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chemistry
chemistry as unit 1

Topic 1: Formulae, Equations and Amount of Substance

IAL AS Chemistry – Revision Notes (Exam-Oriented)

IAL AS Chemistry – Topic 1: Formulae, Equations and Amount of Substance

Edexcel International AS Chemistry – Revision Notes, Exam Practice & Formula Sheet

1. Key Definitions

  • Atom: Smallest unit of an element retaining its chemical properties.
  • Element: Substance made of only one type of atom.
  • Ion: Charged particle formed by loss or gain of electrons.
  • Molecule: Two or more atoms covalently bonded.
  • Compound: Two or more different elements chemically bonded.
  • Empirical formula: Simplest whole-number ratio of atoms.
  • Molecular formula: Actual number of atoms in a molecule.

2. The Mole and Avogadro Constant

The mole (mol) is the unit for amount of substance.

Avogadro constant:

6.02 × 1023 mol−1

One mole of any substance contains 6.02 × 1023 particles.

3. Chemical Equations

Balanced Equations

  • Mass and charge must be conserved
  • Correct stoichiometric coefficients required

State Symbols

  • (s) solid
  • (l) liquid
  • (g) gas
  • (aq) aqueous

Ionic Equations

  • Only reacting ions shown
  • Spectator ions removed
  • Charges must balance

4. Relative Masses

  • Relative atomic mass (Ar): Based on carbon-12 scale
  • Relative molecular mass (Mr): Sum of Ar values
  • Relative formula mass: Used for giant ionic/covalent structures
  • Molar mass: Mass per mole (g mol−1)

Parts Per Million (ppm)

Used for very low concentrations, especially atmospheric gases.

5. Concentration of Solutions

mol dm−3:

Concentration = moles ÷ volume (dm3)

g dm−3:

Concentration = mass (g) ÷ volume (dm3)

Titration calculations are not required.

6. Empirical and Molecular Formula

  1. Convert mass to moles
  2. Divide by smallest number
  3. Convert to whole-number ratio
  4. Use molar mass to find molecular formula

7. Reacting Masses

  • Balance the equation
  • Convert given quantity to moles
  • Apply mole ratio
  • Convert back to mass or volume

8. Volumes of Gases

Molar volume at RTP: 24.0 dm3 mol−1

Ideal Gas Equation

pV = nRT

  • p = pressure (Pa)
  • V = volume (m3)
  • n = moles
  • R = 8.31 J mol−1 K−1
  • T = temperature (K)

9. Percentage Yield & Atom Economy

Percentage yield:

(actual yield ÷ theoretical yield) × 100

Atom economy:

(molar mass of desired product ÷ total molar mass of all products) × 100

10. Exam-Style Questions (Worked)

Question 1

Calculate the number of moles in 9.0 g of water (H2O).

Answer:

Mr(H2O) = (2 × 1) + 16 = 18

Moles = 9.0 ÷ 18 = 0.50 mol

Question 2

What volume will 0.25 mol of CO2 occupy at RTP?

Answer:

Volume = moles × 24.0

Volume = 0.25 × 24.0 = 6.0 dm3

Question 3

A compound contains 40% carbon, 6.7% hydrogen and 53.3% oxygen. Find the empirical formula.

Answer:

C: 40 ÷ 12 = 3.33

H: 6.7 ÷ 1 = 6.7

O: 53.3 ÷ 16 = 3.33

Divide by smallest (3.33):

C : 1, H : 2, O : 1

Empirical formula = CH2O

11. Quick Formula Sheet

  • moles = mass ÷ molar mass
  • concentration = moles ÷ volume
  • volume of gas = moles × 24.0
  • pV = nRT
  • % yield = actual ÷ theoretical × 100
  • atom economy = desired ÷ total × 100

12. MCQ Practice

1. How many particles are in 1 mol of a substance?

A. 6.02 × 1022
B. 6.02 × 1023
C. 12.0
D. 24.0

Answer: B

2. Which unit is correct for molar mass?

A. g dm−3
B. mol dm−3
C. g mol−1
D. kg mol−1

Answer: C

3. What volume does 1 mol of gas occupy at RTP?

A. 22.4 dm3
B. 18.0 dm3
C. 24.0 dm3
D. 1.0 dm3

Answer: C

End of Topic 1 – IAL AS Chemistry

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