# Relative Masses and Reacting Masses (No Mole)

> CIE IGCSE Chemistry · 0620 2026-2028
> Source: https://www.owlsprep.com/study/cie-0620-u3-relative-masses-and-reacting-masses/

This Core CIE IGCSE Chemistry 0620 guide teaches you to calculate relative atomic and formula masses, and use mass ratios from balanced equations to find unknown reacting masses, no mole concept required.

**Prerequisites:** [Understanding of balanced chemical equations](https://www.owlsprep.com/study/cie-0620-u3-balanced-equations/); [Knowledge of chemical formulae for common compounds](https://www.owlsprep.com/study/cie-0620-u2-chemical-formulae/)

## Learning objectives

- Define relative atomic mass ($A_r$) and relative formula mass ($M_r$) for Core level
- Calculate $M_r$ of any compound from given $A_r$ values
- Use $M_r$ ratios from balanced equations to calculate unknown reacting masses
- Apply the law of conservation of mass to verify reaction mass calculations

## 1. Relative Atomic Mass ($A_r$) and Relative Formula Mass ($M_r$)

**Relative Atomic Mass ($A_r$)** — The average mass of one atom of an element, compared to 1/12 the mass of one carbon-12 atom.

*Example:* $A_r$ of hydrogen = 1, $A_r$ of oxygen = 16

Relative formula mass ($M_r$) is the sum of the relative atomic masses of all atoms in a compound's chemical formula. $A_r$ and $M_r$ are relative values, so you do not need to write units for them in your answers.

**Worked example:** Calculate the relative formula mass of glucose, $C_6H_{12}O_6$, using $A_r$ values: C=12, H=1, O=16.

1. Step 1: Count the number of each atom in the formula: 6 carbon, 12 hydrogen, 6 oxygen atoms
2. Step 2: Multiply each atom count by its $A_r$ value: 6×12 = 72, 12×1 = 12, 6×16 = 96
3. Step 3: Sum the values to get $M_r$: 72 + 12 + 96 = 180
4. $$M_r(C_6H_{12}O_6) = (6 \times 12) + (12 \times 1) + (6 \times 16) = 180$$

> **Exam tip**
>
> Always double-check atom counts, especially for formulae with brackets: e.g. for $Ca(OH)_2$, you have 2 oxygen and 2 hydrogen atoms, not 1 of each.

## 2. Law of Conservation of Mass

**Conservation of Mass** — No atoms are created or destroyed during a chemical reaction, so the total mass of reactants equals the total mass of products in a closed system.

*Example:* If 4g of hydrogen reacts fully with 32g of oxygen, 36g of water is produced.

> **tip**
>
> If a reaction appears to lose mass, a gaseous product has escaped into the air. If it appears to gain mass, a gaseous reactant from the air has been used in the reaction.

**Worked example:** For the reaction: $2Mg + O_2 \rightarrow 2MgO$. If 48g of magnesium reacts fully with 32g of oxygen, what mass of magnesium oxide is formed?

1. Step 1: Apply conservation of mass rule: total reactant mass = total product mass
2. Step 2: Add the masses of the two reactants: 48g + 32g = 80g
3. Step 3: The mass of magnesium oxide formed is 80g

## 3. Calculating Reacting Masses Using Mass Ratios

For balanced chemical equations, the ratio of the total relative masses of reactants and products equals the ratio of their actual reacting masses. You can use this ratio to find unknown masses without using the mole concept.

**Worked example:** Use the balanced equation $2H_2 + O_2 \rightarrow 2H_2O$ to find the mass of water formed when 8g of hydrogen reacts fully with excess oxygen. Use $A_r$ values: H=1, O=16.

1. Step 1: Calculate total relative mass for each substance (multiply $M_r$ by balancing number): 2$H_2$ = 2×(2×1) = 4, 2$H_2O$ = 2×18 = 36
2. Step 2: Write the mass ratio of the substances you need: $H_2 : H_2O = 4 : 36 = 1 : 9$
3. Step 3: Scale the ratio to match the known mass: 1 part $H_2$ = 8g, so 9 parts $H_2O$ = 8 × 9 = 72g
4. Step 4: Verify with conservation of mass: 8g $H_2$ + 64g $O_2$ = 72g $H_2O$, which is correct.

> **Exam tip**
>
> Always multiply each substance's $M_r$ by its balancing coefficient before calculating the mass ratio, otherwise your calculation will be incorrect.

## Common pitfalls

- **Wrong:** Forgetting to multiply $A_r$ by the number of atoms in a formula when calculating $M_r$
  - Why it fails: Leads to incorrect $M_r$ values that break all subsequent calculations
  - Correct: List each atom count first, multiply by its $A_r$, then sum all values to get $M_r$
- **Wrong:** Ignoring balancing numbers when calculating reacting mass ratios
  - Why it fails: The mass ratio only matches real reacting masses if you account for the number of molecules of each substance in the balanced equation
  - Correct: Multiply each substance's $M_r$ by its balancing coefficient before calculating the ratio
- **Wrong:** Adding units to $A_r$ or $M_r$ values
  - Why it fails: $A_r$ and $M_r$ are relative values with no units, so adding units will lose marks
  - Correct: Only add units (usually grams) to actual reacting mass values in your final answer
- **Wrong:** Using an inverted ratio (e.g. product:reactant instead of reactant:product)
  - Why it fails: Leads to an incorrect scaled mass value for the unknown substance
  - Correct: Write the ratio clearly with labels for each substance before substituting the known mass

## Cheatsheet

| Concept | Method | Example |
| --- | --- | --- |
| Relative Atomic Mass ($A_r$) | Provided on exam front page, no calculation needed | $A_r(C) = 12$ |
| Relative Formula Mass ($M_r$) | Sum of (number of each atom × $A_r$ of atom) in formula | $M_r(CO_2) = 12 + (2×16) = 44$ |
| Conservation of Mass | Total mass of reactants = Total mass of products | 12g C + 32g $O_2$ = 44g $CO_2$ |
| Reacting Mass Calculation | 1. Calculate total $M_r$ × balancing coefficient for relevant substances 2. Simplify to mass ratio 3. Scale ratio to match known mass | Ratio $H_2:H_2O = 4:36 = 1:9$, so 8g $H_2$ makes 72g $H_2O$ |

## What's next

Now that you have mastered relative masses and reacting mass calculations without the mole concept, you are ready to move on to more Core stoichiometry content for CIE IGCSE Chemistry 0620. This foundational knowledge is critical for understanding how chemical reactions follow quantitative rules, which will appear in both Paper 1 (multiple choice) and Paper 2 (structured) Core exams. You will build on these skills when you learn about concentration calculations in g/dm³, another Core stoichiometry outcome, before moving to Extended content if you are taking the Extended tier. Practice as many structured reacting mass questions as possible to build confidence, and always cross-check your answers using the conservation of mass rule to catch calculation errors early.

---

From [OwlsPrep](https://www.owlsprep.com) — free study guides for A-Level, IB, AP and IGCSE, written against the official syllabus. Canonical page: https://www.owlsprep.com/study/cie-0620-u3-relative-masses-and-reacting-masses/