Chemical Calculations (2, 3, 4, 5)

Question 1

What is the definition and value of the Avogadro constant?

Answer 1

The amount of particles in one mole of a substance:
6.02 × 10²³

Question 2

What are relative atomic mass (Aᵣ) and relative molecular mass (Mᵣ)?

Answer 2

The ratio of the average mass of an atom/molecule to ¹⁄₁₂ the mass of one carbon-12 atom

Relative formula mass (also Mᵣ) is the proper term for ionic compounds

Question 3

What is the definition and formula of molar mass?

Answer 3

The mass of one mole of a substance:
molar mass (g/mol) = mass (g) / number of moles (mol)

Question 4

What is the definition and formula of percentage composition?

Answer 4

The percentage by mass of an element in a compound:
% composition = (number of atoms of element × Aᵣ of element / Mᵣ of compound) × 100%

Aᵣ and Mᵣ can be replaced with their respective molar masses

Question 5

What is the definition of an empirical formula?

Answer 5

The simplest formula of a compound
e.g. empirical formula of C₆H₁₂O₆ is CH₂O, all numbers are divided by 6

Question 6

How do you calculate a compound’s empirical formula?

Given the percentage composition of each element

Answer 6

1. Let total mass be 100g
2. Find number of moles of each element
3. Find mole ratio of elements
4. Find ratio of elements (round mole ratios to whole numbers if they are close)
5. (not always necessary) multiply ratio to obtain whole numbers

it is recommended to present calculations in a table; refer to pg 33

Question 7

How do you calculate a compound’s molecular formula?

When given its molecular mass and empirical formula

Answer 7

1. Let n be the number multiple of the subscript in its empirical formula e.g. (CH₂O)ₙ
2. Calculate molecular mass of empirical formula
3. n = molecular mass of compound / molecular mass of empirical formula
4. Multiply empirical formula subscripts by n
   e.g. molecular formula = (CH₂O)₆ =C₆H₁₂O₆

Question 8

What is the volume of 1 mole of gas at r.t.p.?

Answer 8

24 dm³

Hence a gas’s density is 24dm³/mol, not 24mol/dm³!

Question 9

What is the definition of a limiting reactant?

Answer 9

The reactant that is used up first in a reaction

Question 10

How do you calculate the limiting reactant?

Answer 10

1. Find mole ratio of reactants
2. Determine amount of moles of first reactant needed to completely react the second reactant
3. Based on amount of first reactant given, determine if it is the limiting reactant or in excess (other reactant is limiting reactant)
   if more is given than is needed, it is in excess. else, it is the limiting reactant

When amount of only one reactant is given, can assume there is no excess

Question 11

What are two assumptions made in chemical calculations using stochiometric ratios?

Calculating mass of reactants/products, moles of reactants/products etc.

Answer 11

• Reactants are 100% pure substances
• All reactants are converted to products

Question 12

What is the definition and formula of percentage yield?

Answer 12

The percentage by mass OR moles of a product that is obtained compared to how much would ideally be obtained:
% yield = (actual yield (g OR mol) / theoretical yield (g OR mol) ) × 100%

Actual yield and theoretical yield must be in the same units

Question 13

What is the definition and formula of percentage purity?

Answer 13

The percentage by mass of a substance that is pure/reacted:
% yield = (amount of substance reacted (g) / amount of substance used (g)) × 100%

Question 14

What is the definition and formula of mass concentration?

Answer 14

The amount of grams of solute in one dm³ of solution:
mass concentration (g/dm³) = mass (g) / volume (dm³)
OR
mass concentration (g/dm³) = molar concentration (mol/dm³) × molar mass (g/mol)

Question 15

What is the definition and formula of molar concentration?

Also known as the molarity of a solution

Answer 15

The amount of moles of solute in one dm³ of solution:
molar concentration (mol/dm³) = number of moles (mol) / volume (dm³)
OR
molar concentration (mol/dm³) = mass concentration (g/dm³) / molar mass (g/mol)

When asked for molarity, use M as units instead of mol/dm³

Question 16

What remains constant when a solution is diluted?

This topic (dilution) is not tested in 2024 SA1

Answer 16

The number of moles of solute

Question 17

What is the formula for concentration/volume of a solution before/after dilution?

This topic (dilution) is not tested in 2024 SA1

Answer 17

C (conc) × V (conc) = C (dil) × V (dil)

C, V, conc, dil are concentration, volume, concentrated, diluted

write conc and dil as subscripts

Question 18

What is the formula for concentration of ions in a solution?

Given the concentration of the ionic compound in the solution

Answer 18

concentration of ion (mol/dm³) = amount of ion in compound × concentration of compound in solution (mol/dm³)
e.g. concentration of H⁺ in 0.500mol/dm³ H₂SO₄ = 2 × 0.500 = 1.00mol/dm³

Question 19

What is the equivalence point and end point of a solution?

This topic (indicators) is not tested in 2024 SA1

Answer 19

Equivalence point is the point where the acid has been completely neutralized by the base
End point is the point where the indicator being used changes colour

Ideally, they are as close together as possible

Question 20

What are the requirements of a reaction suitable for titration?

Answer 20

1. Reaction must be fast
2. Reaction must go to completion
3. Only one reaction should take place
4. It must be possible to determine when the solution has been completely neutralized

(yes, this means our prac test did not meet the requirements)

Question 21

How do you calculate the concentration of a solution in titration?

For this example, refer to the solution being analysed as the acid

Answer 21

1. Write a balanced chemical equation for the reaction
2. Calculate the number of moles of the base used
3. Calculate the number of moles of the acid (mole ratio)
4. Divide number of moles by volume to get concentration of acid