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Chemistry Calculations

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- 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?

Each element has a Mass Number

e.g. Na = 23, O = 16

Task:

Use your data book to find the mass numbers for:

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

If we add up all the mass numbers present

in a chemical we get the

Formula Mass number

Example:

H2O =

2 X Hydrogen + 1 X Oxygen

Mass Numbers

= 2 X 1 + 1 X 16

= A formula mass of 18

- NaOH
- HCl
- O2
- HNO3
- H2SO4
- (NH4)3PO4

Solution:

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

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

Example:

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.

- 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.

If I could weigh 6.02 X 1023molecules of water

it would weigh 18g

Q) What is the formula mass of water?

18

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

etc

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

(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?

(n)

M = n X FM

M = 0.25 X 28

M = 7g

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

have?

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?

- 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?

M = n X FM

n = M/FM

FM = M/n

(n)

- 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

- What is concentration?
- What are the units for concentration?
- How do we calculate moles in solutions?

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)”

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)

To work out the exact concentration of a

solution we use this triangle:

Number of

Moles

Volume

(L)

Concentration

(mol/L)

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

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

For some calculations we need to use

both triangles together

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