كيف يحسب عدد المولات والكتلة المولية؟ How to calculate number of moles?

يحسب عدد المولات بالقانون: n = m / M_r (الكتلة بالجرام مقسومة على الكتلة المولية بالجرام/مول). وللغازات عند الظروف المعيارية STP: الحجم = عدد المولات × 22.4 لتر. [الصيغة الرمزية الموزونة: n = m / M_r | N = n × N_A (6.022 × 10²³) | M = n / V | V_gas = n × 22.4 L (STP)]

4.2Stoichiometry

Formulas and equations — EST 2 Chemistry Revision Notes & Topical Questions

5 min read 8 parts Lesson 2 of 3 1 quiz

📚 EST 2 Chemistry Hub / Topical Revision

Curriculum Equation Key قوانين المول والحسابات الكيميائية - Moles & Stoichiometry FormulasFast Exam Solution

n = m / M_r  |  N = n × N_A (6.022 × 10²³)  |  M = n / V  |  V_gas = n × 22.4 L (STP)

Word Equation & Conditions: عدد المولات = الكتلة بالجرام ÷ الكتلة المولية | عدد الجسيمات = عدد المولات × عدد أفوجادرو

Location & Specifications: كيمياء التحصيلي وثانوي وبكالوريا و Cambridge IGCSE Chemistry.

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Lesson summary

Formulas and equations EST Chemistry notes: Write and name formulas.Never change subscripts to balance an equation — change only coefficients.Balance polyatomic ions as units when they appear unchanged.State symbols(s), (l), (g), (aq).C6H12O6 has empirical formula CH2O.Ionic form…

Ionic formulas are charge-balanced
Common polyatomic ions
A compound is 40.0% C, 6.7% H, 53.3% O; molar mass 180. Find formulas
Balance C3H8 + O2 → CO2 + H2O
Formulas and equations EST Chemistry: what to do next
Molecular formula

Section 1 of 8Formulas and equations

Balance C, then H, then O in combustion.

Common polyatomic ions

NO₃⁻, SO₄²⁻, PO₄³⁻, CO₃²⁻, NH₄⁺, OH⁻.

Acids

HCl hydrochloric, HNO₃ nitric, H₂SO₄ sulfuric.

Heating a hydrate removes water; mass loss gives the water ratio.

Check that total charge also balances in ionic equations.

Section 2 of 8Process & sequence

Convert percent to grams (assume 100 g).

Convert grams to moles.

Divide by the smallest number of moles.

Multiply to get whole numbers: empirical formula.

Molecular formula = empirical × (molar mass ÷ empirical mass).

Finding an empirical formulawrite these steps in this order; each step is a marking point.

Percent to grams

Grams to moles

Divide by smallest

Whole-number ratio

Scale to molecular

Section 3 of 8Compare

Naming ionic compounds
Formula	Name	Rule
NaCl	Sodium chloride	Metal + nonmetal-ide
FeCl₃	Iron(III) chloride	Roman numeral for variable charge
CaSO₄	Calcium sulfate	Polyatomic ion name
CuSO₄·5H₂O	Copper(II) sulfate pentahydrate	Hydrate prefix

Covalent prefixes
Number	Prefix	Example
1	mono-	CO carbon monoxide
2	di-	CO₂ carbon dioxide
3	tri-	SO₃ sulfur trioxide
4	tetra-	CCl₄ carbon tetrachloride
5	penta-	N₂O₅ dinitrogen pentoxide

Section 4 of 8Worked examples

1A compound is 40.0% C, 6.7% H, 53.3% O; molar mass 180. Find formulas.

Moles 3.33 : 6.7 : 3.33 → CH₂O (30); 180 ÷ 30 = 6 → C₆H₁₂O₆.

2Balance C₃H₈ + O₂ → CO₂ + H₂O.

C₃H₈ + 5O₂ → 3CO₂ + 4H₂O.

Section 5 of 8Keep going

Formulas and equations EST Chemistry: what to do next

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

Practice: Formulas and equations EST Chemistry practice questions

Previous lesson: 4.1 The mole

Next lesson: 4.3 Reaction calculations

Course: EST 2 Chemistry — all units

Official syllabus: OpenStax free science textbooks

Formulas and equations 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

Empirical formula · Molecular formula · Chemical equation · Coefficient · Law of conservation of mass · Hydrate

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)

Answer exactly what is asked; extra correct material does not earn credit, and contradictory statements can cancel credit (AP FRQ).

Use precise vocabulary (e.g. “active site”, “concentration gradient”, “allele”), never “it/stuff/things”.

Show units and working in calculations.

In MCQs, predict the answer before reading the options, then eliminate.

Last updated: 18 October 2026