Basic Chemistry - The Mole

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UNIT ONE: THE MOLE 1

The ole

Outcome Ugly OK Super

Define the term mole

Calculate the Molar mass of compounds

Calculate the number of particles present in a sample

Calculate the number of moles in a sample

Calculate the mass of a sample given the number of moles

present

Determine the percentage composition by mass of a

sample

Calculate the percentage composition of hydrated

compounds

Determine the empirical formula of a compound

Determine the molecular formula of a compound

Recap:

12

24

If you had a dozen eggs, how many eggs do you have?

If you had a kilo of bacon, how much bacon do you have in grams?

The Mole If we take the relative atomic mass of an atom and write it as grams, every

element that we do this for has exactly the same number of atoms in it.

Eg 20.18 g of neon has the same number of atoms as 39.95 g of argon.

This number of atoms is called a mole (abbreviation mol).

The number of particles in a mole is 6.02 X 1023

.

This is called Avagadro’s Number. (NA).

Mg

This is the number. It is always the of the 2

numbers. It tells you how many or how many are in

an atom.

In an atom the amount of protons = the amount of .

This is why all atoms have no charge.

This is the . It tells you how much an atom

weighs. It is calculated by adding the number of and

together.




Exercise: Calculate the number of particles in the following

a. 1 mole of Silver

b. 2 moles of Silver

c. 1 mol of water

d. 2.5 mol of water

e. 0.5 mol of water

f. 0.2 mol of water

Exercise: Calculate the following

a. The number of oxygens in 1 mol of water

b. The number of hydrogens in 1 mol of water

c. The number of legs in 1 mol of spiders

d. The number of legs in 5 mol of spiders

e. The number of legs in 0.2 mol of spiders

Moles of particles from a given formula

The formula of a compound tells the ratio of moles of atoms or ions in a

sample.

1 molecule of H2SO4 contains 2 hydrogen atoms, 1 sulfur atom and 4 oxygen

atoms, therefore 1 mol of H2SO4 molecules contains:

o mol of hydrogen atoms

o mol of sulfur atoms

o mol of oxygen atoms.




Formula for calculating number of particles

Number of moles = number of particles

Avagadro’s number

Exercise 1: How many moles are there in 3.3 X 1024

helium atoms?

Step 1 - Write the formula.

Step 2 - Substitute into the formula.

Exercise 2:a) How many nitrogen atoms are there in one molecule of nitrogen gas, N2?

b) How many nitrogen atoms are there in ten molecules of N2?

c) How many nitrogen atoms are there in 6.02 X 1023

nitrogen molecules?

d) How many nitrogen atoms are there in 1 mole of nitrogen molecules?

e) How many nitrogen atoms are there in 4 moles of nitrogen molecules?

Exercise 3

A sample of 3 mol of CuSO4.5H2O contains how many moles of

(i) S atoms (ii) H atoms

(ii) SO42

ions (iv) H2O molecules

(v) O atoms (vi) atoms altogether

Exercise 4

If a sample contains 3 x 1024

molecules of CH3COOH, calculate

(i) moles of CH3COOH

(ii) moles of C atoms

(ii) moles of atoms altogether




Molar Mass

If we take the relative atomic mass of an atom and express it in grams, this is

the mass of 1 mole

This is called the molar mass (M).

To find the molar mass of a molecular compound we add the relative atomic

masses together and write it in grams.

M(O2) =

M(CO2) =

M(CuSO4) =

Calculating the number of moles you have in a given sample can be done

using the following formula.

Example

How many moles of ethanol (C2H5OH) are there in 4.6 g of ethanol?

Step 1 Calculate the molar mass of ethanol.

Step 2 Write the formula.

Step 3 Substitute into the formula.

Exercise – Calculate the number of moles in the following samples

a. 80g of NaOH

b. 100g of ZnSO4

c. 100g of H2O




The formula can be rearranged in the following ways and used to calculate

the unknown.

Example

What is the mass of 2.5mol of ethanol (C2H5OH)?

Step 1 Calculate the molar mass of ethanol.

Step 2 Write the required formula.Step 3 Substitute into the formula.

