10.1 The Mole: A Measurement of Matter

• Three methods for measuring the amount of a substance are by count, by mass, and by volume.
• A mole of any substance always contains Avogadro’s number of representative particles, or 6.02 × 10²³ representative particles.
• The atomic mass of an element expressed in grams is the mass of a mole of the element.
• To calculate the molar mass of a compound, find the number of grams of each element contained in one mole of the compound. Then add the masses of the elements in the compound.

10.2 Mole–Mass and Mole–Volume Relationships

• The molar mass of an element or compound is the conversion factor for converting between the mass and the number of moles of a substance.
• One mole of any gas occupies a volume of 22.4 L at standard temperature and pressure. One mole of any substance contains Avogadro’s number of particles, so 22.4 L of any gas at STP contains 6.02 × 10²³ representative particles of that gas.

10.3 Percent Composition and Chemical Formulas

• To determine the percent by mass of any element in a given compound, divide the element’s mass by the mass of the compound and multiply by 100%.
• An empirical formula of a compound is the simplest whole-number ratio of atoms of the elements in the compound.
• The molecular formula of a compound is either the same as its experimentally determined empirical formula, or it is a simple whole-number multiple of it.