Summary
Atoms are far too small to count one by one, so chemistry counts them in moles: one mole is 6.022 · 10²³ particles (Avogadro’s number). The molar mass says how much a mole weighs and is found by adding the atomic masses in the formula: water, H₂O, has 2 · 1 + 16 = 18 g/mol. With it you convert grams to moles (n = m ÷ M) and moles to grams.
A balanced equation is read in moles: 2 H₂ + O₂ → 2 H₂O means 2 mol of hydrogen react with 1 of oxygen. To calculate, convert the data to moles, apply the ratio of the coefficients and convert back to the unit asked for. The reactant that runs out first is the limiting one and decides how much product forms; in practice less forms (yield) and samples are not pure (purity). For gases, PV = nRT with temperature always in kelvin, and 1 mol takes up 22.4 L at 0 °C and 1 atm.
Key points
- 1 mol = 6.022 · 10²³ particles (Avogadro’s number).
- Molar mass: the sum of the atomic masses in the formula, in g/mol.
- n = m ÷ M: grams to moles; m = n · M: moles to grams.
- Lavoisier: mass is conserved. Proust: each compound has a fixed mass ratio.
- The coefficients of a balanced equation are mole ratios, not gram ratios.
- The limiting reactant is the one that runs out first, not the one with fewer moles.
- Yield = obtained ÷ theoretical · 100; it never exceeds 100 %.
- Gases: PV = nRT, with T in kelvin; 1 mol takes up 22.4 L at 0 °C and 1 atm.