Mathematics of Chemical Formulas Objectives: determine the atomic mass of elements and the formula mass of compounds determine the gram atomic mass of elements and the gram formula mass of compounds Calculate the mass of a given number of moles of a substance Calculate the number of moles in a given mass of a substance Calculate the number of molecules in a given number of moles of a substance Calculate the volume of a given mass of a gaseous substance at STP Determine the percentage composition of a substance from its formula Determine the empirical formulas and molecular formulas of compounds. Created by C. Ippolito Dec 2006

Stoichiometry study of the quantitative relationships that can be derived from chemical formulas and chemical equations chemical formulas represent substances chemical equations represent reactions between substances Created by C. Ippolito Dec 2006

Formula Mass Formula Mass (molecular mass) sum of the atomic masses of all of the atoms represented by the formula Find the Formula Mass of Ca(OH)2 Element Mass Atoms Total Ca 40 amu x 1 = 40 amu O 16 amu x 2 = 32 amu H 1 amu x 2 = 2 amu 74 amu Created by C. Ippolito Dec 2006

Gram Atomic Mass Gram atomic mass Gram formula mass that quantity of an element that has a mass in grams equal to its atomic mass Gram formula mass that quantity of a substance that has a mass in grams equal to its formula mass Gram molecular mass equivalent to formula mass for molecular substances Created by C. Ippolito Dec 2006

Mole 6.02 x 1023 atoms (a.k.a Avogadro’s number) # of atoms in a gram atomic mass of any element # of formula units in a gram formula mass of any substance gaseous substances at STP [0oC and 1 atm (101.3 kPa)] has a volume 22.4 liters 12/21 section 1 & 2 Created by C. Ippolito Dec 2006

Percentage Composition the percentage by mass of each element in a compound determined by experimentation Created by C. Ippolito Dec 2006

Formula Determination Formula determined if one knows: mass of each element in a sample or the percentage composition Example: In a 9.2-g sample 2.8 g is nitrogen and 6.4 g is oxygen. What is the empirical formula? Created by C. Ippolito Dec 2006

Formula Determination from Masses Example: In a 9.2-g sample 2.8 g is nitrogen and 6.4 g is oxygen. What is the empirical formula? Find mole ratio: nitrogen  oxygen Created by C. Ippolito Dec 2006

Formula Determination from Percentage Composition Example: What is the empirical formula of a compound with a percentage mass of 65.2% arsenic and 34.8% oxygen by mass? Find mole ratio: arsenic  oxygen sect 3 jan 2 Created by C. Ippolito Dec 2006

Empirical Formula (con’t) Created by C. Ippolito Dec 2006