Exercise – Calculate the mass of the following samples

a. 3 mol of NaOH

b. 1.5 x 10-2

mol of ZnSO4

c. 0.5 mol of H2O

Exercise 2 Now combine the two formula you have learnt so far – Calculate the mass

of the following samples

a. 3.01 x 10-23

molecules of HCl

b. 5.22 x 10-24

molecules of CO2

c. 5 x 10-23

molecules of NO2




Percentage Composition

The percentage composition of a substance is the percentage that an

element contributes to the total mass of a compound

E.g. SnF2 is made up of 1 Sn atom and 2 F atoms, but by mass it is made up by

75.7% Sn and 24.3% F

We can work out percentage composition by using the formula:

% of x in compound = mass of x in 1 mol of compound X 100

molar mas of compound

Example: Calculate the percentage composition of F in SnF2

Step 1 Calculate the molar mass of the compound.

Step 2 Calculate the mass of fluorine in SnF2 Step 3 Substitute into the formula.

Exercise Calculate the percentage composition of each element in the following:

a. H2O

b. Na2SO4

c.

HCNd. C6H12O6

Percentage composition of hydrated compounds

Hydrated compounds as those that have water attached to them in their

crystal lattice structure

Hydrated copper(II) sulfate has the formula CuSO4·5H2O.

This means that for each Cu2+ ion and SO42 – ion in the crystal lattice

structure, five water molecules are also included.

Heating the copper(II) sulfate crystals will remove the water molecules to

leave a white powder known as anhydrous copper(II) sulfate, CuSO4.

The percentage composition of the water in the hydrated compound can be

calculated using the same equation above

Example – Calculate the percentage composition of water in CuSO4·5H2O

Step 1 Calculate the molar mass of the compound.

Step 2 Calculate the mass of water in CuSO4·5H2O

Step 3 Substitute into the formula.




Empirical and Molecular Formula

The molecular formula gives the total number of atoms of each type in a

molecule.

E.G. Ethane = C2H6

Sulfuric acid = H2SO4.

The empirical formula is a formula which gives the lowest whole number

ratio of atoms in a molecule.

The empirical formula for ethane (C2H6) is CH3

Empirical formula for sulfuric acid is the same as the molecular formula,

H2SO4, since this cannot be simplified further.

Chemists determine empirical formulas of a compound by measuring the

percentage composition of each element present in the compound and then

going through the steps in the example below.

Example: A compound of carbon and oxygen is found to contain 27.3% carbon. What

is the empirical formula of the compound?

C O

Write the ratio by % or mass

Divide by atomic mass to change

to mole ratio

Divide by the smallest number to

change the ratio into whole numbers

The empirical formula is

Example 2 Washing soda crystals may be used to bleach linen. When dissolved in

water, washing soda (sodium carbonate, Na2CO3) forms crystals of a hydrated ionic

compound. When 5.00 g of washing soda crystals were dried 1.85 g of sodium

carbonate remained. Calculate the empirical formula of the hydrated compound.

Step 1 Calculate the mass of water present in the hydrated compound.

Step 2 We already know the formula for sodium carbonate, so we only need to find

out the ratio between sodium carbonate and water, so write the formula for these

two compounds at the top.

Step 3 Write the masses beneath the formulas.

Step 4 Divide by the molar masses.

Step 5 Divide by the smallest number.

Step 6 Write the formula.




H2O Na2CO3

Write the ratio by % or mass

Divide by atomic mass to change

to mole ratio

Divide by the smallest number to

change the ratio into whole numbers

The empirical formula is

Exercise 1: Hydroquinone is a liquid that is used as a photographic

developer. Determine the empirical formula of hydroquinone given that itcontains 65.4% carbon and 29.1% oxygen with the remainder being hydrogen.

Exercise 2: A 0.942 g sample of MgSO4 crystals was heated to drive off the

water of crystallisation. When completely dry, a residue of 0.461 g was

obtained. Determine the empirical formula of the hydrated compound.




Molecular Formula calculation

We can work out the molecular formula if we know the empirical formula

and the molar mass.

Example

A compound has the empirical formula CH. Its molar mass is 78 g/mol. What is the

molecular formula?

Step 1 Work out the empirical formula mass (ie like the molar mass, but just for the

empirical formula)

Step 2 Divide the molar mass by the empirical formula mass.

Step 3 Write the molecular formula.

Exercise: Mothballs are made of the organic compound naphthalene. It is a

hydrocarbon containing 93.7% carbon and 6.3 % H and has a molar mass of 128. Find

the empirical and molecular formula for naphthalene.

Homework: Review Questions Pg 65-66 + Mole Calculations worksheet

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Basic Chemistry - The Mole