CHAPTER 3 - AMOUNT OF SUBSTANCE

Question 1

What is amount of substance?

Answer 1

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

Question 2

What is avogadro’s constant?

Answer 2

The number of particles in each mole of carbon-12

Question 3

how do you calculate moles

Answer 3

Mass - g
————————————
Molar mass (mr) -g mol-1

Question 4

Define molecular formula

Answer 4

The number of atoms of each element in a molecule

Question 5

What is empirical formula?

Answer 5

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

Question 6

Define relative molecular mass

Answer 6

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

Question 7

Define relative formula mass

Answer 7

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

Question 8

What is water of crystallisation?

Answer 8

Water molecules that are bonded into the crystalline structure of a compound

Question 9

What are 2 assumptions that are made about the hydrated copper sulphate equation (water of crystallisation) ?

Answer 9

1. Assumes all water is lost - salt changes colour but only surface is seen / some water may be left inside
2. Assumes there is no further decomposition- some salts decompose further when heated / difficult to judge when no colour change has been seen

Question 10

Define concentration of a solution

Answer 10

The amount of solute, in moles, dissolved in each 1dm3 of solution

Question 11

How do you calculate concentration?

Answer 11

N (moles) = c (Moldm-3) x v (dm3)

Question 12

What is a standard solution?

Answer 12

A solution of known concentration

Question 13

How is a standard solution prepared

Answer 13

Dissolve exact mass of solute in solvent and making up solution to an exact volume

Question 14

How do you work out mass concentration?

Answer 14

Mass concentration (gdm-3) = n (moles) x mol concentration (moldm-3)

Question 15

When do equal volumes of different gases contain the same number of molecules?

Answer 15

At the same temperature and pressure

Question 16

What is molar gas volume (Vm) ?

Answer 16

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

Question 17

How do you calculate molar gas volume (dm3) ?

Answer 17

n = v dm3
————
24.0

Question 18

How do you calculate molar gas volume (cm3) ?

Answer 18

n = V cm3
————
24 000

Question 19

What are the assumptions made for the molecules making up an ideal gas?

Answer 19

• random motion
• elastic collisions
• negligible size
• no intermolecular forces

Question 20

What is the ideal gas equation?

Answer 20

pV = nRT

• p = pressure (Pa)
• V = volume (m3)
• n = amount of substance (mol)
• R = ideal gas constant (8.314 mol-1k-1)
• T = temperature (K)

Question 21

How do you convert dm3 or cm3 into m3 ?

Answer 21

dm3
———
1000

cm3
—————
1,000,000

Question 22

How do you convert MPa and kPa to Pa ?

Answer 22

kPa x 1000 =Pa

MPa x 1,000,000 =Pa

Question 23

How do you convert degrees Celsius to kelvin?

Answer 23

temp ( in celsius) + 273 = K

Question 24

What is stoichiometry of a reaction?

Answer 24

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

Question 25

What are balanced equations used to find?

Answer 25

• the quantities of reactants required to prepare a required quantity of a product
• the quantities of products that should be formed from certain quantities of reactants

Question 26

What is theoretical yield?

Answer 26

The maximum possible amount of product

Question 27

State three reasons why it is difficult to achieve 100% yield?

Answer 27

1. The reaction may not have gone to completion
2. Other reactions (side reactions) may have taken place alongside the main reaction
3. Purification of the product may result in the loss of some product

Question 28

How do you calculate percentage yield?

Answer 28

Actual yield
—————————— x 100
Theoretical yield

Question 29

What is a limiting reagent?

Answer 29

The reactant that is not in excess and will be completely used up first and stop the reaction.

Question 30

What is atom economy?

Answer 30

A measure of how well atoms have been utilised

Question 31

What happens to reactions with high atom economies?

Answer 31

• produce larger proportion of desired products and few unwanted waste products
• important for sustainability as they make the best use of natural resources

Question 32

Why can’t atom economy be over 100% yield.

Answer 32

It is based on the balanced chemical equation for a reaction and assumes a 100% yield.

Question 33

How do you calculate atom economy?

Answer 33

Sum of molar masses (mr) of desired products
—————————————————————— x 100%
Sum of molar masses (mr) of all products

Question 34

What are the benefits of improving atom economy?

Answer 34

• makes industrial processes more efficient
• preserves raw materials
• reduces waste