Ch. 7: Chemical Formulas and Compounds 7.3 Uses of Chemical Formulas

Meaning of Formulas the subscripts in a formula give you: simplest ratio of atoms OR the number of atoms in a molecule ALWAYS provides you with a ratio of moles of atoms Example: C13H18O2 13 moles of C : 18 moles of H : 2 moles of O ratio can be used as a conversion factor

Example If I have 2.00 moles of C13H18O2, how many moles of each atom would I have?

Formula Mass mass of a molecule, ion, or formula unit sum of mass of all atoms in the chemical formula in amu Ex: H2O 18.01528 amu formula mass = molecular mass for molecular compound

Example Find formula mass of potassium chlorate. KClO3 1(39.0983) + 1(35.4527) + 3(15.9994) 122.549 amu

Molar Masses mass of one mole of pure substance numerically equal to formula mass units: g/mol Find molar mass of barium nitrate. Ba(NO3)2 1(137.327) + 2(14.00674) + 6(15.9994) 261.337 g/mol

Molar Mass in Conversions can be used as a conversion factor between grams and moles What is the mass in grams of 2.50 mol of oxygen gas?

Example Ibuprofen, C13H18O2, is the active ingredient in many pain relievers. Find molar mass: (13 x 12.011) + (18 x 1.00794) + (2 x 15.9994) =206.29 g/mol

Example If the tablets in a bottle contain a total of 33 g of ibuprofen, how many moles are in one bottle? How many molecules of ibuprofen are in the bottle?

Example What is the number of moles of carbon in that bottle? What is the total mass in grams of carbon in the bottle?

Conversion with Compounds grams and moles molar mass of compound moles and number of molecules or units Avogadro’s number moles and moles mole ratio from subscripts

Percentage Composition percentage by mass of each element in the compound assume you have 1 mole of sample total convert moles to grams for each element divide grams of each by molar mass of compound multiply by 100

Example Find percent composition for C13H18O2 Assuming 1 mole of sample, find grams of each element

Example Divide each mass by total molar mass and multiply by 100