Download Compounds: Formulas and Molar Masses and more Study notes Chemistry in PDF only on Docsity! Adapted from Workshop Chemistry 1 Compounds: Formulas and Molar Masses The formulas of ionic compounds give the ratio of ions in the crystal lattice. The formula NaCl indicates a one-to-one ratio of Na+ ions to Cl– ions in the NaCl lattice. In contrast, a molecular formula gives the actual description of the atoms in each individual molecule. For example, the molecular formula of hydrogen peroxide is H2O2, and each hydrogen peroxide molecule has the structure H—O—O—H. If hydrogen peroxide were an ionic compound (it’s not), its formula would be written as HO, reflecting the one-to-one ratio of hydrogen atoms to oxygen atoms in the compound. The molar mass of NaCl, sodium chloride, is calculated as the sum of the molar masses of its ions: 22.99 g/mol + 35.45 g/mol = 58.44 g/mol The molar mass of H2O2 is calculated as the sum of the molar masses of the atoms that make up the molecule: 2 × 1.008 g/mol + 2 × 16.00 g/mol = 34.02 g/mol The mole serves as a bridge between the invisible world of atoms & molecules and the tangible, visible world of mass which can be measured on a balance (weight). Molar mass (g/mol: grams per mole) is the unit that relates both worlds. As you saw above, the critical component is an accurate accounting of the number of each of the respective atoms in the molecule and their masses, which can be obtained from the Periodic Table. For iron(II)phosphate, Fe3(PO4)2, the number of atoms and atomic masses in 1 molecule of Fe3(PO4)2 : Fe atoms = 3; 55.85 x 3 = 167.55 amu P atoms = 2; 30.97 x 2 = 61.94 amu O atoms = 8; 16.00 x 8 = 128.00 amu Fe3(PO4)2 = 357.49 amu For iron(II)phosphate, Fe3(PO4)2, the total number of atoms of each respective atom in 1 mole of Fe3(PO4)2: [Fe3(PO4)2 molecules = 6.02 x 1023 molecules/mole Fe3(PO4)2] 1 mole of Fe3(PO4)2 equals: 3 Fe moles ; → 3 x 6.02 x 1023 atoms Fe /mole Fe3(PO4)2 = 1.81 x 1024 atoms Fe 2 P moles; → 2 x 6.02 x 1023 atoms P /mole Fe3(PO4)2 = 1.20 x 1024 atoms P 8 O moles; → 8 x 6.02 x 1023 atoms O /mole Fe3(PO4)2 = 4.82 x 1024 atoms O
