A mole is a counting unit. The confusion starts because three different quantities, particles, moles and grams, are all written as numbers, and it is unclear which one a step needs.
This page helps students new to the mole, or returning to it after a gap. The conversions are taught in converting between mass, moles and particles.
What does it sound like when this happens?
Students describe it in similar words.
- “I do not know whether to multiply or divide.”
- “I memorise the formula but forget it by the exam.”
- “The numbers in the equation are not grams, are they?”
What is usually behind it?
The mole is usually introduced as a formula before the idea. Without the idea, each conversion is a separate rule to remember.
Another cause is using the coefficients of an equation as grams. In 2H₂ + O₂ → 2H₂O, the 2 and the 1 describe particles or moles, not masses.
How do you see the mole as a bridge?
Think of moles in the middle, with particles on one side and mass on the other.
| From | To | Use |
|---|---|---|
| Mass (g) | Moles | ÷ molar mass (g mol⁻¹) |
| Moles | Mass (g) | × molar mass |
| Moles | Particles | × 6.02 × 10²³ |
| Moles | Gas volume (dm³) | × molar volume (dm³ mol⁻¹) |
The equation speaks in moles. To use an equation you must go into moles first and out of moles last.
Worked example: magnesium and hydrochloric acid
Question: “2.4 g of magnesium reacts with excess hydrochloric acid. What volume of hydrogen forms at room conditions, if the molar volume is 24 dm³ mol⁻¹?”
Equation: Mg + 2HCl → MgCl₂ + H₂
- Mass to moles: Ar of Mg is 24, so moles = 2.4 ÷ 24 = 0.10 mol.
- Equation ratio: 1 mol Mg gives 1 mol H₂, so 0.10 mol H₂ forms.
- Moles to volume: 0.10 × 24 = 2.4 dm³.
The path is mass to moles, through the equation, then out to volume. At no step are the coefficients treated as grams.
What can you do this week?
- Draw the bridge table on a card and keep it beside you.
- Write the units on every line of a calculation.
- Check that the units cancel to give the quantity asked.
- Do five short questions each day, with mass to moles only, then add the equation.
The mole and stoichiometry steps tool shows the same order. The lesson on solving reacting mass and gas volume calculations practises the full path.
When might one-to-one tuition help?
If the bridge makes sense when you read it but you still hesitate in a question, a teacher can walk through live problems with you and ask which quantity you hold at each line. That habit is hard to build alone.
See online one-to-one Chemistry tuition for how starting works. Keep a mistake log to see whether the confusion is in conversion or in reading the equation.