A unit cell of an ionic crystal (a) shares some ions with other unit cells; (b) is the same as the formula unit; (c) is any portion of the crystal that has a cubic shape; (d) must contain the same number of cations and anions.

Chapter 3: Composition of Substances and Solutions 3.1 Formula Mass and the Mole Concept Formula mass – sum of average atomic masses of all atoms in the substance’s formula Covalent formulas represent # and types of atoms in a molecule; formula mass = molecular mass Ionic compounds contain cations and anions but do not represent the composition of a discrete molecule; formula mass is not molecular mass Measured in amu The Mole – the amount of substance containing the sam12number of discrete entities as the numbers of atoms in a sample of pure C weighing exactly 12 g An amount unit like pair or dozen Avogadro’s number (N ) = 6.02 x 10 23entities composing a mole A Molar mass – mass in gram