Moles Moles Moles …Chapter 7.3

How do we use these?

These indicate which of the elements make up a substance.

These also indicate the number of ions or atoms that make up a given substance.

C6H12O6

Chemical Formulas

Atomic mass is the weighted average of the masses of the isotopes of that element.

This reflects the mass and the relative abundance of the isotopes as they occur in nature.

Remember atomic mass?

A mole is a unit of measurement of number of atoms of an element◦ 602,000,000,000,000,000,000,000 atoms are in

one mole of an element.◦ 1 mole = 6.02 x 1023 particles◦ This is called Avogadro’s number

The atomic mass of an element is the mass of one mole of that element in grams g/mol.

Using moles allows us to work with formulas in a measurable way – using grams.

Remember the mole?

“Atomic masses and chemical formulas” Take out your periodic tables Complete this assignment

Worksheet Assignment

We will review this assignment in five minutes

One mole of an element is equal to its atomic mass in grams

How can we use moles to measure amounts?

Equals

Equals

Equals

1 mole of Nitrogen

6.02 x 1023 atoms of Nitrogen 14.007 g of

Nitrogen

Moles are a way to make working with teeny tiny particles manageable.

Have you ever purchased a dozen eggs? Moles are not that much different. These are both ways of counting a number of items

How can we use moles and grams?

The mole is how we can measure something … …very small (atoms of an element) …in a manner that we can see (mass in grams).

Moles and Atomic Masses

For Bromine, the mass of one mole is

79.9 grams.This is also the mass

of 6.02 x 1023 particles of Bromine …

AND one mole of Bromine is 6.02 x 1023 particles of Bromine

This activity will help you to see the similarities between the mole (which is unfamiliar) and the dozen (which is quite familiar).

The Dozen and the Mole Activity

Now that we know how to work with single elements’ atomic masses, how can we apply this to more complicated substances?

If we can determine the mass of mole of a single element in grams, we can determine the mass of a mole of a compound in grams

Moving on …

As you know, a mole reflects 6.02 x 1023

representative particles.

What is a representative particle? A representative particle is one piece of the

substance … Depending on what you are talking about, it

might be ◦ An atom of a single element◦ A molecule of a substance◦ An ion◦ Anything!

Representative Particles

The molar mass of a substance is the mass of one mole of a substance in grams.

To determine it, you must know the chemical formula for the substance, and the atomic masses for each of the elements in it.

Molar Mass

MgCl2 Each particle of this compound contains:

◦ 1 atom of Magnesium◦ 2 atoms of chlorine

The molar mass of MgCl2 = ◦ 1 x atomic mass of Mg plus◦ 2 x atomic mass of Cl =

◦ 1x 24.305 g Mg/mole

◦ +2x35.453 g Cl/mole = 95.211 g/mol MgCl2

Determining Molar Mass

NaCl and HNO3

Remember, the sum of the atomic masses times the number of atoms of each kind of element is equal to the mass of one mole of the substance.

Examples:Na = 22.990 g/mol + Cl = 35.453 g/mol Therefore, NaCl has a molar mass of 58.4743 g/mol

Nitric acid is HNO3. Its molar mass isH = 1.0079 x 1 = 1.0079 g/mol N = 14.007 x 1 = 14.007 g/molO = 15.9997 x 3 = 47.9991 g/mol

Total = 63.014 g/mol HNO3

The subscripts in the formula tell you how many of each atom to include in your calculations

Ca3N2 will have three Ca atoms and 2 N atoms

If there are parenthesis in a formula, remember that the number outside the parenthesis acts as a multiplier for everything within the parenthesis.

Mg(OH)2 particles contain 1 Mg, 2 O and 2 H atoms per particle

Things to keep in mind

There are certain compounds called hydrates which are salts that bind water molecules in their crystal structure.

For example, consider ZnSO4 * 7 H2O

Zinc sulfate heptahydrate When determining the molar mass of a

hydrate, you add the molar masses of that number of water molecules to the salt portion of the formula

Molar Masses of Hydrates

ZnSO4 * 7 H2OMolar mass of zinc sulfate + 7 x molar mass of water

ZnSO4 molar mass =

1 Zn (65.39 g/mol) + 1 S (32.06 g/mol) + 4 O (15.999 g/mol) = 161.4476 g/mol zinc sulfate

2H + 1 O = 18.0148 g/mol water x 7 (hepta) = 126.1036 g/mol H2O

Total molar mass is 161.4476 g/mol zinc sulfate plus 126.1036 g/ 7 mol H2O

287.5512 g/mol ZnSO4 * 7 H2O

Zinc sulfate heptahydrate

Now that we know the amount of mass contributed by each individual element in a formula, we can determine the percentage of that element by mass

Mass of element in one mole x 100 = % element in compound

molar mass of compound

Calculating Percent Composition

Using Formulas in Problem SolvingThe percentage composition of each element in a compound can be determined using only the correct formula and the atomic masses.

