Concentration and Volume

Question 1

What is concentration in chemistry?

Answer 1

It tells you how much solute (stuff) is dissolved per unit volume of solution.

Question 2

What is the standard unit for concentration?

Answer 2

mol/dm³ (moles per cubic decimetre)

Question 3

Conversion: How many cm³ are in 1 dm³?

Answer 3

1 dm³ = 1000 cm³ = 1 litre

Question 4

Formula for concentration using moles and volume?

Answer 4

Concentration (mol/dm³) = Moles ÷ Volume (dm³)

Question 6

Example: How many moles are in 250 cm³ of 3 mol/dm³ NaCl?

Answer 6

Convert volume: 250 cm³ = 0.25 dm³
Moles = 3 × 0.25 = 0.75 moles

Question 8

Example: 3 g NaCl in 100 cm³ of water — what’s the concentration?

Answer 8

100 cm³ = 0.1 dm³
Concentration = 3 ÷ 0.1 = 30 g/dm³

Question 9

How to convert mol/dm³ to g/dm³?

Answer 9

Use the formula:
Mass (g) = Moles × Relative Formula Mass (Mr)

Question 10

Example: 0.04 mol/dm³ of H₂SO₄ — what is this in g/dm³?

Answer 10

Mr of H₂SO₄ = (1×2) + 32 + (16×4) = 98
Mass = 0.04 × 98 = 3.92 g/dm³

Question 11

What is molar volume of a gas at RTP (room temp & pressure)?

Answer 11

24 dm³ per mole (or 24,000 cm³)

Question 12

Formula to calculate gas volume at RTP?

Answer 12

Volume (dm³) = Moles × 24

Question 13

Example: What is the volume of 4.5 moles of chlorine at RTP?

Answer 13

4.5 × 24 = 108 dm³

Question 14

Example: How many moles in 8280 cm³ of hydrogen gas at RTP?

Answer 14

8280 cm³ = 8.28 dm³
Moles = 8.28 ÷ 24 = 0.345 moles

Question 15

Steps to calculate gas volume from mass in a reaction?

Answer 15

1. Calculate moles using mass ÷ Mr
2. Use balanced equation for mole ratio
3. Multiply moles of gas × 24 to get volume (dm³)

Question 16

Example: Volume of CO₂ from 2.4 g carbon burnt in oxygen?

C + O₂ → CO₂

Answer 16

• Equation: C + O₂ → CO₂
• Mr of C = 12
• Moles = 2.4 ÷ 12 = 0.2
• Mole ratio = 1:1, so CO₂ = 0.2 mol
• Volume = 0.2 × 24 = 4.8 dm³