Stoichiometry and moles quiz: from grams to moles, and back.
Test in ItalianThe questions of this simulator are in Italian, as in the real exam.
15 questions in 40 minutes, no penalty for wrong answers: molar mass, balancing, limiting reagent, yield, empirical formula, gas volume and molarity, with the calculations marked. The correct answer to every question and your weak topics arrive by email for free; with an account you see them straight away here, together with the score, which is already on the page for everyone. The guided step-by-step solution unlocks with your first lesson.
A free high-school chemistry quiz on stoichiometry and the mole, the topic of the Year 3 test in November.
| Topic | What it covers |
|---|---|
| Molar mass and the mole | n = m/M, Avogadro’s number, comparing samples |
| Balancing and stoichiometric ratios | coefficients, grams → moles → grams, law of definite proportions |
| Limiting reagent | which reagent runs out first and how much product you get |
| Percentage yield | theoretical mass, actual mass, efficiency |
| Composition and empirical formula | percentages by mass, empirical and molecular formula |
| Gases and molarity | molar volume, Avogadro’s hypothesis, concentration, dilutions |
15 questions drawn at random from a bank of 48 · 40 minutes · no penalty (blank = wrong = 0 points) · options shuffled on every attempt. Original questions, recalculated one by one.
Two worked examples from the test
Qual è la massa molare dell'acido solforico ? (Masse atomiche: H = 1, S = 32, O = 16 u)
Solution
La massa molare è la somma delle masse atomiche di tutti gli atomi della formula, ciascuna moltiplicata per il suo pedice.
Passo 1 — Conto gli atomi. In ci sono 2 atomi di H, 1 di S e 4 di O.
Passo 2 — Sommo.
Errori tipici: contare tre ossigeni invece di quattro (dà 82), dimenticare gli idrogeni (dà 96), oppure dividere per due senza motivo (dà 49).
Quanti atomi in totale sono contenuti in 1 mol di acqua ? (Numero di Avogadro: mol)
Solution
1 mol di acqua contiene molecole, ma ogni molecola è fatta di 3 atomi (2 di H e 1 di O).
La risposta conta le molecole, non gli atomi; conta solo gli atomi di idrogeno.
In the online practice test you will find 13 more questions like these; the correct answer to every question arrives by email.
Where you lose marks in a stoichiometry test.
The stoichiometry test comes in November, just as the class has moved from nomenclature to calculations, and almost every problem is the same problem in a different costume: a mass going in, a reaction to balance and a mass or a volume coming out. The route is always grams → moles → ratio of the coefficients → moles → grams (or litres). Whoever has made it automatic solves in three lines what looks like a brand-new exercise to everyone else.
The first place where marks are lost is skipping the balancing: a stoichiometric ratio read from an unbalanced reaction is wrong from the start, even if the later calculations are flawless. The second is the limiting reagent: the smaller quantity in grams or in moles is not automatically the one that limits, because it has to be compared with its own coefficient. The third is the yield: the percentage is calculated on the theoretical product, never on the mass of the starting reagent.
In the simulator every question comes with the atomic masses already written, because in a class test you almost always copy them from the periodic table. After the test you see the breakdown by topic: if all the lost points are on the limiting reagent and the yield, the problem is set-up; if they are scattered over simple calculations, it is attention to signs and powers of ten.
To revise the theory: the guide to moles and stoichiometry for Year 3.
Frequently asked questions.
Do I need a calculator?
Yes, as in a class test. The numbers are chosen so that the sums work out without too many digits, but a few questions (molarity, percentages) need a division that is awkward to do in your head.
Are the atomic masses the ones in my periodic table?
Every question gives the atomic masses used in its text, rounded. If your table has more precise ones, the difference is tiny: the answer options are far enough apart that a different rounding does not change the right one.
Can I retake the quiz?
Yes, as many times as you like. On every attempt 15 different questions are drawn from a larger bank and the options change order: you cannot memorise a letter, you have to redo the calculation.
Does the quiz also cover reactions in solution and titrations?
It covers molarity and dilutions and one precipitation in solution. Acid-base titrations are in the pH and redox quiz.
What do I see at the end of the practice test?
The correct answer to every question and your weak topics arrive by email for free; with an account you see them straight away here, together with the score, which is already on the page for everyone. The guided step-by-step solution unlocks with your first lesson.
Is stoichiometry not adding up for you?
The quiz shows you whether the problem is balancing, the limiting reagent or the yield. In lessons we start from the topic where you lose the most marks, with graded exercises and timed practice tests.