1 Moles and molar mass
Key ideas
- A mole is a number: 6.022 × 1023 particles (atoms, molecules or formula units).
- The molar mass M is the mass of one mole, in g mol−1. Add up the atomic mass of every atom in the formula.
- n = mM so m = nM. Moles = mass in grams over molar mass.
Worked example
(a) How many moles are in a 10.0 g stick of chalk?
(b) What is the mass of 0.250 mol of calcium carbonate?
Molar mass builder
Pick a formula, then click atoms in until you have built it. Each block in the bar is one atom, as wide as its mass, so you can see where the molar mass comes from.
Practice
New numbers every time. Pick a level; use a hint if you are stuck. Give answers to 3 significant figures unless the question says otherwise.
2 Concentration
Key ideas
- Concentration c is the number of moles dissolved in each litre of solution, in mol L−1.
- c = nV so n = cV and V = nc. The volume must be in litres.
- There are 1000 mL in 1 L. To change mL into L, divide by 1000: 250 mL is 0.250 L.
- Adding water changes the volume, not the number of moles, so the concentration goes down.
Worked example
(a) What is the concentration of the solution?
(b) What volume of this solution, in mL, contains 0.0100 mol of NaCl?
Beaker lab
Each dot is 0.01 mol of dissolved substance, and the colour deepens as the concentration rises. Beaker A stays put; you control beaker B. Try the challenges, or just play: what happens to the dots when you add water?
Practice
New numbers every time. Pick a level; use a hint if you are stuck. Give answers to 3 significant figures unless the question says otherwise.
3 Stoichiometry
Key ideas
- A balanced equation has the same number of each kind of atom on both sides. Balance it by changing the numbers in front, never the small numbers inside a formula.
- The numbers in front give the mole ratio: in 2Mg + O2 → 2MgO, 2 mol of Mg react with 1 mol of O2 to make 2 mol of MgO.
- The route: grams of A → moles of A → (ratio) → moles of B → grams of B.
- The ratio step: n(B) = number in front of Bnumber in front of A × n(A). What you want goes on top.
Worked example
(a) Balance the equation and write the mole ratio.
(b) What mass of chlorine reacts with 4.60 g of sodium?
(c) What mass of sodium chloride forms?
Equation balancer
Change the numbers in front with − and +. Each little cluster is one molecule or formula unit, so you can watch the atoms on each side. The table turns green when an element balances.
Practice
New numbers every time. Pick a level; use a hint if you are stuck. Give answers to 3 significant figures unless the question says otherwise.
4 Limiting reactant
Key ideas
- Reactants are rarely mixed in exactly the right ratio. The one that runs out first is the limiting reactant: it decides how much product forms. The other is in excess, and some of it is left over.
- The sandwich rule: a sandwich takes 2 slices of bread and 1 slice of cheese. With 10 slices of bread and 3 of cheese you make only 3 sandwiches. The cheese is limiting and 4 slices of bread are left over, even though there was less cheese to start with.
- To find the limiting reactant, work out the moles of each and divide by its number in front: nnumber in front. The smallest result is the limiting reactant.
- Then do every calculation from the limiting reactant.
Worked example
(a) Which reactant is limiting?
(b) What mass of water forms?
(c) What mass of the other reactant is left over?
Reaction mixer
Pick a reaction and set how many molecules of each reactant you have. Predict which runs out first, then see what forms and what is left over.
Practice
New numbers every time. Pick a level; use a hint if you are stuck. Give answers to 3 significant figures unless the question says otherwise.
5 Titration
Key ideas
- A titration finds an unknown concentration. A measured volume of the unknown is pipetted into a conical flask, and a standard solution of known concentration is run in from a burette until they have exactly reacted.
- The end point is the first lasting colour change of the indicator. Phenolphthalein is colourless in acid and pink in alkali.
- Read the burette at the bottom of the meniscus, to 0.05 mL: two decimal places, ending in 0 or 5. The numbers increase downwards. Titre = final reading − start reading.
- The first titre is a rough one. Repeat until 2 or 3 titres agree within 0.10 mL (concordant), and average only those.
- The route: titre and c of the standard → n = cV → mole ratio → n of the unknown → c = nV with the pipetted volume.
Worked example
(a) What is the concordant titre?
(b) What is the concentration of the hydrochloric acid?
Burette lab
Start with the guided run: a pink mark shows where the end point is, and each step is explained as you go. Then titrate an unknown hydrochloric acid on your own, repeat until you have concordant titres, and work out its concentration.
Practice
New numbers every time. Pick a level; use a hint if you are stuck. Give concentrations to 3 significant figures, and burette volumes to two decimal places.
Challenge
Key ideas: the mole map
Every calculation in this lab is a route across this map. Find where you start and where you want to end up, and plan the route before you touch any numbers. To get from one substance to another you must go through moles, and cross the bridge with the mole ratio.
- Two reactants with amounts given? Find the moles of each, then the limiting reactant, before crossing the bridge.
- Volumes in litres, every time. A sense check at the end catches most slips.
Worked example
(a) Plan the route to the volume of acid the tablet neutralises.
(b) What volume of acid is that?
(c) What mass of carbon dioxide does the tablet release?
Route planner
No calculating yet: build the route. Click the steps in the order you would use them (a step can be used more than once), then check.
Practice
No section labels here: decide the route yourself. Mixed draws questions from every section; Multi-step crosses sections; Year 12 ready is the hardest. Give answers to 3 significant figures.