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Chemistry · 02 / 10

The mole: connect mass and particles

High school foundations · Worked example and 4 exercises with solutions, at your pace.

Need a foundation?Units: numbers have dimensionsBalance a reaction equation
UnderstandSee an exampleGuided exercise3 progressive challenges

The principle, simply

Amount of substance counts entities in defined-size groups called moles. Molar mass M is the mass of one mole in g/mol. Thus n = m/M. A chemical formula specifies atom ratios, not a mass in grams.

n=m/M
THE PICTURE TO REMEMBERThe mole: connect mass and particles
  1. 01Mass m
  2. 02Mass per mole M
  3. 03Ratio m/M
  4. 04Amount n

WORKED EXAMPLE

From principle to reasoning

5.844 g of NaCl at 58.44 g/mol gives 0.100 mol; the units cancel to mol.

Solve a problem in this subject

  1. Identify species

    Write formulas and useful states. Distinguish atoms, ions, molecules and amount of substance. Total charge is conserved.

  2. Connect quantities

    Use m = nM and C = n/V with consistent units. V is the final solution volume.

  3. Use coefficients

    Balance before building an extent table. Compare initial amounts divided by coefficients to identify the limiting reactant.

  4. Check context

    Remaining amounts cannot be negative. Check conservation, units and assumptions: dilution, medium, completion or equilibrium.

YOUR TURN

01 · Try before checking

Find the mass of 0.2 mol with molar mass 18 g/mol.

One hint

Isolate m in n = m/M.

Reveal and understand the solution

m = nM = 3.6 g: one fifth of a mole.

Compare method, conditions and answer; identify the exact part to revisit.

IMPROVE THROUGH PRACTICE

Three additional challenges with solutions

Start with the first challenge. Write your method before opening a hint; use the solution to find the first step that needs work.

Reflect after practising
02 · Recognise and understandLink to this exercise

A substance has M = 40 g/mol. Find the amount in 8 g.

A hint to get started

n = m/M.

Step-by-step solution
  1. n = 8/40.
  2. n = 0.20 mol.
  3. Mass and amount of substance use different units.

Which step did you miss? Retry without help, then explain why the method works.

03 · Apply the methodLink to this exercise

Find the mass of 0.5 mol water with M(H₂O) = 18 g/mol.

A hint to get started

Use the inverse relation m = nM.

Step-by-step solution
  1. m = 0.5 × 18.
  2. m = 9 g.
  3. Using g/mol produces a mass in grams.

Which step did you miss? Retry without help, then explain why the method works.

04 · Go furtherLink to this exercise

Compare 1 mol CO₂ and 1 mol H₂O: same mass and same molecule count?

A hint to get started

A mole fixes particle count, not mass.

Step-by-step solution
  1. Each contains Avogadro’s number of molecules.
  2. Their molar masses differ: about 44 and 18 g/mol.
  3. They have equal molecule counts but different masses.

Which step did you miss? Retry without help, then explain why the method works.

The trap to avoid

Keep mass, amount and molar mass distinct using their units.

Without notes, can you…

  • Explain the principle in your own words?
  • Rebuild the diagram and its connections?
  • Solve the exercise and justify the method?

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