2.1.3: Amount of substance

Question 1

What is an empirical formula?

Answer 1

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

Question 2

How to calculate empirical formula

Answer 2

● Divide the amount of each element by its molar mass
● Divide the answers by the smallest value obtained
● If there is a decimal, divide by a suitable number to make it into a whole number

Question 3

What is the unit used to measure amount of substance?

Answer 3

Mole

Question 4

What does the Avogadro constant represent?

Answer 4

The number of atoms per mole of the carbon-12 isotope

Question 5

How to calculate the mass of 1 mole of the element?

Answer 5

Mass of 1 mole = relative atomic mass in grams

Question 6

How to calculate moles when mass and molar mass (mr) are given?

Answer 6

Moles (mol) = mass (g) / molar mass (mr) (g mol -1)

Question 7

State Avogadro’s law

Answer 7

Under the same temperature and pressure, one mole of any gas would occupy the same volume.

Question 8

How much volume does a gas occupy, at room temperature and

pressure?

Answer 8

24 dm3 or 24000 cm3

Question 9

Define molar gas volume

Answer 9

The volume per mole of gas molecules

Question 10

Why do different gas particles occupy the same volume?

Answer 10

The gas particles are very spread out,
hence individual differences has no
effect.

Question 11

How to calculate moles when gas volume is given?

Answer 11

Moles (mol) = volume (dm3) / 24

Moles (mol) = volume (cm3) / 24000

Question 12

What are the ideal ways in which gases behave?

Answer 12

● They are in continuous motion
● No intermolecular forces experienced
● Exert pressure when they collide with each other or
container
● No kinetic energy is lost in the collisions
● When temperature increases, kinetic energy of gases also increase

Question 13

Write down the ideal gas equation (in words and symbols, including the units?

Answer 13

pV = nRT
Pressure (Pa) x volume (m3) = number of
moles (mol) x gas constant (8.314 J/mol-1)x temperature (K)

Question 14

1 atmospheric pressure is equal to how many pascal?

Answer 14

1 atm = 101325 Pa

Question 15

0℃ is equal to how many kelvin?

Answer 15

273 K

Question 16

What does concentration of a solution mean?

Answer 16

It is the amount of solute dissolved in 1

dm3 of solvent

Question 17

How do you calculate moles when concentration and volume are

given?

Answer 17

Moles (mol) = concentration (mol dm-3) x

volume (dm3)

Question 18

What is a standard solution?

Answer 18

A solution of known concentration

Question 19

Write down the steps to prepare a standard solution

Answer 19

1. Weigh the solute using the weigh by difference method
2. In a beaker dissolve the solute using the solvent
3. Pour the solution into a volumetric flask
4. Rinse the beaker using the solution and add it to the flask
5. Add solvent to the flask carefully until it reaches the graduation line
6. Mix the solution thoroughly to ensure complete mixing

Question 20

What does the terms concentrated and dilute mean?

Answer 20

Concentrated - large amount of solute per dm3 of solvent

Dilute - small amount of solute per dm3 of solvent

Question 21

What is a species in a chemical reaction?

Answer 21

Any particle that takes part in a reaction

Question 22

What does percentage yield mean?

Answer 22

The efficiency of which reactants are

converted into products

Question 23

What are the reasons for not obtaining 100% yield? (5 marks)

Answer 23

● Reaction may be at equilibrium
● The reactants may be impure
● Side reactions could happen
● Reactants or products may be left behind while
transferring
● Loss of products during separation and purification

Question 24

How is percentage yield calculated?

Answer 24

Percentage yield = (actual amount of product (mol) / theoretical amount of product (mol)) x 100

Question 25

What does atom economy tell us about?

Answer 25

The proportion of desired products
compared with all the products formed in
the reaction

Question 26

How is atom economy calculated?

Answer 26

Atom economy = (molecular mass of desired product / sum of the molecular masses of all products) x 100

Question 27

Does 100% yield mean 100% atom economy?

Answer 27

No, even if all the reactants are converted into products, not all products of the reaction will be the required products

Question 28

Which type of reaction has 100% atom economy?

Answer 28

Addition reactions (two or more reactants are combined to form a product)