IB Stoichiometry Revision: Moles & Calculations

Overview

• Revision session for IB Chemistry Topic 1: Moles, calculations, and ratios.
• Suitable for both HL and SL levels of IB Chemistry.
• Focus: Equations in the unit and types of questions you might encounter.
• Tools recommended: revision worksheet, IB Periodic Table, and data booklet.

Key Equations

1. Solid Substances:

   • Equation: N (moles) = Mass (g) / Molar Mass (g/mol)
   • Molar mass can be found in the data booklet (periodic table).

2. Avogadro's Number:

   • Equation: Number of Particles = Moles * Avogadro’s Number
   • Avogadro’s Number: 6.02 x 10^23 (per mole)
   • Found in the data booklet under constants.

3. Percentage Yield:

   • Equation: Percentage Yield = (Actual Amount / Theoretical Amount) * 100
   • Ensure both amounts are in the same units.

4. Gases:

   • Equation: Moles = Volume (dm³) / Molar Volume
   • Standard temperature and pressure: 1 mole of gas = 22.7 dm³.

5. Ideal Gas Equation (PV = nRT):

   • Only equation in the data booklet for gases.
   • Units:
     • P (pressure) in Pascals.
     • V (volume) in meters cubed.
     • R (gas constant) = 8.31 Joules/(K*mol).
     • T (temperature) in Kelvin (°C + 273).
   • Conversions:
     • 1 dm³ = 10^-3 m³.
     • 1 cm³ = 10^-6 m³.

6. Solutions:

   • Equation: Moles = Concentration (mol/dm³) * Volume (dm³).

Sample Problems and Solutions

Calculating Molar Mass

1. Example Problem:

   • Amount in moles for 2g of NaOH.
   • Equation used: Moles = Mass / Molar Mass.
   • Calculation: Na (approx. 23) + O (16) + H (1) ≈ 40.
   • Moles = 2/40 = 0.05 mol.

2. Hydrated Compounds:

   • Example: Hydrated CuSO₄:
     • Cu (64) + S (32) + O₄ (416) + 5 (H₂O) = 64 + 32 + 64 + 90 = 250 g/mol.
     • Answer: 250 g/mol (D).

Number of Particles

1. Given: 1.8 x 10²² molecules of O₂.
   • Equation: Moles = Number of Particles / Avogadro's Number.
   • Moles = 1.8 x 10²² / 6 x 10²³ = 0.03 mol.
   • Answer: B.

Empirical Formula

1. Definition: Simplest ratio of atoms within a compound.
2. Example:
   • Given: Percentages of C, H, O.
   • Convert % to mass (assuming a 100g sample).
   • Calculate moles: moles = mass / atomic mass.
   • Find ratio by dividing by the smallest number of moles.
   • Example: C2H4O (simplest ratio).

Water of Crystallization

1. Example: Hydrated copper chloride.
   • Given masses before and after heating.
   • Find mass of water lost, then calculate moles for both CuCl2 and H₂O.
   • Ratio determination leads to formula CuCl₂.2H₂O.

Reacting Mass Calculations

1. Example: LiOH and CO₂ reaction.
   • Given mass of CO₂, find moles.
   • Use stoichiometric ratio to find moles of LiOH.
   • Calculate mass using Moles * Molar Mass.
   • Answer: 12g.

Limiting Reagents

1. Example: Al(OH)₃ and H₂SO₄.
   • Determine limiting reagent by comparing moles divided by coefficients.
   • Use limiting reagent to calculate product mass.

Ideal Gas Behavior and Deviation

1. Assumptions:
   • Particles have no volume.
   • No intermolecular forces between particles.
2. Deviations:
   • At low temperatures and high pressures, gases deviate significantly.
   • Example: Water deviates due to strong hydrogen bonding compared to Nitrogen.

Titration Calculations

1. Simple Example: HCl and CaCO₃.

   • Use given mass and concentration.
   • Convert volume to dm³ and use moles = concentration * volume.

2. Complex Example: Unknown metal carbonate and HCl.

   • Use titration data to calculate moles of reactants.
   • Determine formula mass and identify the metal.

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This revision session covers essential concepts for mastering stoichiometry in IB Chemistry and is crucial for both SL and HL students. Ensure to practice different types of problems to reinforce the learning.