Stoichiometry & Gas Volume Calculator

Master mole calculations, gas volumes at RTP ($24\text{ dm}^3/\text{mol}$), and molar concentration for IGCSE Chemistry (0620).

1. Known Substance (A) Data
grams ($\text{g}$)
2. Stoichiometry Ratio
:
3. Target Substance (B) $M_r$
Calculated Answers
Moles of A ($n_A$): --
Moles of Target B ($n_B$): --
Target B Mass (g): --
Target B Gas Vol (RTP): --
Mark Scheme Working Out
⚠️ Quick Tip: Divide volume in $\text{cm}^3$ by $1000$ to get $\text{dm}^3$ before calculating moles!

Click "Calculate & Show Working" to generate step-by-step solutions.

IGCSE Chemistry Revision: Stoichiometry Masterclass

1. Essential Stoichiometry Formulas Summary

Choose the correct formula based on the physical state of the substance given in the question:

State of Matter Known Quantity Core Formula Unit Conversion Tip
Solid / Pure Substance Mass ($m$) $$\text{Moles } (n) = \frac{\text{Mass (g)}}{M_r}$$ Mass must always be in grams ($\text{g}$).
Gas at RTP Volume ($V$) $$\text{Moles } (n) = \frac{\text{Volume (dm}^3\text{)}}{24\text{ dm}^3/\text{mol}}$$ $1\text{ dm}^3 = 1000\text{ cm}^3$ (Divide $\text{cm}^3$ by $1000$).
Solution / Aqueous Concentration ($C$) & Vol ($V$) $$\text{Moles } (n) = C \times V\text{ (in dm}^3\text{)}$$ Ensure volume is converted to $\text{dm}^3$ before multiplying.

2. Past Paper Worked Example (Cambridge IGCSE 0620)

Question: $4.8\text{ g}$ of magnesium reacts completely with excess dilute hydrochloric acid. Calculate the volume of hydrogen gas produced at RTP in $\text{dm}^3$.
$$\text{Mg (s)} + 2\text{HCl (aq)} \rightarrow \text{MgCl}_2\text{ (aq)} + \text{H}_2\text{ (g)}$$

Step Working Method Calculation & Result
Step 1: Moles of Known Find moles of $\text{Mg}$ using $n = \frac{m}{A_r}$ $$n(\text{Mg}) = \frac{4.8}{24} = \mathbf{0.20\text{ mol}}$$
Step 2: Molar Ratio Equation ratio $\text{Mg} : \text{H}_2 = 1 : 1$ $$n(\text{H}_2) = 0.20\text{ mol}$$
Step 3: Target Volume Gas volume at RTP $= n \times 24$ $$V(\text{H}_2) = 0.20 \times 24 = \mathbf{4.80\text{ dm}^3}$$

3. Common Substance $M_r$ Quick Reference

Memorizing or quickly deriving these high-frequency IGCSE values speeds up calculations:

Substance Name Chemical Formula Relative Molecular Mass ($M_r$)
Water$\text{H}_2\text{O}$18
Carbon Dioxide$\text{CO}_2$44
Hydrochloric Acid$\text{HCl}$36.5
Sulfuric Acid$\text{H}_2\text{SO}_4$98
Sodium Hydroxide$\text{NaOH}$40
Calcium Carbonate$\text{CaCO}_3$100
Ammonia$\text{NH}_3$17

4. Equation Balancing & Ionic Equations Tips

Before doing any stoichiometry math, your chemical equation must be correctly balanced. Follow these rules:

  • Never change small subscripts in a formula (e.g., change $\text{H}_2\text{O}$ to $\text{H}_3\text{O}$ is strictly illegal); only adjust big coefficients in front.
  • Balance metals first, then non-metals (except hydrogen and oxygen), and leave $\text{H}$ and $\text{O}$ for the very end.
  • Ionic Equations: Omit spectator ions (ions that remain unchanged on both sides of the ionic equation, e.g., $\text{Na}^+$ or $\text{NO}_3^-$ in many neutralization reactions).

5. Top Exam Traps & Examiner Insights

❌ Forgetting Unit Conversions

Plugging $\text{cm}^3$ directly into $n = C \times V$. Always divide $\text{cm}^3$ by $1000$ to convert to $\text{dm}^3$.

❌ Diatomic Gas $M_r$ Errors

Using $A_r = 16$ for Oxygen gas instead of $M_r(\text{O}_2) = 32$. Remember gases like $\text{H}_2, \text{O}_2, \text{Cl}_2$ are diatomic.

❌ Misusing $24\text{ dm}^3/\text{mol}$

Applying the $24\text{ dm}^3$ rule to liquid water or solutions. It applies ONLY to gases at room temperature & pressure.

6. Core Knowledge & Key Definitions (IGCSE Chemistry 0620)

Essential syllabus knowledge required for Cambridge IGCSE examinations regarding moles, gases, and stoichiometry:

1. Definition of the Mole ($n$) The mole is the amount of substance that contains as many elementary entities (atoms, molecules, ions, or electrons) as there are atoms in exactly $12\text{ g}$ of carbon-12. One mole of any substance contains the Avogadro constant number of particles ($6.02 \times 10^{23}\text{ mol}^{-1}$).
2. Relative Masses
  • Relative Atomic Mass ($A_r$): The average mass of the isotopes of an element compared to $\frac{1}{12}$th of the mass of an atom of carbon-12.
  • Relative Molecular Mass ($M_r$): The sum of the relative atomic masses of all the atoms in a molecular formula. (For ionic compounds, we use Relative Formula Mass).
3. Molar Gas Volume at RTP At Room Temperature and Pressure (RTP, typically taken as $20^\circ\text{C}$ and $1\text{ atmosphere}$), one mole of any gas occupies exactly $24\text{ dm}^3$ (or $24,000\text{ cm}^3$). This relationship is independent of the chemical identity of the gas.
4. Solution Concentration & Molarity Concentration measures the amount of solute dissolved in a specific volume of solvent. In IGCSE chemistry, it is predominantly expressed in $\text{mol/dm}^3$. The relationship is defined as $\text{Concentration} = \frac{\text{Moles}}{\text{Volume (in dm}^3\text{)}}$.

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