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 = 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 ft? More difficult… How much does it cost to drive 438 miles if your car gets 23 miles per gallon and gas costs $2.79 per gallon?

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

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

Molar Mass molar mass: mass in grams of 1 mole of any substance (usually written as # g/mol) Just “add up the masses” of the atoms in the formula to find the molar mass. What are the molar masses of Se, BaCl 2, Fe 2 (CO 3 ) 3 and NaC 2 H 3 O 2 ∙3H 2 O?

Objective 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 mol S?

Converting g ↔ mol What is the mass of mol CaCl 2 ? How many moles are in 4.83 g Sr(NO 3 ) 2 ?

Why Do We Use Moles? Suppose you want to react Fe and S to make FeS. The ratio of atoms is 1:1. You can’t simply react equal number of grams. You must react equal numbers of moles (same number of atoms). For example, 0.25 mol Fe and 0.25 mol S. You must calculate how many grams of Fe and S are needed from these amounts.

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 K 2 SO 4 ?

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: g Cu reacts with 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 numbers of the atoms in a molecule (empirical formula is lowest whole-number ratio) Example: butane empirical = C 2 H 5 molecular = C 4 H 10 The empirical formula of a compound is CH 2 O. The actual mass of its molecules is g/mol. What is the molecular formula of the compound?