The Mole

Objectives Be able to write conversion factors. Be able to make conversions using dimensional analysis.

Dimensional Analysis Dimensional analysis is a method for converting values and units in the sciences. equality: two equal amounts 1 cup = 8 fl oz 1.61 km = 5280 feet 760 mm Hg = 101.3 kPa conversion factor: a ratio made from an equality; always = 1

Dimensional Analysis Problems Easy… How tall is Mt. Everest in km if its elevation is 29035 ft? More difficult… How much does it cost to drive 1400 miles if your car gets 32 miles per gallon and gas costs $3.49 per gallon?

Objectives Understand and explain the concept of the mole. Be able to determine the molar mass of a substance.

The Mole What are some terms that represent a specific number? pair = 2 dozen = 12 gross = 144 google = 10100 Avogadro’s Number mole = 6.02 x 1023 Why this number?? 1 atom Cu = 63.5 u 1 molecule H2O = 18.0 u 1 mole of Cu atoms = 63.5 g 1 mole of H2O = 18.0 g Different masses, but the same number of particles!

Molar Mass molar mass: mass in grams of 1 mole of any substance (usually written as # g/mol) Just “add up the masses” to find the molar mass of the following Se BaCl2 Fe2(CO3)3 NaC2H3O2∙3H2O

Why Do We Use Moles? Suppose you want to react Cu and S to make Cu2S. The ratio of atoms is 2:1. You can’t simply react a 2:1 ratio of grams You must react a 2:1 ratio of moles For example, 0.50 mol Cu and 0.25 mol S. You must calculate how many grams of Cu and S are needed from these amounts.

Objectives Be able to convert from moles to grams or from grams to moles for a given amount of substance. Be able to calculate and measure needed amounts of substance in the lab.

Converting g ↔ mol How many moles of copper are in a 12.4 g sample? What is the mass of 0.0472 mol S?

Converting g ↔ mol What is the mass of 0.0500 mol CaCl2? How many moles are in 4.83 g Sr(NO3)2?

Objectives Be able to calculate the percentage composition for a compound. Be able to determine the empirical formula of a compound from mass data.

Percentage Composition percentage composition: shows mass percentage of each element in a chemical compound find molar mass divide mass of each element by total molar mass What is the percentage composition of K2SO4?

Empirical Formulas empirical formula: a chemical formula determined from experimental data; shows the lowest whole-number ratio of the atoms convert masses to moles find lowest whole-number ratio—this is the formula Example: 1.587 g Cu reacts with 0.402 g S to form a compound. What is the empirical formula (and stock name) of the compound?

Empirical Formulas 0.56 g chromium is placed in a crucible and burned. The mass of the resulting chromium oxide is 0.82 g. What is the empirical formula?

Empirical Formulas What is the empirical formula of “red lead,” a compound that contains 90.7% lead and 9.33% oxygen? just use grams for % values

Objectives Be able to explain the difference between an empirical formula and a molecular formula. Be able to determine the molecular formula of a compound when given the molar mass and the empirical formula.

Molecular Formulas molecular formula: shows the actual # of the atoms in a molecule empirical formula: lowest whole-number ratio Example: butane molecular = C4H10 empirical = C2H5 The empirical formula of a compound is CH2O. The actual mass of its molecules is 180.0 g/mol. What is the molecular formula of the compound?