Mole calculations - Higher
A balanced chemical equationA chemical equation written using the symbols and formulae of the reactants and products, so that the number of units of each element present is the same on both sides of the arrow. shows the amounts in moleThe amount of substance that contains the same number of particles as there are atoms in 12 g of carbon-12 (contains the Avogadro's constant 6.0 脳10虏鲁 number of particles). of reactantA substance that reacts together with another substance to form products during a chemical reaction. that react and the amounts of productA substance formed in a chemical reaction. that are made. From these amounts, the massThe amount of matter an object contains. Mass is measured in kilograms (kg) or grams (g). of reactants and products can be calculated.
Amounts from equations
In a balanced equation, the coefficients (the numbers in front of the formulae) show the amounts of the reactants and products. If there is no coefficient, the amount is one mole.
For example, the equation below shows that one mole of zinc reacts with two moles of hydrogen acid to make one mole of zinc chloride and one mole of hydrogen molecules.
Zn(s) + 2HCl(aq) 鈫 ZnCl2(aq)+ H2(g)
Worked example
Use the equation below to calculate the amount in moles of oxygen molecules that reacts with 2 mol of magnesium metal.
2Mg(s) + O2(g) 鈫 2MgO(s)
The equation shows that 2 mol of magnesium metal reacts with 1 mol of oxygen molecules.
Masses in equations
The masses of substances shown in a balanced equation can be calculated using the equation:
mass = relative formula mass 脳 amount
For example, in the equation 2Mg(s) + O2(g) 鈫 2MgO(s) the masses of the substances shown are:
| Magnesium, Mg | 24 脳 2 = 48 g |
| Oxygen, O2 | 2 脳 16 = 32 g |
| Magnesium oxide, MgO | (24 + 16) 脳 2 = 80 g |
| Magnesium, Mg |
| 24 脳 2 = 48 g |
| Oxygen, O2 |
| 2 脳 16 = 32 g |
| Magnesium oxide, MgO |
| (24 + 16) 脳 2 = 80 g |
Calculating masses from equations and given masses
Balanced equations and relative formula mass values can be used to calculate:
- the mass of product made from a given mass of reactant
- the mass of reactant needed to make a given mass of product
Worked example
In the reaction shown by the equation below, what mass of sulfur dioxide can be made from 16 g of sulfur? (Mr of SO2 = 64)
S(s) + O2(g) 鈫 SO2(g)
\(amount~of~S = \frac{mass}{relative~atomic~mass}\)
\(= \frac{16}{32}\)
= 0.5 mol
The equation shows that 1 mol of sulfur reacts with 1 mol of oxygen molecules to make 1 mol of sulfur dioxide. This means that 0.5 mol of sulfur makes 0.5 mol of sulfur dioxide.
mass of SO2 = relative formula mass 脳 amount
= 64 脳 0.5
= 32 g
Question
In the reaction shown by the equation below, what mass of nitrogen, N2, is needed to make 120 g of nitrogen monoxide, NO? (Mr of NO = 30 and Mr of N2 = 28)
N2(g) + O2(g) 鈫 2NO(g)
\(Amount~of~nitrogen~dioxide = \frac{mass}{relative~atomic~mass}\)
\(= \frac{120}{30}\)
= 4.0 mol
1 mol of N2 makes 2 mol of NO.
This means that 2 mol of N2 makes 4.0 mol of NO.
Mass of 2 mol of N2 = 2 脳 28
= 56 g