Convert between mass, amount and number of atoms with explicit units.
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Method and worked examples
A mole counts a specified entity. One mole contains exactly 6.02214076×1023 entities; use the rounded classroom value 6.02×1023mol−1 here unless specified otherwise. Equal moles of different elements contain equal numbers of atoms, but generally have different masses. Always state whether atoms, molecules, ions or formula units are counted.
Convert mass to amount with n=Mm, using grams and gmol−1. Convert amount to particle number with N=nNa, or reverse it with n=NaN. Keep extra digits during calculations and round the final answer to the precision supported by the given measurements and rounded constants.
Example 1
Find the amount in 4.86g Mg using M=24.3gmol−1.
n=Mm=24.34.86=0.200 mol Mg atoms.
Example 2
How many atoms are in 0.0500 mol helium using Na=6.02×1023mol−1?
N=nNa=0.0500×6.02×1023=3.01×1022 atoms.
Watch out:Every mole has the same entity count, but different atoms have different molar masses.
Counting atoms with the mole practice | Year 11 Chemistry | Ascent Arcade