Use moles as the bridge: convert grams to moles using molar mass, then convert moles to particles using Avogadro’s constant. Reverse these operations when converting particles back to grams.
This is part of Structure 1.4: Counting particles by mass and is assessed at both SL and HL.
The molar mass, , is the mass of one mole of a substance, measured in . The Avogadro constant, , gives the number of particles in one mole:
This value is provided in the Chemistry data booklet.
The mole is an amount-of-substance unit, so both conversions depend on identifying the chemical entity. Molar mass connects a measurable macroscopic mass to moles, while the Avogadro constant connects moles to a sub-microscopic count. Keep the full calculator value until the final step to reduce rounding error.
| Starting quantity | Operation | Result |
|---|---|---|
| Grams, | Divide by molar mass: | Moles, |
| Moles, | Multiply by molar mass: | Grams, |
| Moles, | Multiply by : | |
| Particles, | Divide by : |
For a direct grams-to-particles conversion, always perform two steps: grams moles particles.
Calculate the number of water molecules in of .
First, calculate the molar mass:
Then convert mass to moles:
Finally, convert moles to molecules:
For ionic compounds, count formula units rather than molecules. For example, one mole of sodium chloride contains formula units of , but also sodium ions and chloride ions. If asked for total ions, multiply the formula-unit count by two.
A common misconception is that “particles” always means atoms. The required entity may be atoms, molecules, ions, or formula units, depending on the substance.
For a calculate question, show each conversion stage, include units, and give the final answer to appropriate significant figures. Always identify the type of particle requested; writing “molecules” instead of “atoms” can determine whether the final mark is awarded.
| Number of particles, |
| Moles, |
