Atoms, molecules and stoichiometry

• What is the definition of relative atomic mass?


  The weighted average mass of an atom of an element compared with 1/12 of the mass of an atom of carbon-12.
• How is relative isotopic mass defined?


  The mass of an atom of an isotope compared to 1/12 of the mass of an atom of carbon-12.
• What does relative molecular mass represent?


  The weighted average mass of one molecule of an element or compound compared with 1/12 of the mass of an atom of carbon-12.
• What is relative formula mass?


  The weighted average mass of one unit of a substance compared with 1/12 of the mass of an atom of carbon-12.
• What is one mole defined as?


  The amount of any substance containing as many particles as there are carbon atoms in exactly 12 g of carbon-12.
• What is Avogadro's constant?


  The number of particles in one mole of any substance, Avogrado constant = 6.0 x 10^23
• How do you calculate the number of particles from moles?


  Number of particles = Avogadro’s constant x moles.
• What does mass spectrometry provide information about?


  The abundance of different elements on a mass spectra.
• What does a mass spectrum of boron indicate?


  It shows the presence of different isotopes of boron.
• What is the mass to charge ratio (m/z) of a +1 ion with a mass of 10?


  The m/z ratio is 10.
• What is the empirical formula?


  The simplest whole number ratio of atoms of each element in a compound.
• What is the molecular formula?


  The actual number of atoms of each element in a compound.
• What are the steps to calculate the empirical formula from masses?


  1. Divide the mass (or percentage by mass) of each element by its molar mass to calculate the molar ratio.
  2. Divide each number in the ratio by the smallest number to get the simplest ratio of elements.
  3. If the ratio contains decimal numbers, multiply it as appropriate to obtain whole numbers.
• What are the steps to calculate the empirical formula from combustion data if the compound contains only carbon and hydrogen?


  1. Divide the mass of CO₂ produced by 44 to find the moles of carbon.
  2. Divide the mass of water produced by 18 to find the moles of hydrogen (double this value).
  3. Divide the moles of hydrogen and carbon by the smallest value to find the simplest molar ratio.
• What is the empirical formula of a sample that produces 0.069 g of CO₂ and 0.0113 g of H₂O when burned?


  The empirical formula is C₅H₄.
• What are the steps to calculate the empirical formula if the compound contains other elements as well as carbon and hydrogen?


  1. Calculate moles of carbon from CO₂ and mass of carbon present.
  2. Calculate moles of hydrogen from water and mass of hydrogen present.
  3. Compare total mass of carbon and hydrogen to initial mass; if different, calculate mass of oxygen.
  4. Divide moles of each element by the smallest value to obtain the empirical formula.
• How do you calculate the molecular formula from the empirical formula?


  Divide the relative formula mass of the compound by the relative formula mass of the empirical formula.
• What is the molecular formula if the empirical formula is HO and the relative formula mass is 34?


  The molecular formula is H₂O₂.
• How should quantities be reported in chemistry calculations?


  To the same number of significant figures as the least accurate measured quantity.
• What is molar mass?


  The mass per mole of a substance calculated by adding the relative atomic masses of all the atoms in a formula.
• How do you calculate the number of moles using mass and molar mass?


  Number of moles = mass (g) ÷ molar mass.
• What volume does one mole of any gas occupy at room temperature and pressure?


  24 dm³.
• How do you calculate the number of moles of a gas at room temperature and pressure?


  Moles of gas = Volume (dm³) ÷ 24.
• What is concentration in terms of solutions?


  The amount of solute dissolved in a given volume of solution.
• How do you calculate the number of moles from concentration and volume?


  Number of moles = concentration (mol dm⁻³) x volume (dm³).
• What is stoichiometry?


  The proportions of reactants and products in a chemical reaction.
• How can stoichiometry be determined?


  By using a balanced chemical equation.