Chapter 3 - Amount of substance

Question 1

What is the amount of substance ?

Answer 1

A quantity (n) used to count the number of particles in a substance

Question 2

What is a mole ?

Answer 2

The amount of any substance that contains 6.02x10^23 particles

Question 3

What is avogadros constant ?

Answer 3

• Avogadro’s constant is 6.02x10^23

- It is the number of particles in each mole of carbon-12

Question 4

What is molar mass (Mr) ?

Answer 4

Molar mass gives the mass in grams in each mole of the substance

Question 5

What equation links molar mass, mass and amount ?

Answer 5

• n = m÷Mr
• n - number of moles (mol)
• m - mass (g)
• Mr - molar mass (g/mol)

Question 6

How do you work out the amount of particles ?

Answer 6

(m÷Mr) x avogadros constant

Question 7

How do you work out how many atoms there are ?

Answer 7

Add together all the atoms and then multiply by avogadros constant

Question 8

What is the molecular formula ?

Answer 8

The number of atoms of each element in a molecule.

Question 9

What is the empirical formula ?

Answer 9

The simplest whole-number ratio of atoms of each element in a compound.

Question 10

What is relative molecular mass?

Answer 10

The mass of a molecule compared with the mass of an atom of carbon-12.

Question 11

How do you calculate relative molecular mass ?

Answer 11

Worked out by adding together the relative atomic masses of the elements making up a molecule

Question 12

What is relative formula mass ?

Answer 12

It compares the mass of a formula unit with the mass of an atom of carbon-12

Question 13

How do you calculate the relative formula mass ?

Answer 13

It is calculated by adding together the relative atomic masses of the elements in the empirical formula

Question 14

How do you work out molecular formula from composition by mass ?

Answer 14

• Use the mass and mr to find out the number of moles

- Then use the ratio of moles to work out how many molecules there are of each element

Question 15

How do you work out molecular formula from composition by percentage ?

Answer 15

• Use the mass (percentage) and mr to find out the number of moles
• Then use the ratio of moles to work out how many molecules there are of each element

Question 16

In hydrated salts, what is the name given to water molecules?

Answer 16

Water of crystallisation

Question 17

What is the concentration of a solute ?

Answer 17

The amount of solute dissolved in a solution

Question 18

What is the equation for finding a solutions concentration ?

Answer 18

• n =cv

n - number of moles (mol)

c - concentration (mol/dm^3)

v - volume (dm^3)

Question 19

How do you get from mol/dm^3 to g/dm^3 ?

Answer 19

You multiply it by the mr

Question 20

What is a standard solution ?

Answer 20

A solution of known concentration

Question 21

How is it prepared ?

Answer 21

Dissolving an exact mass of the solute in a solvent and making up the solution to an exact volume

Question 22

What is molar gas volume ?

Answer 22

The volume per mole of gas molecules at RTP

Question 23

What is the value of molar gas volume at RTP ?

Answer 23

24 dm^3/mol

Question 24

What are the conditions of RTP ?

Answer 24

• 20 degrees

- 101 kPa (1 atm)

Question 25

What is the equation for molar gas volume ?

Answer 25

• n = v/mgv
  n - number of moles (mol)
  v - volume (dm^3)
  mgv - molar gas volume (dm^3/mol)

Question 26

What are the assumptions made about an ideal gas ?

Answer 26

• Random Motion
• Elastic collisions
• Negligible size
• No IMFs

Question 27

What is the equation for the ideal gas law ?

Answer 27

pV = nRT

Question 28

What is the value of the ideal gas constant ?

Answer 28

8.314 J/mol/K

Question 29

What are the standard conditions ?

Answer 29

The standard conditions are 298K and 100kPa

Question 30

cm3 to m3

Answer 30

x 10^-6

Question 31

dm3 to m3

Answer 31

• x 10^-3

Question 32

degrees C to K

Answer 32

+ 273

Question 33

KPa to Pa

Answer 33

x 10^3

Question 34

What is stoichiometry?

Answer 34

The balancing numbers that give us the moles of each substance in a balanced equation

Question 35

What are balanced equations used to find ?

Answer 35

• The quantities of reactants required to prepare a required quantity of a product.
• The quantities of products formed

Question 36

What is the actual yield ?

Answer 36

The actual amount of product that you get

Question 37

What is the theoretical yield ?

Answer 37

The maximum possible amount of product

Question 38

What is the percentage yield ?

Answer 38

The actual yield divided by the theoretical yield expressed as a percentage.

Question 39

Why is the theoretical yield so hard to achieve ?

Answer 39

• The reaction can be incomplete
• Side reactions can give off different products
• There can be a loss of product when handling the substance or during purification
• The reaction could be reversible

Question 40

What is the limiting reagent ?

Answer 40

• The reactant that is not in excess

- Limits the reaction

Question 41

How can it be worked out ?

Answer 41

Can be worked out by working out the amount in moles of each reactant and comparing that with the equation

Question 42

What is atom economy ?

Answer 42

Atom economy is a measure of how well atoms have been utilised

Question 43

How do you work out atom economy ?

Answer 43

The mr of desired products divided by the mr of all products, expressed as a percentage.

Question 44

What are the benefits of a high atom economy ?

Answer 44

• Produce a larger proportion of desired products and few waste products.
• Important for sustainability, the best use of natural resources.