Quantitative chemistry, Year 10: reacting masses, limiting reactants and concentration

6

提示The big numbers in front of the formulae give the recipe.

为什么The equation says that one mole of magnesium reacts with two moles of hydrochloric acid. Three moles of magnesium therefore need 3 × 2 moles of acid.

0.5; 0.5; 20

提示Three steps: grams to moles, then the ratio in the equation, then moles back to grams.

为什么Moles of Mg = 12 ÷ 24. The equation shows 2 mol of Mg making 2 mol of MgO, a 1 : 1 ratio. The Mr of MgO is 24 + 16 = 40, so the mass is the moles of MgO × 40.

6 g

提示28 g is exactly one mole of N2, so read the amounts straight from the balancing numbers.

为什么One mole of N2 has a mass of 2 × 14 = 28 g. It reacts with three moles of H2, and each mole of H2 has a mass of 2 × 1 = 2 g, so the mass of hydrogen is 3 × 2 g.

The 2 is a ratio of moles, not of masses

提示Balancing numbers count particles, and the particles of the two substances do not weigh the same.

为什么The equation says 1 mol of Mg reacts with 2 mol of HCl. In grams that is 24 g of magnesium with 2 × 36.5 = 73 g of hydrogen chloride. Masses have to be turned into moles before the ratio can be used.

160 g

提示Work backwards: turn the product into moles first, then use the ratio to find the reactant.

为什么Moles of Fe = 112 ÷ 56 = 2. The equation shows 1 mol of Fe2O3 giving 2 mol of Fe, so 1 mol of Fe2O3 is needed. Its Mr is (2 × 56) + (3 × 16) = 160, so the mass is 160 g.

moles

提示Balancing numbers count particles, so you need the chemist's counting unit.

为什么Divide each mass by the relative formula mass of that substance. The amounts you get are then turned into the simplest whole-number ratio, and those whole numbers are the balancing numbers.

0.2; 0.6; 0.4; 3

提示Divide each mass by its Mr, then divide all three results by the smallest.

为什么Moles = mass ÷ Mr for each substance. Dividing the three amounts by the smallest gives the ratio 1 : 3 : 2, and these are the balancing numbers.

2 : 3

提示Dividing by the smaller amount leaves a half in the ratio, so scale up once more.

为什么Dividing both amounts by 0.4 gives 1 : 1.5. Balancing numbers must be whole, so multiply both by 2. The equation is 2Al + 3Cl2 → 2AlCl3.

To get the simplest whole-number ratio

提示Amounts such as 0.2 and 0.6 are in the right proportion, but an equation is never written with them.

为什么Dividing by the smallest amount turns it into 1 and scales the others to match, which usually gives whole numbers straight away. If a half appears, doubling everything clears it.

44

提示Rearrange moles = mass ÷ Mr so that Mr is the subject.

为什么Multiplying both sides of moles = mass ÷ Mr by Mr and then dividing by moles gives Mr = mass ÷ moles = 22 ÷ 0.5. The specification expects you to be able to change the subject of an equation like this.

The limiting reactant

提示Its name comes from the fact that it sets a ceiling on what can be made.

为什么Once this reactant has run out, the reaction stops, however much of the other reactant is left. The amount of product therefore depends on how much of it there was at the start.

excess

提示It means more than is needed.

为什么If there is more than enough of one reactant, the other one is certain to react completely. The one that is used up is then the limiting reactant, and some of the other is left over at the end.

Magnesium: it needs only 0.2 mol of acid, so acid is left over

提示Use the ratio in the equation to see how much of one the other requires.

为什么0.1 mol of magnesium reacts with 0.1 × 2 = 0.2 mol of acid. There is 0.3 mol of acid, more than enough, so the magnesium runs out first and 0.1 mol of acid is left unreacted.

It stays the same

提示Ask which substance runs out first, and whether that has changed.

为什么The amount of product is set by the limiting reactant. The magnesium still runs out at the same point, so the same amount of hydrogen is made; there is simply more acid left over.

4.4

提示The acid is plentiful, so only the solid decides the answer: grams to moles, ratio, moles to grams.

为什么Calcium carbonate is the limiting reactant. Its Mr is 40 + 12 + (3 × 16) = 100, so 10 g is 0.1 mol. The equation gives 1 mol of CO2 per mole of CaCO3, so 0.1 mol of CO2 forms, with a mass of 0.1 × 44.

volume

提示It is the amount of space the solution takes up.

为什么Concentration says how much solute is packed into each unit of solution. More solute in the same space, or the same solute in less space, both give a more concentrated solution.

1000

提示A decimetre is 10 cm, and a cube has three dimensions.

为什么A cube 10 cm along each edge holds 10 × 10 × 10 cubic centimetres. One cubic decimetre is the same volume as one litre.

0.25; 15

提示Convert the volume first, then multiply by how much each whole cubic decimetre holds.

为什么Divide by 1000 to change cm³ into dm³. Then mass of solute = concentration × volume = 60 × 0.25. This is the concentration equation rearranged to make mass the subject.

40

提示Change the volume into cubic decimetres before you divide.

为什么500 cm³ is 0.5 dm³, and 20 ÷ 0.5 = 40. Another way to see it: a whole cubic decimetre is twice as much solution, so it would hold twice as much sugar.

40 g/dm³: the same mass is in half the volume

提示Think about what happens to the result of a division when the number you divide by gets smaller.

为什么Concentration = mass ÷ volume. The mass of solute has not changed, but it is now divided by half the volume, so the answer doubles. In the same way, doubling the mass of solute in the same volume doubles the concentration.