front

Calculate the formulae of simple compounds from reacting masses.

#### Back

1. divide mass by formula mass to find moles 2. divide by the smaller amount of moles to get a ratio 3.use the ratio to form the simplified empirical formula

### Card 3

#### Front

Deduce: the empirical formula of a compound from the formula of its molecule

### Card 4

#### Front

Work out the the molecular formula of a compound when given the empirical formula and its relative molecular mass.

### Card 5

#### Front

Describe an experiment to determine the empirical formula of a simple compound such as magnesium oxide.