Topic 5 Formulae, Equation & Amounts of substance

Question 1

What is the ideal gas equation?

Answer 1

PV = nRT

P = Pressure (Pa)
V = Volume (M^3)
n = Amount of gas (Moles)
R = Gas constant (8.31 J mol-1 K-1)
T = Temperature (Kelvins - Celsius + 273)

Question 2

What assumptions need to be made when using the ideal gas equation?

Answer 2

• Particles are not attracted to each other.

- Particles have no volume.

Question 3

What is the equation for moles?

Answer 3

Moles = Mass / Mr

Mass is in grams

Question 4

What is Avogadro’s constant?

Answer 4

6.02 x 10^23

Question 5

What does Avogadro’s constant mean?

Answer 5

One mole of a substance is made up of 6.02 x 10^23 of those things. E.g. One mole of bananas is 6.02 x 10^23 bananas / One mole of oxygen is 6.02 x 10^23 atoms of oxygen.

Question 6

What is a limiting reagent?

Answer 6

A limiting reagent is a reactant in a reaction that dictates how much product is formed.

Question 7

What is the equation for % yield?

Answer 7

% yield =
Mass of product formed / Maximum theoretical mass
x 100

Question 8

What is the equation for atom economy?

Answer 8

Atom economy = 
Mass of wanted product from equation
/
Total mass of products from equation
x 100

Question 9

What is the equation for concentration?

Answer 9

Concentration = Moles / Volume
or
Concentration = Mass / Volume

Conc = mol dm⁻³ (mol/dm³)
Vol = dm³
Mass = g

Question 10

How do you convert different volumes into m^3?

Answer 10

dm³ / 1000 = m³

cm³ / 1000000 = m³

Question 11

How do you convert different units of pressure into Pa?

Answer 11

kPa x 1000 = Pa
MPa x 1000000 = Pa
1 atm of pressure = 100kPa

Question 12

How do you convert degrees Celsius to Kelvins?

Answer 12

Celsius + 273 = Kelvins

Question 13

How do you calculate back titrations?

Answer 13

Using the ratio of acid:alkali , calculate the moles of alkali that the acid reacts with (should tell you the moles of acid needed to neutralise) in the sample. Then calculate how many moles of alkali react in the resulting solution. Then calculate the final moles of unknown reactant (alkali should be in excess so: amount added - amount reacted = moles of unknown)

Question 14

What is the definition of empirical formula?

Answer 14

Simplest ratio of each atom in each compound.

Question 15

What is the definition of molecular formula?

Answer 15

The actual ratio of each atom in each compound.

Question 16

How do you find the molecular formula from the empirical formula and formula mass?

Answer 16

Empirical formula x
(Formula mass (Mr) / Formula mass of empirical formula)

E.g. Empirical = CH2 Mr = 42 Mr of empirical = 14
42 / 14 = 3
CH2 x 3 = C3H6

Question 17

What is the amount of a substance?

Answer 17

The physical quantity of a substance (n), measured in mol.

Question 18

What is the relative atomic mass?

Answer 18

The average mass of an element relative to 1/12 the mass of an atom of the 12C isotope.

The values are relative so have no units.

Question 19

What is the molar mass of a substance?

Answer 19

The mass of one mole of the chemical. The unit for this is g mol⁻¹.

Question 20

What is the percentage composition?

Answer 20

The percentage by mass of each of the elements in the sample compound.

Question 21

What is the species?

Answer 21

A collective noun used to refer generally to atoms, molecules or ions.

Question 22

What are the gas laws?

Answer 22

The behaviour of gases and are summarised by the ideal gas equation.

Question 23

What is an ideal gas?

Answer 23

A gas which obeys the ideal gas equation.

However, in practice real gases deviate to at least some extent from ideal behaviour.

Question 24

What are SI units?

Answer 24

The internationally agreed units for measurement in science.

Question 25

What is pressure?

Answer 25

The force per unit area. This is measured in pascals (Pa).

1 Pa is very small so pressure is often seen in kPa
1 atm = 100,000 Pa = 100 kPa

Question 26

What is volume?

Answer 26

The amount of space taken up by a sample. This is generally measured in dm³.

1 dm³ = 1 litre
1 dm³ = 1000 cm³
1 m³ = 1000 dm³
1 m³ = 1000000 cm³

Question 27

What is kelvin?

Answer 27

The SI unit for temperature on the absolute or kelvin temperature scale.

0K = absolute zero
273K = Freezing temp of water (0ᵒc)
373K = Boiling temp of water (100ᵒc)

Question 28

What is the molar volume of a gas?

Answer 28

The volume of one mole of the gas under stated conditions.

At room temperature and atmospheric pressure, the molar volume of all gases is 24 dm³ mol⁻¹.

Question 29

What is a titration?

Answer 29

A volumetric analysis technique for finding the concentrations of solutions and for the investigating the amounts of chemicals involved in reactions.

Question 30

What is a standard solution?

Answer 30

A solution with an accurately known concentration.

Question 31

What is a primary standard?

Answer 31

A chemical which can be weighed out accurately to make up a standard solution.

Question 32

What is the end-point of a titration?

Answer 32

The point at which a colour change shows that enough of the solution in the burette has been added to react with the chemical in the conical flask.

Question 33

What is the equivalence point of a titration?

Answer 33

The point at which the amount of reactant added from a burette is just enough to react exactly with all the measured amount of the chemical in the conical flask.

Question 34

What is a strong acid?

Answer 34

An acid that is fully ionised when dissolved in water.

E.g. Hydrochloric Acid

Question 35

What is a weak acid?

Answer 35

An acid that is only partially ionised when dissolved in water.

E.g. Ethanoic Acid

Question 36

What is an accurate measurement?

Answer 36

A measurement that is precise and free from bias.

Question 37

What are precise measurements?

Answer 37

Measurements that have values that are close together/concordant.

Precise measurements have a small random error.

Question 38

What is bias in measurments?

Answer 38

This arises from systematic errors which affect all measurements in the same way, by making them all higher or lower than the true value.

Question 39

What is yield?

Answer 39

The efficiency of a chemical process.

Question 40

What is atom economy?

Answer 40

A measure of how efficiently a chemical reaction converts the atoms in its reactants to atoms in the product.