Chemistry The Mole: MAC NOTES: The mole is the base unit in the metric system to measure the amount of a substance. Conversion factors are equalities with different units; they can be inverted, based on the needs of a problem. Avogadro’s conversion factor, 1 mol./6.02 X 1023 particles, is used to change from moles to a particle count and vice versa. Common particles in chemistry are atoms, ions, molecules, formula units. The molar mass conversion factor, 1 mol./grams of a pure substance—either as an element or compound—is used to change from moles to mass in grams and vice versa. The two conversion factors can be used together to solve problems starting with particles and ending in mass and vice versa. Number sense: Molar mass values are rounded to the hundredths place; most final answers have three significant digits; don’t forget your units and labels when working problems! 1

Chemistry The Mole: MAC NOTES: Determining the Molar Mass of Compounds: Step ONE: Determine the molar mass of each element in the compound using a periodic table (average atomic mass in grams, rounded to the 100th place). Step TWO: Multiply this molar mass per mole by an element’s subscript the if there is more than one mole (a subscript applies to the element to its LEFT) Step THREE: Use a subscript OUTSIDE a bracketed part of a chemical formula as a multiplier of the number of moles for each element INSIDE the bracket only. Step FOUR: Sum the molar masses of each element. 2

Chemistry The Mole: MAC NOTES: Determining Percent Composition: Step ONE: Find the molar mass of the compound. Step TWO: Calculate the mass of EACH element in the compound.  Step THREE: Divide the mass due to each element by the total molar mass of the compound and multiply by 100.  Determining Chemical Formulas: An empirical formula is the smallest whole-number ratio of moles that make up a compound. A molecular formula is the actual whole-number ratio of moles that make up a compound. They can be the SAME. 3

Chemistry The Mole: MAC NOTES: Determining Chemical Formulas CONT.: An empirical formula determination: Convert percentage or mass date per element in the compound into moles using the molar mass conversion factor. Divide all mole values by the smallest of the mole values. If the resulting moles per element can NOT be rounded—such as 0.25; 0.33; 0.50; or 0.75—then use an appropriate multiplier of all mole values (i.e, 2, 3, or 4) to make ALL whole numbers. A molecular formula determination: Determine the molar mass of the empirical formula. Divide the molar mass of the molecular formula (a given) by the molar mass of the empirical formula to get ‘n’ (a multiplier). Use ‘n’ to multiply the empirical formula subscripts by to get the actual number of moles for each element in the compound. 4