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4.1Stoichiometry

The mole — EST 2 Chemistry Revision Notes & Topical Questions

5 min read 8 parts Lesson 1 of 3 1 quiz

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Lesson summary

The mole EST Chemistry notes: Convert between mass, moles and particles.Molar mass in g/mol equals the formula mass in u.One mole of H2O has a mass of 18.02 g.Moles are the 'bridge' in all stoichiometry.Count atoms using subscripts1 mol CO2 contains 2 mol O atoms.Percent composit…

  • Diatomic elements
  • How many molecules are in 36.0 g of water?
  • Percent of O in CO2 (44.0 g/mol)
  • The mole EST Chemistry: what to do next
  • Avogadro's number
  • Representative particle

Section 1 of 8The mole

Use periodic-table masses to at least one decimal.
Diatomic elements
H2, N2, O2, F2, Cl2, Br2, I2.
Ionic compounds are counted as formula units.
Hydrates include water in their molar mass.
Dimensional analysis prevents unit errors.

Section 2 of 8Process & sequence

1
Write the given quantity with units.
2
Convert to moles first.
3
Use the correct factor from the mole map.
4
Convert to the wanted unit.
5
Check significant figures.

The mole mapwrite these steps in this order; each step is a marking point.

  1. Mass
  2. ÷ molar mass
  3. Moles
  4. × NA
  5. Particles

Section 3 of 8Compare

Mole conversions
FromOperationTo
Mass (g)÷ molar massMoles
Moles× 6.022 × 1023Particles
Particles÷ 6.022 × 1023Moles
Moles (gas, STP)× 22.4 LVolume
Moles× molar massMass (g)

Section 4 of 8Worked examples

1How many molecules are in 36.0 g of water?
  • 36.0 ÷ 18.0 = 2.00 mol; 2.00 × 6.022 × 1023 = 1.20 × 1024.
2Percent of O in CO2 (44.0 g/mol).
  • 32.0 ÷ 44.0 × 100 = 72.7%.

Section 5 of 8Keep going

The mole EST Chemistry: what to do next

These The mole EST Chemistry notes covered the key ideas, worked examples and exam technique for this topic. Check your The mole EST Chemistry understanding with the practice questions below, then reread any The mole EST Chemistry point you were unsure about before moving on.

1
Practice: The mole EST Chemistry practice questions
2
Previous lesson: 3.4 Precipitation and other reactions
3
Next lesson: 4.2 Formulas and equations
4
Course: EST 2 Chemistry — all units
5
Official syllabus: OpenStax free science textbooks

The mole EST Chemistry revision notes

Section 6 of 8Key terms

Section 7 of 8Command terms

Command terms
Identify / Describe (FRQ)
A short, specific statement; describing requires relevant characteristics, not a single word.
Explain / Justify (FRQ)
Give the cause-and-effect reasoning or evidence; a restated fact does not score.
Calculate
Show setup, substitution and the answer with units.
Predict
State the expected outcome; add a reason only if asked to justify.
MCQ technique
Read every option; eliminate distractors that are true statements but do not answer the question; watch for NOT/EXCEPT.

Section 8 of 8Exam technique

Keywords the mark scheme looks for in this lesson

Keywords the mark scheme looks for in this lesson
Mole · Avogadro's number · Molar mass · Representative particle · Molar volume
Learn each definition in the Key words tab exactly; a missing keyword in a definition usually loses the mark.

How to get full marks (examiner guidance)

1
Answer exactly what is asked; extra correct material does not earn credit, and contradictory statements can cancel credit (AP FRQ).
2
Use precise vocabulary (e.g. “active site”, “concentration gradient”, “allele”), never “it/stuff/things”.
3
Show units and working in calculations.
4
In MCQs, predict the answer before reading the options, then eliminate.

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