formulae & equations

• What is the mole a unit of measurement for?

  Substances
• What does the Avogadro Constant (L) allow us to find?

  The number of particles present in a sample of a substance with known mass
• What is the formula for calculating moles?


  n = moles
• What are the units for molar mass?

  g mol^-1
• How is 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

  
• What is 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

  
• What is the term used for relative molecular mass in ionic compounds?

  Relative formula mass
• What is the empirical formula?

  The simplest whole number ratio of atoms of each element in a compound
• How can the molecular formula be determined?

  Using the Mr of the empirical formula and the true Mr of the molecule

  
• What is the relationship between pressure and temperature for gases under standard conditions?

  Pressure is proportional to temperature
• What is the ideal gas equation?

  pV = nRT
• What are the standard units for pressure, volume, and temperature in the ideal gas equation?

  Pressure in Pascals, volume in m³, temperature in Kelvin

  
• What is the value of the ideal gas constant (R)?

  1. 31 J K^-1mol^-1
• Why must chemical equations be balanced before calculations?

  To ensure the reacting ratios are correct
• What are the four state symbols used in chemical equations?


  (s), (l), (g), (aq)
• What does the percentage yield indicate?

  How much of the maximum amount of product was obtained during an experiment

  
• What does a low percentage yield suggest?

  It could indicate an incomplete reaction or loss of product during purification
• What is atom economy?

  A measure of efficiency that indicates the proportion of reactant atoms converted into the desired product
• Why is high atom economy desirable in industrial chemical processes?

  It means there is little or no waste product, making the process more economically viable
• What is the purpose of a titration?

  To determine the concentration of a solution of unknown concentration
• What are the steps in preparing a volumetric solution?

  1. Weigh the sample bottle containing the solid.
  2. Transfer solid to beaker and reweigh.
  3. Record the difference in mass.
  4. Add distilled water and stir until dissolved.
  5. Transfer to a volumetric flask with washings.
  6. Make up to the 250 cm³ mark with distilled water.
  7. Shake flask.

  
• What are the common errors in preparing a volumetric solution?

  • Systematic errors on the balance
  • Lost substance during transfer
  • Overfilling of the volumetric flask
• What are the steps in performing a titration?

  1. Fill the burette with the standard solution.
  2. Transfer 25 cm³ of unknown solution to a conical flask.
  3. Add indicator.
  4. Record initial burette reading.
  5. Titrate until a permanent colour change occurs.
  6. Record final burette reading and calculate titre volume.
  7. Repeat until at least two concordant results are obtained.

  
• What is the purpose of calculating percentage uncertainty in a titration?

  To assess the reliability of the measurement
• How do you calculate the percentage uncertainty when a 25 cm³ volume is recorded using a burette with a 0.5 cm³ uncertainty?

  Percentage uncertainty = (0.5 cm³ / 25 cm³) * 100 = 2%
• Why is it important to carry out a risk assessment before practical work?

  To analyze equipment, the lab environment, and chemicals used, and suggest methods for reducing risk
• What should be done if an accident occurs during practical work?

  Follow the suggested methods for reducing risk and report the incident
• What is the unit for the amount of substance?

  Mole
• What does one mole of a substance represent in terms of carbon-12?

  It is the amount of chemical species found in 12 g of $^{12}C$.
• What is Avogadro’s constant?

  1. 02 x $10^{23} \text{ mol}^{-1}$
• How is molar mass expressed?

  In gmol$^{-1}$
• What is an empirical formula?

  It is the smallest whole number ratio of atoms of each element in a compound.
• What is the empirical formula of benzene (C$_{6}H_{6}$)?


  CH
• What is a molecular formula?

  It is the actual number of atoms of each element in a molecule.
• What are spectator ions?

  They are ions that do not take part in the reaction.
• In the reaction NaOH + HCl → NaCl + H$_{2}O$, what are the spectator ions?


  Na$^{+}$, Cl$^{-}$
• What is a hydrate?

  A compound that has molecules of water of crystallisation.
• Give an example of a hydrate.

  MgSO$_{4} \cdot 7 H_{2}O$
• What is a solution composed of?

  A solute that is dissolved and a solvent that dissolves the solute.
• What is a standard solution?

  A solution with accurately known concentration.