Example: Sodium chloride or NaClNa = 22.99 % Na = 22.99 x 100 = 39.3%NaCl = 35.453 58.443 58.443g/mol

% Cl = 35.453 x 100 = 60.7% Cl 58.443

Notice that the total of the percentages is always equal or very close to 100%.

Percent of phosphorus in phosphate (PO4) In each phosphate, there are 1 P and 4 O

atoms

1 P x 30.97376 g/mol = 30.97376 g/mol 4 O x 15.9994 g/mol = 63.9976 g/mol 94.97136 g/mol PO4

30.97376 g P *100 = 32.6% P 94.97136 g PO4

Example of Percent Composition

What is the percent of water in ZnSO4 * 7 H2O?

From our previous calculations, the total molar mass is 161.4476 g/mol zinc sulfate plus +126.1036

g/ 7 mol H2O 287.5512 g/mol ZnSO4 * 7 H2O

126.1036 g/ 7 mol H2O x 100 = 43.9% water 287.5512 g/mol ZnSO4 * 7 H2O

Percent composition of water in a hydrate

We use dimensional analysis to work with moles, grams and particles.

Atomic masses, one mole, and Avogadro’s number can be used as conversion factors to convert between moles, grams and particles of an element

Calculations with moles

How can this concept help to convert between moles, mass and particles?

Equals

Equals

Equals

1 mole of Nitrogen

6.02 x 1023 atoms of Nitrogen 14.007 g of

Nitrogen

How many grams are there in 5.40 moles of Nitrogen?

Converting from MOLES to GRAMSThe conversion factor you will use is:1 mole N = 14.007 g N

5.40 moles N x 14.007 g N = 1 mole

Sample problem

75.6 g N

How many atoms are there in 40.6 g Nitrogen? Converting from GRAMS to ATOMS The conversion factor that you will use is

14.007 g Nitrogen = 6.02 x 1023 atoms

40.6 g N x 6.02 x 1023 atoms = 14.007 g N

Another sample problem

1.74 x 1024 atoms of N

How many moles are there in 2.3 x 1023 atoms of Nitrogen?

Converting from ATOMS to MOLES The conversion factor that you will use is: 1 mole Nitrogen = 6.02 x 1023 atoms

2.3 x 1023 atoms x 1 mole = 6.02 x 1023 atoms

Another sample problem

0.38 moles N

What is an empirical formula?

The empirical formula represents the lowest whole number ratio between elements in a compound.

For example, peroxide has 1 H for every 2 O in the compound

Empirical Formula

BUT … the empirical formula may not match the way that the actual molecules form.

For that, we have the molecular formula, which represents the whole number ratio of atoms in a molecule.

Sometimes these do match!

Ex: For water, H2O, the empirical and molecular formulas are the same.

Molecular Formulas

Let’s work through a problem in which you need to determine the empirical and molecular formulas for a substance.

Peroxide is a substance made from hydrogen and oxygen atoms.

We will determine the empirical and molecular formulas for peroxide:

Sometimes they don’t match

A 6.8 g sample of a compound contains 4.0 g H and 6.4 g O. It has a molar mass of 34.0146 g/mol.

Step 1. Determine the number of moles of each of the elements in the formula.

Figure out the empirical formula, and its molar mass.

From our previous calculation, we determine that there is a one-to-one ratio between H and O in this substance.

0.4 moles H = 0.4 moles O = 1:1 ratio

But this does not match the molar mass! HO = 1.0079 g/mol H + 15.9994 g/mol O =

17.0073 g/mol HO

Compare actual molar mass to the empirical formula’s molar mass

But ….. 17.0073 g/mol X 2 does equal 34.0146 g/mol.

So, the correct molecular formula for peroxide is H2O2.

Multiple the empirical formula until it has the correct molar mass.

We will work through some problems on this topic.

And, this is the last topic in this chapter, so we will move into chemical reactions next!

Practice with these