CHAPTER 3 - AMOUNT OF SUBSTANCE

Question 1

What is the Avogrado constant?

Answer 1

6.02 x10^23

The number of partices in each mole of Carbon-12

Question 2

What is the equation to calculate moles?

Answer 2

Mass (m)/Molar mass (Mr)

(Mr M(e)n)

(Worked equation page 23)

Question 3

What is molecular formula?

Answer 3

Number of atoms of each element in a molecule

Question 4

What is empirical formula

Answer 4

Simplest whole-number ratio of atoms of each element in a compound

Question 5

What is relative molecular mass?

Answer 5

Compares the mass of a molecule with the mass of an atom of carbon-12

Question 6

What’s relative formula mass?

Answer 6

The mass of a formula unit compared with the mass of an atom of Carbon-12

Question 7

Describe the practical method to find out the formula of a hydrated salt

Answer 7

Weigh an Empty crucible

Add hydrated salt into the weighed crucible. Weigh the crucible and the hydrated salt

Using a pipe-clay triangle, support the crucible containing the hydrated salt on a tripod.

Heat the crucible and contents gently for about one minute, then heat strongly for a further three minutes

Leave the crucible to cool. Then weigh the crucible and anhydrous salt

Question 8

How to work out the formula of a hydrated salt

Answer 8

Calculate the amount, in mol, of anhydrous CuSO4.

Calculate the mass and amount, in mol, of water

Find the smallest whole-number ratio

Write down the value of x and the formula of hydrated CuSO4

(worked example pg 24)

Question 9

What are two assumptions that have been made during the calculation of Formula of Hydrated salt (Water of Crystallisation)

Answer 9

1 - All of the Water has been lost

2 - No further Decomposition

Question 10

A student carried out an experiment to calculate the water of crystallisation of two hydrated salts. For the first salt, the student did not remove all the water of crystallisation. For the second salt, the student removed all of the water of crystallisation but unfortunately the salt composed further.

For each salt, explain whether the students calculated value of x, the number of water molecules in the formula unit, would be greater or smaller than the actual value of x in data books

Answer 10

The first salt would not lose all H2O so x would appear to be smaller than it should be

The second salt loses more mass than just water so x would appear to be greater than it should be

Question 11

Unit conversions Between a cubic centremetre to millilitre
to Cubic decimetre to litre

Answer 11

1 cm3 = 1ml
1000ml = 1 litre
1dm3 = 1000ml = 1 Litre

Question 12

How would you work out the Concentration of a solution

Answer 12

c = n/V

(worked solution page 26)

Question 13

What is a standard solution?

Answer 13

A solution with a known concentration

Question 14

What is molar gas volume?

Answer 14

he volume per mole of gas molecules at a stated temperature and pressure

Question 15

What does RTP mean?

Answer 15

Room Temperature and Pressure

Question 16

What is RTP values?

Answer 16

20 degrees celsius

101 kPa (1 atm) pressure

Question 17

What is Vm at RTP?

Answer 17

24.0 dm3mol-1

Question 18

What is the ideal gas equation?

Answer 18

pV = nRT

P = Pressure
V = Volume
n = Moles
R = Gas constant = 8.31
T = Temperature in Kelvin

(Worked example page 29)

Question 19

What are the assumptions for the Ideal gas equation?

Answer 19

Random Motion
Elastic Collisions
Negligible size
No Intermolecular Forces

Question 20

Describe an experiment to find a relative molecular mass

Answer 20

Add sample of volatile liquid to a small syringe via a needle
Weigh small syringe

Inject the Sample into a gas syringe through the self-sealing rubber cap. Reweigh the small syringe to find the mass of the volatile liquid added to the gas syringe.

Place gas syringe in a boiling water bath at 100 degrees C. The liquid vaporises producing a gas. The pressure is recorded.

Question 21

What is Stoichiometry?

Answer 21

The balancing of numbers to give the ratio of the amount, in moles, of each substance

Question 22

Describe the method of Identifying an unknown metal

Answer 22

Set Up Gas syringe attached to a beaker,

Weigh Sample of metal and add to flask

Using a measuring cylinder, add 25.0cm3 of 1M HCl (dilute) (an excess) to the flask and quickly replace the bung.

Measure the maximum volume of gas in the syringe

(Worked example page 34)

Question 23

What is Theoretical Yield

Answer 23

The maximum possible amount of product in a reaction

Question 24

Why is the Theoretical yield so difficult to achieve?

Answer 24

The Reaction may not have gone to completion

Other reactions (side reactions) may have taken place alongside the main reaction

Purification of the product may result in loss of some product

Some of the Products may have escaped

Question 25

What is Actual Yield

Answer 25

The actual amount of product collected from experiment

Question 26

How would you calculate percentage yield?

Answer 26

Percentage yield = Actual yield/ Theoretical yield x 100

(worked example page 35)

Question 27

What is the limiting reagent?

Answer 27

The reactant that is not in excess and will be completely used up during he reaction

Question 28

What is Atom economy?

Answer 28

A measure of how well atoms have been utilised

Question 29

Why are reactions with high atom economies beneficial?

Answer 29

Produce large proportion of desired products and few unwanted waste products

Important for sustainability as they make but use of natural resources

Question 30

How would you calculate atom economy?

Answer 30

Atom economy = Sum of molar masses of desired products / Sum of molar masses of all products x 100

(worked example page 36)