L23. Mass, Moles, and Particles

The Mole

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L23. Mass, Moles, and Particles

How can grams tell us the number of particles in a sample?

Central Idea:

A mole is a counting unit chemists use to connect mass and particles: 1 mole = 6.022 × 10^23 particles (Avogadro’s number) and molar mass links grams to moles (for example, the molar mass of water is 18.02 g per mole). This gives the practical equality 18.02 g H2O = 1.00 mol H2O = 6.022 × 10^23 H2O molecules; the first equality (18.02 g = 1.00 mol) means that one mole of water weighs 18.02 grams, and the second equality (1.00 mol = 6.022 × 10^23 molecules) means one mole contains Avogadro’s number of water molecules. Use these equalities to convert: grams ÷ molar mass = moles, and moles × Avogadro’s number = particles, so you can count enormous numbers of molecules using ordinary laboratory masses.

How To Convert:

Measure or note the mass in grams (weigh the sample on a balance). Divide the mass by the molar mass (g ÷ g·mol−1 = mol) to get amount in moles; for water use 18.02 g·mol−1 (for example 36.04 g ÷ 18.02 g·mol−1 = 2.00 mol). Multiply moles by Avogadro’s number (mol × 6.022 × 10^23 = particles) to find particles. To reverse, divide the particle count by Avogadro’s number to get moles, then multiply by molar mass to get grams. Keep units with every number and cancel units during calculation to check work. Report answers with appropriate significant figures based on the measured mass.

Apply To Practice:

When you see a problem, decide whether you need mass, moles, or particle count. Quick rule: if grams are given, go grams → moles → particles; if particles are given, reverse. Tests to check your work: convert back to the original units, write units at every step, and for molecules compute atoms per molecule to compare atom counts. Also confirm you used the correct molar mass for the right chemical formula and check arithmetic carefully.

Synthesize Ideas:

Mass, moles, and particles are linked by molar mass and Avogadro’s number; use precise vocabulary like 'amount of substance' for moles and 'molar mass' for conversion. Always check work by writing short conversions (for example 18.02 g H2O = 1.00 mol H2O = 6.022 × 10^23 H2O molecules) and by explaining at the particle level what a mole count means. In writing, include units, show the arithmetic, and explain what the number tells you about molecules and their atoms.

Before accepting an answer: check units and arithmetic; tell the micro-level story (how many molecules and atoms are present); and convert back to the original units to confirm consistency. If possible, estimate whether the result is reasonable, and compare it to a known sample in grams.