Amount of substance

• What is meant by the term isotope?


  Isotopes are atoms of the same element with different numbers of neutrons.
• Why do different isotopes have the same chemical properties?


  They have the same number of electrons in their outer shell.
• What is the definition of relative atomic mass?

  Relative atomic mass is the weighted average mass of an element's isotopes compared to 1/12th of the mass of Carbon-12.
• How do you calculate the relative atomic mass of element X?


  By averaging the masses of its isotopes based on their natural abundance.
• What are the relative charges and numbers of particles in a X²⁺ ion with a mass number of 150 and an atomic number of 40?


  • Proton: +1 charge, 40 particles
  • Neutron: 0 charge, 90 particles (150 - 40)
  • Electron: -1 charge, 38 particles (40 - 2)
• What are some common polyatomic ions and their charges?


  • Ammonium: NH₄⁺
  • Hydroxide: OH^-
  • Carbonate: CO₃^2-
  • Phosphate: PO₄^3-
  • Nitrate: NO₃^-
  • Sulfate: SO₄^2-
  • Nitrite: NO₂^-
  • Sulfite: SO₃^2-
  • Hydrogencarbonate: HCO₃^-
  • Dichromate (VI): Cr₂O₇^2-
  • Manganate (VII): MnO₄^-
• What is a common mistake regarding isotopes in exams?


  Stating that isotopes are the same elements with different numbers of neutrons and the same number of electrons.
• Why do isotopes have the same chemical properties?


  Because they have the same number of protons, which determines their chemical behavior.
• What is a common misconception about relative atomic mass?


  That it is the mean mass of an element compared to 1/12th of one mole of Carbon-12.
• How do you calculate the moles in 96.0 g of substance X₁₂?


  By using the formula: moles = mass / molar mass.
• How do you calculate the empirical formula of a sample containing 1.46 g of carbon, 0.482 g of hydrogen, and 1.69 g of nitrogen?


  By converting the masses to moles and finding the simplest whole number ratio.
• How do you determine the molecular formula of a compound with given percentages and relative molecular mass?


  By converting the percentages to moles and then using the molar mass to find the molecular formula.
• How do you calculate the formula mass of hydrated magnesium sulfate and the value of n?


  By subtracting the mass of anhydrous magnesium sulfate from the mass of the hydrated crystals.
• How do you calculate the mass of Na₂CO₃ required to prepare a 0.250 mol dm^-3 solution?


  By using the formula: mass = concentration × volume × molar mass.
• How do you calculate the volume of 0.150 g of SO₂ at RTP?


  By using the ideal gas equation: PV = nRT.
• How do you determine the volume of hydrogen gas produced when sodium reacts with water?


  By using stoichiometry based on the balanced equation.
• How do you calculate the concentration of potassium hydroxide formed from potassium reacting with water?


  By dividing the moles of potassium hydroxide by the volume of the solution in dm³.
• How do you calculate the percentage yield of sodium chloride from sodium reacting with chlorine?


  By dividing the actual yield by the theoretical yield and multiplying by 100.
• How do you calculate the atom economy for making oxygen from hydrogen peroxide?


  By dividing the molar mass of the desired product by the total molar mass of the reactants and multiplying by 100.
• How do you calculate the mass of cyclobutanol needed to prepare 10.00 g of cyclobutane with a 55% yield?


  By using the formula: mass = desired mass / percentage yield.

  
• How do you calculate the number of rhodium atoms in 0.0027 g of rhodium?


  By using Avogadro's number and the molar mass of rhodium.
• How do you calculate the number of oxygen atoms in 88 g of CO₂?


  By converting grams to moles and then multiplying by Avogadro's number.
• What improvements can be made to obtain more accurate gas volume measurements in experiments?


  Using calibrated equipment and ensuring proper sealing of gas collection apparatus.
• Why is the percentage yield of a reaction never 100%?


  Due to factors like incomplete reactions, side reactions, and losses during product recovery.
• What is a common mistake regarding the units in the pV=nRT equation?


  Students often confuse the units for pressure, volume, and temperature.
• What is a common mistake when calculating significant figures in answers?


  Students often do not round their answers to the appropriate number of significant figures.
• What should students do when faced with complicated calculation questions?

  Always work out the moles of any compounds given to score at least one mark.
• How do you find the concentration from a titration involving sodium hydroxide and sulfuric acid?


  By using the volume and concentration of sulfuric acid to find the moles of sodium hydroxide.
• How do you identify a metal carbonate from titration results?


  By calculating the relative formula mass of the metal carbonate from the titration data.
• What should a student do to reduce random error in a titration?


  Repeat the titration multiple times and take the average of concordant results.
• What is the difference between a base and an alkali?


  A base is a substance that can accept protons, while an alkali is a soluble base that produces hydroxide ions in solution.
• What are the sources of error in making a standard solution and how can they be mitigated?


  • Sources of error:
  • Impurities in reagents
  • Inaccurate measurements
  • Incomplete dissolution
  • Mitigation:
  • Use high-purity reagents
  • Calibrate equipment
  • Ensure complete mixing
• How should a student prepare a standard solution of X?


  By accurately weighing the solute, dissolving it in a volumetric flask, and diluting to the mark.
• What is the effect of washing a pipette with water instead of the solution before use?


  This mistake dilutes the solution, leading to inaccurate titration results.
• What are the special cases of oxidation numbers for hydrogen and oxygen?


  Hydrogen is -1 in metal hydroxides, and oxygen is -1 in peroxides.
• What redox processes occur in the reaction Mg(s) + CuSO₄(aq) → Cu(s) + MgSO₄(aq)?


  Magnesium is oxidized (loses electrons) and copper is reduced (gains electrons).
• What is a common mistake regarding the transfer of standard solutions?


  Students often state that standard solutions should be transferred to a beaker instead of a volumetric flask.
• What is a common mistake when calculating the mean titre from titration results?


  Students often use all titre values instead of only concordant titres.
• What is the correct way to express titre readings?


  Titre readings should always be given to two decimal places.
• What is the resolution of a standard burette?


  The resolution of a standard burette is 0.1 cm³.