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)?

  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 processes?

  It means there is little or no waste product, making the process more economically viable
• What is the purpose of acid-base titrations?

  To determine the concentration of a solution of unknown concentration
• What is the standard method for making up a volumetric solution?

  1. Weigh the sample bottle containing the solid.
  2. Transfer solid to beaker and reweigh sample bottle.
  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 common errors can occur in the volumetric solution method?

  Systematic errors on the balance, lost substance in transfer, and overfilling of the volumetric flask
• What are the steps in the titration method?

  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 color change occurs.
  6. Record final burette reading and calculate titre volume.
  7. Repeat until at least two concordant results are obtained.

  
• How can uncertainties in titration be reduced?

  By increasing the titre volume needed for the reaction
• What should be done before undertaking any practical work?

  Carry out a risk assessment
• What does the percentage uncertainty of a measurement indicate?

  The uncertainty of the instrument used for the measurement

  
• How do you calculate the percentage uncertainty when using a burette with a 0.5 cm³ uncertainty for a 25 cm³ volume?

  Percentage uncertainty = (0.5 cm³ / 25 cm³) * 100 = 2%

  
• What is the purpose of including state symbols in chemical equations?

  To clarify what might be observed during the reaction
• What is the unit for the amount of substance in chemistry?

  Mole
• What does one mole of a substance correspond to 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 defined?

  It is the mass of one mole of the substance expressed in $g \text{ mol}^{-1}$.
• 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, which ions 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$_4 \cdot 7 H_2O$
• What is a solution composed of?

  A solution is composed of a solute and a solvent.