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 convertingvalues and units in the sciences.

equality: two equal amounts1 cup = 8 fl oz1.61 km = 5280 feet760 mm Hg = 101.3 kPa

conversion factor: a ratio made from an equality; always = 1

Dimensional Analysis Problems

Easy…How tall is Mt. Everestin km if its elevation is29035 ft?

More difficult…How much does it costto drive 438 miles if your car gets 23 miles per gallonand gas costs $2.79 pergallon?

Objectives

• Understand and explain the concept of the mole.

• Be able to determine the molar mass of a substance.

The Mole

pair = 2dozen = 12gross = 144google = 10100

What are some terms that represent a specific number?

mole = 6.02 x 1023

Why this number??1 atom Cu = 63.5 u1 mole of Cu atoms = 63.5 g

1 molecule H2O = 18.0 u1 mole of H2O = 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 formulato find the molar mass.What are the molar masses of Se, BaCl2, Fe2(CO3)3

and NaC2H3O2 3H∙ 2O?

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 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?

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 percentageof 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 Formulasempirical formula: a chemical formula determined fromexperimental data; shows the lowest whole-numberratio 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 forma 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 Formulasmolecular formula: shows the actual numbers ofthe atoms in a molecule (empirical formula is lowest whole-number ratio)Example: butaneempirical = C2H5 molecular = C4H10

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?