Chemical Formulas & Formula Mass

Subscripts in Chemical Formulas Subscripts show # atoms each element in formula No subscript – understood to be 1 Subscripts refer to element immediately preceding them Subscripts after parentheses refer to everything inside ( )

Coefficients Numbers in front of formulas Refer to everything in formula to count atoms: coefficient x subscript

Chemical Formulas H2O: 2 hydrogen, 1 oxygen NH3: 1 nitrogen, 3 hydrogen CH4 has ? CO2 has ? AgNO3 has ? Al(NO3)3 has ? 1 carbon, 4 hydrogen 1 carbon, 2 oxygen 1 silver, 1 nitrogen, 3 oxygen 1 aluminum, 3 nitrogen, 9 oxygen

No. of atoms in formulas Subscript outside ( ) x subscript inside ( ) = # of atoms Al(NO3)3 Al = 1 N = 1 X 3 = 3 O = 3 X 3 = 9 Total = 13

No. of atoms in formulas Mg3(PO4)2 Mg = 3 P = 2 X 1 = 2 O = 4 X 2 = 8 Total = 13

Molar mass of an element Molar mass = mass in grams of 1 mole of any pure substance Molar mass of element = to atomic mass units change from amu to grams per mole

Molar mass of elements Na: atomic mass of 23 amu so molar mass = 23grams/mole K: atomic mass of 39 amu so molar mass = 39 grams/mole Ca: atomic mass of 40 amu so molar mass = 40 grams/mole

Molar Mass of Compounds = sum of masses of atoms in formula Round atomic masses to whole number Ex: H2O 2 X H = 2 X 1 = 2 1 X O = 1 X 16 = 16 Sum = 2 + 16 = 18 g/mol

Molar Mass of Compounds Try CH4 1 x C = 1 X 12 = 12 4 x H = 4 X 1 = 4 Sum = 4 + 12 = 16 g/mol