Quantitative Chemistry

• What does the Law of conservation of mass state?


  The mass of reactants is the same as the mass of products.
• Why are atoms not lost or gained in a chemical reaction?


  Atoms are rearranged but not created or destroyed.
• What is the chemical equation for the thermal decomposition of magnesium carbonate?


  Magnesium carbonate → Magnesium oxide + carbon dioxide
• If 120g of magnesium carbonate decomposes, what are the masses of magnesium oxide and carbon dioxide produced?


  90g of magnesium oxide and 30g of carbon dioxide.
• What type of reaction is the decomposition of magnesium carbonate?


  This reaction is an example of a thermal decomposition reaction.
• What is the chemical equation for the decomposition of calcium carbonate?


  CaCO₃(s) → CaO(s) + CO₂(g)
• Why does the mass decrease in the decomposition of calcium carbonate?


  CO₂ is a gas that is released to the surroundings.
• What is the chemical equation for the reaction of magnesium with oxygen?

  2Mg(s) + O₂(g) → 2MgO(s)
• Why does the mass increase in the reaction of magnesium with oxygen?


  O₂ is a gas that has reacted with Mg.
• Write the balanced chemical equation for the reaction of phosphorus and oxygen.

  P₄ + 5O₂ → P₄O₁₀
• Write the balanced chemical equation for the synthesis of ammonia.


  N₂ + 3H₂ → 2NH₃
• What is resolution in the context of measurements?


  Resolution is the smallest difference that gives a chance to the reading.
• How is uncertainty calculated in measurements?

  Uncertainty = $\frac{range}{2} +$$Mean$
• What is the mean of the following density measurements: 960 kg/m³, 1120 kg/m³, and 1040 kg/m³?


  Mean = $\frac{960 + 1120 + 1040}{3} =$$1080 \text{ kg/m}^3$
• What is the range of the density measurements: 960 kg/m³, 1120 kg/m³, and 1040 kg/m³?


  Range = 1120 - 960 = 160 kg/m³
• What is the relative atomic mass (Ar) of an element?


  Ar is the weighted average mass of an atom of an element compared to 1/12th of the mass of carbon-12.
• What are isotopes?


  Isotopes are atoms of the same element with the same number of protons and electrons but different numbers of neutrons.
• Ar = (mass number x % of isotope 1) + (mass number x % of isotope 2) / 100
• What is the relative formula mass (Mr) of a compound?


  Mr is the sum of the relative atomic masses of the atoms in a compound.
• Calculate the relative formula mass (Mr) of aluminium oxide, Al₂O₃.


  Mr = 54 + 48 = 102 g/mol
• Calculate the relative formula mass (Mr) of H₂SO₄


  Mr = 2 + 32 + 16(4) = 98 g/mol
• How do you calculate the percentage by mass of an element in a compound?


  Percentage by mass = $\frac{Ar \, of \, element}{Mr \, of \, compound} \times 100$
• Calculate the percentage by mass of iron in FeSO₄.


  Percentage by mass = $\frac{56}{56 + 32 + 16 + 16} \times 100 =$$46.7\%$
• What are moles in chemistry?


  Moles are a unit of measurement for the amount of substance.
• How can moles be calculated using mass and relative formula mass?


  Moles = $\frac{mass \, (g)}{Mr \, (g/mol)}$
• Calculate the number of moles in 14,520 g of ammonium nitrate (NH₄NO₃).


  Moles = $\frac{14520}{80} =$$181.5 \text{ mol}$
• How many moles are in 20 g of sodium chloride (NaCl)?


  Moles = $\frac{20}{58.5} \approx 0.34 \text{ mol}$
• What is the Avogadro constant?


  One mole of any substance contains exactly $6.02 \times 10^{23}$ particles.
• How do you determine the number of particles in a sample using moles?


  Number of particles = $6.02 \times 10^{23} \times moles$
• Calculate the number of ions in 0.724 mol of calcium ions.


  Number of ions = $6.02 \times 10^{23} \times 0.724 =$$4.36 \times 10^{23} \text{ ions}$
• How is the mass of a single atom calculated?

  Mass of single atom = $\frac{Ar}{6.02 \times 10^{23}}$
• Calculate the mass of a single lithium atom.


  Mass = $\frac{7}{6.02 \times 10^{23}} =$$1.16 \times 10^{-23} \text{ g}$
• How do you calculate the number of moles of carbon dioxide produced from burning a hydrocarbon?


  Moles of CO₂ = $\frac{mass \, of \, CO₂}{Mr \, of \, CO₂}$
• Calculate the number of moles of carbon dioxide produced from 6.60 g of carbon dioxide.


  Moles = $\frac{6.60}{44} =$$0.15 \text{ mol}$
• What is the method to determine the mass of an unknown substance from a known substance?


  1. Work out moles of known substance. 2. Use molar ratio to determine moles of unknown. 3. Determine mass of unknown.
• M = m / Mr

  M = moles (mol), m = mass of substance (g), Mr = molar mass of substance (g/mol)