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The mole is the SI unit for amount of substance. One mole contains the Avogadro constant number of specified entities, such as atoms, molecules, ions, or electrons.

$N = nNₐ where Nₐ = 6.022 × 10²³ mol⁻¹$

Identify which entity the question asks for, then multiply the amount in moles by Avogadro's constant and by the number of those entities in each formula unit or molecule when needed.

Specify the entity before calculating. One mole of H₂O contains one mole of molecules, two moles of H atoms and one mole of O atoms; multiplying by Nₐ without the formula-unit multiplier answers a different particle question.

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How many ions are present in 0.20 mol of $\left(\mathrm{NH}_{4}\right)_{2} \mathrm{SO}_{4}$ ?

Retrieve the quantitative chain: count entities with mN_A, sum relative masses from formulae, convert mass with n=m/M, derive formula ratios, use n=VC for solutions, and apply gas-volume ratios at the same temperature and pressure.

Before finalising, check the requested entity, formula subscripts, units, dm³ conversion, balanced-equation coefficients, and any limiting reactant.

1.4.1—Mole (mol)

The Mole and Particle Number

The Mole Summary

Converting Moles to Entities