Chemistry calculations
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Chemistry Calculations. Learning Intentions. How can we work out the formula mass of a chemical? What is a mole? How can we use formula mass and moles to work out grams of a chemical?. Part 1: Formula Mass. Each element has a Mass Number e.g. Na = 23, O = 16. Task:

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Chemistry Calculations

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Chemistry calculations

Chemistry Calculations

Learning intentions

Learning Intentions

  • How can we work out the formula mass of a chemical?

  • What is a mole?

  • How can we use formula mass and moles to work out grams of a chemical?

Part 1 formula mass

Part 1: Formula Mass

Each element has a Mass Number

e.g. Na = 23, O = 16


Use your data book to find the mass numbers for:

Ca, Cl, S, H, N, C, Ar

Formula mass

Formula Mass

If we add up all the mass numbers present

in a chemical we get the

Formula Mass number


H2O =

2 X Hydrogen + 1 X Oxygen

Mass Numbers

= 2 X 1 + 1 X 16

= A formula mass of 18

Chemistry calculations


  • NaOH

  • HCl

  • O2

  • HNO3

  • H2SO4

  • (NH4)3PO4

Nh 4 3 po 4



Split into 2 halves

2) PO4

P = 31 X 1

O = 16 X 4

PO4 = 95

1) (NH4)3

N = 14 X 1

H = 1 X 4

NH4 = 18

(NH4)3 = 18 X 3 = 54

54 + 95 = a formula mass of 149

Part 2 the mole

Part 2: The Mole

Units of Measurement:

We use units of measurement in life to give us an

easier way to see large numbers.

metre = 100 cm, litre = 1000 ml, kilogram = 1000 g

The mole is a unit of measurement

In chemistry we use it to tell us how many

atoms or molecules there are

The mole

The Mole

The next slide is Higher Chemistry but will help you understand what a mole is

The mole1

The Mole


If I had 1 mole of grapefruits I would have

602,214,150,000,000,000,000,000 (6.02 X 1023) grapefruits

which would be the same size as

If I had 1 mole of H2O I would have

6.02 X 1023 molecules of water.

How big is a mole

How big is a mole?

  • A mole of sand grains would cover the United States in approximately one centimetre of sand. 

  • A human body contains roughly one hundred trillion cells; there are roughly six billion people on Earth; so the total number of human cells on the planet is very close to one mole. 

  •  If you had exactly one mole of sheets of paper, you could make one million equal stacks from sea level on the earth that would pass the sun. 

  • If you had a mole of pennies, you could give out enough money to everyone in the world so that they could spend a million dollars every hour, day and night, for the rest of their lives.

The mole2

The Mole

If I could weigh 6.02 X 1023molecules of water

it would weigh 18g

Q) What is the formula mass of water?


Q) What is the relationship between the

formula mass and 1 mole of a chemical?

1 mole of a chemical is equal to

the Formula Mass in grams



I have 40g of NaOH

The Formula Mass (FM) = 40

Therefore I have 1 mole NaOH

FM = 40

1 mole = 40g

0.5 moles = 20g

0.25 moles = 10g


Part 3 mole calculations

Part 3: Mole Calculations

We can use moles and formula mass to work

out mass of chemicals needed/used

M = n X FM

n = M/FM

FM = M/n




If I have 9g of water, how many moles do I have?

n = M/FM

n = 9g/18

n = 0.5

What is the mass of

0.25 moles N2?


M = n X FM

M = 0.25 X 28

M = 7g

Chemistry calculations


1) If I have 9g water how many moles do I


2) If I have 5g NaOH how many moles do I have?

3) If I have 0.5 moles O2 how many grams do I have?

Chemistry calculations 2

Chemistry Calculations 2

Last lesson

Last Lesson

  • How can we work out the formula mass of a chemical?

  • What is a mole?

  • How can we use formula mass and moles to work out grams of a chemical?

Mole calculations

Mole Calculations

M = n X FM

n = M/FM

FM = M/n


Chemistry calculations


  • What is the formula mass of Mg(NO3)2

  • What is the formula mass of potassium carbonate?

  • Calculate the number of moles of 5.6g of CO

  • Calculate the number of moles of 8.2g calcium nitrate

  • Calculate the mass of 0.2 moles of magnesium oxide

  • Calculate the mass of 0.25 mole of NaCl

Learning intentions1

Learning Intentions

  • What is concentration?

  • What are the units for concentration?

  • How do we calculate moles in solutions?

Part 1 concentration

Part 1: Concentration

This bottle of acid is labelled “conc.”

What does that mean?

Today we learn how to work out how “strong” a solution is



“ the amount of solute (the thing that gets dissolved) dissolved in a volume of solvent (the thing that does the dissolving)”

Part 2 units for concentration

Part 2: Units for Concentration

Moles per litre written as mol/L or mol l-1

Note: on bottles of solutions we may see

concentration labelled as “M” (e.g. 1M)

Part 3 mole calculations1

Part 3: Mole Calculations

To work out the exact concentration of a

solution we use this triangle:

Number of








If I want to make 0.5Lof KOH (potassium hydroxide)

that has a concentration of 1mol/L how many moles

KOH do I need to dissolve?

n = C X V

n = 1 mol/L X 0.5L

n = 0.5 moles KOH needed

Example 2

Example 2

What is the concentration of a solution of HCl that

contains 0.1 moles of HCl dissolved in 250cm3

Convert to litres!

C = n/V

C = 0.1/0.25L

C = 0.004 mol/L

Using both triangles together

Using both triangles together

For some calculations we need to use

both triangles together

Chemistry calculations

Calculate the number of grams of NaOH needed to

make 50cm3 of NaOH with a concentration of 2 mol/L

First Triangle

n = ?

C = 2 mol/L

V = 0.05L

n = C X V

n = 2mol/L X 0.05L

n = 0.1 mole NaOH

Second Triangle

M = ?

n = 0.1 moles

FM = 40.

M = n X FM

M = 0.1 moles X 40

M = 4g

4g NaOH to make 50cm3 solution with a concentration of 2 mol/L

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