Calculations (Moles)

Question 1

How is the number of moles calculated using mass and molar mass (Ar or Mr)?

Answer 1

Moles (mol) = Mass (g) / Molar mass (Ar or Mr).

Question 2

How is the number of moles calculated using concentration and volume?

Answer 2

Moles (mol) = Concentration (mol/dm³) × Volume (dm³).

Convert volume from cm³ to dm³ (1 dm³ = 1000 cm³).

Question 3

How is the number of moles calculated using the volume of gas at r.t.p.?

Answer 3

Moles (mol) = Volume (dm³) / 24.

Only use this if the gas is at room temperature and pressure (r.t.p.), or if it’s stated that 1 mole of gas occupies 24 dm³.

Question 4

How is the number of moles calculated using particles (atoms, molecules)?

Answer 4

Moles (mol) = Number of particles / Avogadro’s constant (6.02 × 10²³).

Question 5

What are mole ratios and how are they used?

Answer 5

Mole ratios are the ratios between the coefficients of reactants and products in a balanced chemical equation. These ratios can be used to calculate the moles of substances involved in the reaction.

Question 6

What is an empirical formula?

Answer 6

An empirical formula is the simplest whole number ratio of elements in a compound.
Steps to find it:

1. Divide the mass of each element by its atomic mass (Ar).
2. Divide the ratios by the smallest ratio.
3. Approximate if necessary, but avoid ratios like 0.5, 0.33, or 0.25.

Question 7

Example of finding an empirical formula. If the mass of C is 80% and H is 20%, and Mr = 45

Answer 7

(80 / 12) : (20 / 1)
80% C = 6.67
20% H = 20
Divide by the smallest value (6.67):
1 : 3
Empirical formula = CH₃.

Question 8

How is the molecular formula related to the empirical formula?

Answer 8

Molecular formula = n × Empirical formula.
To find “n”:

1. Calculate the Mr of the empirical formula.
2. Divide the given Mr by the empirical formula’s Mr.
3. Multiply the empirical formula by “n” to get the molecular formula.

Question 9

How do you calculate percentage yield?

Answer 9

Percentage yield = (Actual yield / Theoretical yield) × 100.

Question 10

How do you calculate the percentage of an atom in a compound?

Answer 10

Percentage of atom = (Ar of atom / Mr of compound) × 100.

Question 11

How do you calculate the X value of water in a hydrate (example problem)?

Answer 11

1. Find the mass of substance and water:
2. Write the ratio:
3. Calculate moles of each:
4. Find the ratio:
5. Conclusion: The X value is 10, so the formula is FeSO₄·10H₂O.

Workout:
* Mass of FeSO₄ = 16g - 10g = 6g
* Mass of water = 20g - 16g = 4g
* FeSO₄ : Water = 6g : 4g
* Moles of FeSO₄ = 6 / 152 = 0.04 mol
* Moles of H₂O = 4 / 18 = 0.22 mol
* Divide by the smallest value:
* FeSO₄: 0.04/0.04 = 1
* Water: 0.4/0.04 = 10