SC9

Question 1

How do you calculate relative formula mass (Mᵣ)?

Answer 1

Add the relative atomic masses (Aᵣ) of all atoms in the formula.
Example: Mᵣ of CO₂ = 12 (C) + (2 × 16) (O) = 44

Question 2

What is the empirical formula?

Answer 2

The simplest whole number ratio of atoms in a compound

Question 3

How do you calculate empirical formula?

Answer 3

1. Find the mass or % of each element in the compound
2. Convert masses to moles (divide by Aᵣ)
3. Divide all mole values by the smallest mole number to get the ratio
4. Simplify to whole numbers (round or multiply if needed)

Question 4

Why does mass stay the same in a closed reaction?

Answer 4

Atoms are rearranged, not created/destroyed (law of conservation of mass)

Question 5

In 2Mg + O₂ → 2MgO, why does the product mass increase?

Answer 5

The mass of oxygen gas (O₂) is included in the total mass of MgO

Question 6

What is a mole?

Answer 6

A unit equal to 6.02 × 10²³ particles (Avogadro’s constant)

Question 7

How do you calculate moles from mass?

Answer 7

Moles = Mass (g) ÷ Mᵣ or Aᵣ
Example: Moles in 11g of CO₂ = 11 ÷ 44 = 0.25 mol

Question 8

How many moles are in 50cm³
of 0.2 mol/dm³ NaOH solution?

Answer 8

0.2 × (50 ÷ 1000) = 0.01 mol

Question 9

What is the concentration of solutions equation?

Answer 9

concentration =
mass of solute(g)
———————————-
volume of solution (dm³)

Question 10

convert cm³ to dm³

Answer 10

divide by 1000
1000cm³ =1dm³

Question 11

What is the molecular formular?

Answer 11

The actual number of atoms of each element in a molecule (a multiple of the empirical formula)

Question 12

How do you find the molecular formula?

Answer 12

1. Calculate the empirical formula
2. Find the Mᵣ of the empirical formula.
3. Divide the given Mᵣ of the compound by the empirical Mᵣ to get the multiplier.
4. Multiply the empirical formula by this number