2.1.3 - Amount of Substance

Question 1

What is the relative molecular mass (Mr)?

Answer 1

It compares the mass of a molecule with the mass of an atom of carbon-12.

Question 2

What is the equation linking Amount of Substance, mass, and molar mass?

Answer 2

Amount of Substance = Mass / Molar Mass

Question 3

What is the key difference between a molecular and an empirical formula?

Answer 3

The molecular formula shows the actual number of atoms in a molecule, while the empirical formula shows the simplest ratio of atoms in a compound.

Question 4

What is Molar Mass?

Answer 4

The mass of 1 mole of a substance and is measured in g mol⁻¹

Question 5

What is the first step when finding an empirical formula?

Answer 5

Convert mass or percentage by mass into moles.

Question 6

How do you get from °C to Kelvin?

Answer 6

+273

Question 7

What is ‘Theoretical Yield’?

Answer 7

The theoretical yield is the maximum yield we assume we would produce based on calculations

Question 8

What is ‘water of crystallisation’?

Answer 8

water molecules that are bonded into a crystalline structure of a compound.

Question 9

What is the definition of a molecular formula?

Answer 9

The molecular formula is the number of atoms of each element in a molecule.

Question 10

What is the equation that involves concentration, volume and moles?

Answer 10

𝑛 = c x V
(Mole = Concentration x Volume)
𝑛= mol
c= mol/dm⁻³
V= dm³

Question 11

What does the empirical formula represent in giant structures?

Answer 11

The ratio of atoms or ions in the structure, which remains the same regardless of the size of the crystal.

Question 12

What is the definition of an empirical formula?

Answer 12

The empirical formula is the simplest whole-number ratio of atoms of each element in a compound.

Question 13

What is Avogadro’s Constant?

Answer 13

6.02 × 10²³ atoms/mol

Answer 14

(1) Write down the given data

(2) Calculate the mass of water lost
(Mass of water lost = Mass of hydrated salt - Mass of anhydrous salt)

(3) Calculate moles of anhydrous salt
(Moles of anhydrous salt = Mass of anhydrous salt ÷ Mr of anhydrous salt​)

(4) Calculate the moles of water lost
Molesofwater
(Moles of water = Mass of water lost ÷ 18)

(5) Find the ratio of water to salt
Ratio
(Ratio = Moles of water ÷ Moles of anhydrous salt)
​

(6) Round to the nearest whole number
This gives you the number of water molecules, 𝑥, in the formula:
(Salt⋅𝑥H₂O)

Question 15

How do you calculate Atom Economy?

Answer 15

(Sum of molar mass of desired products ÷ Sum of molar mass of all products) x 100

Question 16

How do you find the empirical formula from mass or % by mass?

Answer 16

Convert mass/% to moles: 𝑛 = 𝑚/𝑀

Divide each mole value by the smallest number of moles

Write the empirical formula from the whole-number ratio

Question 17

What does anhydrous mean?

Answer 17

contains no water

Question 18

How do you get from cm³ to m³?

Answer 18

÷ by 1,000,000

Question 19

What is ‘Actual Yield’?

Answer 19

The actual yield is what we actually produce

Question 20

What does “Mr” stand for?

Answer 20

Relative molecular mass.

Question 21

What does the term ‘Excess Reactant’ mean?

Answer 21

the reactant that is in excess and you have more than you should actually have based on the mole ratio from a balanced symbol equation and therefore some will remain after a reaction has completed.

Question 22

What is the relative formula mass?

Answer 22

It compares the mass of a formula unit (used in giant structures) with the mass of an atom of carbon-12.

Question 23

What are some factors that can affect reaction yield?

Answer 23

Reaction may be reversible

Some of the product may be lost in handling or in separation from the reaction mixture

The reactants may not be pure

The reactants may form unexpected products

Some reactants may be lost

Question 24

What is Molar gas volume?

Answer 24

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

Question 25

What is needed to calculate molecular formula from empirical formula?

Answer 25

The relative molecular mass (Mr).

Question 26

How do you get from dm³ to m³

Answer 26

÷ by 1000

Question 27

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

Answer 27

Calculate the Mr of the empirical formula Divide the substance’s actual Mr by the empirical Mr Multiply the empirical formula by that number

Question 28

For which substances are empirical formulae especially important?

Answer 28

For substances that do not exist as molecules, such as ionic compounds (e.g., NaCl) and giant covalent structures (e.g., carbon and silicon).

Question 29

What does the term 'Limiting Reactant' mean?

Answer 29

the reactant that is not in excess and you have less than you should actually have based on the mole ratio from a balanced symbol equation and therefore will be used up first and stop the reaction.

Question 30

What is Room Temperature & Pressure (RTP)?

Answer 30

RTP is about 20ºC and 101kPa (1 atm) pressure At RTP, 1 mole of gas molecules has a volume of approximately 24.0 dm³. Therefore, at RTP, the molar gas volume is 24.0 dm³ mol⁻¹.

Question 31

What is the Ideal Gas Equation?

Answer 31

pV=𝑛RT (Pressure x Volume = Moles x ideal gas constant x Temperature) p=Pa V=m³ 𝑛=mol R= 8.31 J·mol⁻¹·K⁻¹ T=Kelvin

Question 32

Complete the sentence: We cannot use molecular formula for ___________ structures such as ___________, as the numbers would go into the ___________ of ____________.

Answer 32

We cannot use molecular formula for giant structures such as SiO₂, as the numbers would go into the billions of billions.

Question 33

What does hydrated mean?

Answer 33

Contains water

Question 34

What is 1 mole equal to in atoms?

Answer 34

6.02 × 10²³ atoms/mol

Question 35

How do you get from kPa to Pa

Answer 35

x by 1000

Question 36

What is the equation that links volume and moles at room temperature?

Answer 36

𝑛 = V÷24 (Mole = Volume÷24) 𝑛=mol V=dm³

Question 37

Complete the sentence: We write molecular formulae for __________ molecules such as __________.

Answer 37

We write molecular formulae for small molecules such as CO₂.

Question 38

What is Stoichiometry?

Answer 38

the ratio of the amount, in moles, of each substance in a chemical equation (essentially the ratio of the balancing numbers)

Question 39

What's the equation to calculate Perentage Yield?

Answer 39

(Actual Yield ÷ Theoretical Yield) x 100

Question 40

When is a gas considered ideal?

Answer 40

- Random motion – gas particles move randomly - Elastic collisions – particles do not lose kinetic energy when they collide - Negligible size – particles have negligible volume compared to the total volume of the gas - No intermolecular forces – particles are not affected by forces between molecules

Question 41

Why can't we use molecular formulae for giant structures like SiO₂ or NaCl?

Answer 41

Because these structures contain a vast number of atoms or ions, which would vary depending on the size of the crystal, making molecular formulae impractical.

Question 42

What formula is used to calculate moles?

Answer 42

𝑛=𝑚/𝑀