3. Quantitative Chemistry

Why must chemical equations be balanced?

To show equal numbers of atoms for each element on both sides, respecting conservation of mass.

What does the subscript in a chemical formula represent?

The number of atoms of that element in a molecule or formula unit.

What does the coefficient in front of a formula represent?

The number of molecules or formula units involved.

How do you calculate relative formula mass (Mr)?

Add the relative atomic masses of all atoms in the formula.

Calculate the Mr of H2O.

(2 × 1) + 16 = 18

Calculate the Mr of CO2.

12 + (2 × 16) = 44

What is the formula for moles using mass and Mr?

Moles = mass ÷ Mr

Calculate moles in 36g of water.

36 ÷ 18 = 2 moles

What is Avogadro's number?

6.02 × 10²³ particles/mol

How many particles are in 2 moles of a substance?

2 × 6.02 × 10²³ = 1.204 × 10²⁴ particles

What is the mole ratio in the reaction: 2H2 + O2 → 2H2O?

2 moles H2 : 1 mole O2 : 2 moles H2O

If you start with 4 moles of H2, how many moles of O2 are needed?

2 moles O2 (because ratio is 2:1)

Write the formula to calculate mass from moles and Mr.

Mass = moles × Mr

Calculate the mass of 3 moles of CO2.

3 × 44 = 132g

Explain the limiting reactant concept.

The reactant that runs out first, stopping the reaction and limiting product amount.

If 5g of A reacts with excess B, which is the limiting reactant?

A, because it’s completely used up.

What is concentration?

Mass of solute dissolved per unit volume of solution (g/dm³).

Write the formula relating concentration, mass and volume.

Mass (g) = concentration (g/dm³) × volume (dm³)

Calculate the mass of solute in 0.5 dm³ of 2 g/dm³ solution.

0.5 × 2 = 1g

Explain how to balance equations using moles from masses.

Convert masses to moles, find mole ratio, then use ratios as coefficients.

In a reaction: Mg + 2HCl → MgCl2 + H2, if you have 12g Mg, how many grams of HCl are needed?

• Calculate moles Mg: 12 ÷ 24 = 0.5 mol

• HCl moles needed = 2 × 0.5 = 1 mol

• Mass HCl = 1 × (1+35.5) = 36.5g

So, 36.5g HCl needed.

What is the importance of balanced equations in quantitative chemistry?

They provide mole ratios needed to calculate quantities of reactants/products.

How do gases affect mass in reactions?

Gases can escape or enter, causing apparent mass changes if system isn’t closed.

How to explain mass increase when metal reacts with oxygen?

Oxygen from air combines with metal increasing total mass.

What is a mole in terms of particles?

One mole contains exactly 6.02 × 10²³ particles.

What is the significance of relative atomic mass (Ar)?

It’s the average mass of an atom compared to 1/12th of a carbon-12 atom.

How to calculate empirical formula from mass data?

1. Convert masses to moles.

2. Divide by smallest number of moles.

3. Use ratio to