Chapter 6 · Chemical Composition
6.1How Much Sodium?
7 min · two checks
Predict
Why can’t a nutrition label count sodium atoms one by one?
The idea
- Explain why counting atoms requires a weighing strategy.
- Connect a lab mass to an invisible number of particles.
A label that says 400 mg of sodium is making a particle claim with a balance. Individual sodium atoms have a mass near 3.82 × 10⁻²³ g. Dividing the label mass by that atomic mass gives a count so large that writing it on a label would be useless. Chemistry’s answer is a counting unit sized for the gram scale.
That unit is the mole, defined so that one mole of any element has a mass in grams numerically equal to the atomic mass in atomic mass units. The rest of this chapter is the conversion machinery: grams ↔ moles ↔ particles, then formulas used as additional conversion factors. Sodium on a label is the motivation; the mole is the tool.
Keep these
- Weighable samples contain enormous numbers of atoms.
- The mole connects a mass you can measure to a count you cannot.
- Atomic mass in u and molar mass in g/mol share the same number.