The Mole Concept and Calculations

• The mole is a unit of measurement for substances
• A mole always contains the same number of particles

Avogadro Constant

• The Avogadro Constant (L) is 6.022x10^23 particles
• This number allows calculating the number of particles in a substance with known mass
• The formula: Number of particles = nL (n = moles)
• The mole is an important unit of measurement in many calculations

Molar mass

• Moles = mass/Mr = (concentration x volume)/1000
• Concentration is in mol/dm³, volume is in cm³
• Molar mass is the mass in grams per mole (g/mol)

Relative atomic mass (Ar)

• Defined as the mean mass of an atom of an element, divided by one twelfth of the mean mass of an atom of the carbon-12 isotope

Relative molecular mass (Mr)

• Defined as the mean mass of a molecule of a compound, divided by one twelfth of the mean mass of an atom of the carbon-12 isotope
• For ionic compounds, it is known as relative formula mass

Empirical Formula

• The empirical formula is the simplest whole number ratio of atoms of each element in a compound
• It can be found using molar ratios of each element

Molecular Formula

• It represents the actual number of each atom in a molecule
• Can be determined using the Mr of the empirical formula and the true Mr of the molecule
• This gives a multiplier value to scale up the empirical formula
• True Mr = Mr of empirical formula x multiplier

Ideal Gas Equation

• Under standard conditions, gases and volatile liquids follow trends:
  • Pressure is proportional to Temperature
  • Volume is proportional to Temperature
  • Pressure and Volume are inversely proportional
• Combination of trends gives the ideal gas equation: pV = nRT = (mRT)/Mr

Standard Units for Ideal Gas Equation Variables

• p = pressure in Pascals
• V = volume in m³
• T = temperature in Kelvin
• n = moles
• m = mass in grams
• R is the ideal gas constant, 8.31 J K⁻¹mol⁻¹

Balanced Equations

• Full or ionic chemical equations must be balanced before use in calculations for correct reacting ratios
• A balanced equation must have the same number and type of each atom on both sides
• Include state symbols to indicate observations during reaction:
  • (s) - solid
  • (I) - liquid
  • (g) - gas
  • (aq) - aqueous (dissolved in water)
• Balanced equations calculate reacting masses, percentage yield and atom economy

Ionic Equations

• Ionic equations show reacting particles that undergo change, excluding spectator species
• Equations must be balanced
• Reacting species are shown as dissociated ions

Percentage Yield

• Indicates the amount of product obtained during an experiment
• % yield = (Experimental mass / Theoretical mass) x 100
• A low percentage yield indicates an incomplete reaction, or loss of product in purification

Atom Economy

• Measures the efficiency by the proportion of reactant atoms converted into the desired product
• % atom economy = (Mr of desired product / Total Mr of reactants) x 100
• Desirable to have a high atom economy
• High atom economy means little or no waste product, making the industrial process economically viable

Empirical and Molecular Formulas

• Formulas, and reaction stoichiometries can be calculated with experimental data
• Mol = volume x concentration
• True Mr = Mr of empirical formula x multiplier
• Volume of gas (dm³) = 24 x Number of moles
• At room temperature and pressure
• Number of particles = n x L
• Mass = Mr x mol

Acid-Base Titrations

• A titration is a practical method where a standard solution is reacted with a solution of unknown concentration to determine its concentration

Volumetric Solution Preparation

• Weigh the sample bottle containing the solid on a (2 dp) balance
• Transfer solid to beaker and reweigh sample bottle
• Record the difference in mass
• Add distilled water and stir with a glass rod until all the solid has dissolved
• Transfer to a volumetric flask with washings
• Make up to the 250cm³ mark with distilled water
• Shake flask

Potential Errors in Volumetric Solution Preparation

• Systematic errors on the balance
• Lost substance in transfer processes
• Overfilling of the volumetric flask
• Mitigate errors using washing methods and reading volumes from the bottom of the meniscus

Titration Method

• Fill the burette with the standard solution of known concentration, ensuring the jet space in the burette is filled with no air bubbles
• Transfer 25cm³ of the solution with unknown concentration into a conical flask using a pipette filler and pipette
• Add two to three drops of indicator
• Record the initial burette reading
• Titrate the contents of the conical flask by adding solution from the burette until the indicator displays a definite, permanent color change
• Record the final burette reading and calculate the titre volume
• Repeat until at least two concordant results are obtained (within 0.1cm³ of each other)

Equipment Uncertainty

• The equipment used in a titration come with uncertainty values
• Combine all uncertainty values to find the overall uncertainty in the final answer
• Percentage uncertainty = (instrument uncertainty / measurement) x 100

Reduced Uncertainty in Titration

• Increase the titre volume needed for the reaction
• Increase the volume and concentration of the substance in the conical flask or decrease the concentration of the substance in the burette
• Complete a risk assessment before practical work
• Analyse equipment, the lab environment, the chemicals used, and suggest methods for reducing the risk, and what should be done if an accident occurs

Learn about the mole, Avogadro's constant, and molar mass. Explore how to calculate the number of particles in a substance and understand relative atomic and molecular mass. Discover empirical formula.