Quantitative Chemistry

• What is the law of the conservation of Mass?

  The law of conservation of mass states that no atoms are gained or lost during a chemical reaction.

  mass of products = mass of reactants
• Write a balanced equation for magnesium reacting with hydrochloric acid (₂)

  Mg(s) + 2 HCl(aq) → MgCl₂(aq) + H₂(g)
• Relative Atomic Mass (RAM) = average mass of atoms in an element taking into account mass and abundance of isotopes
• Define Relative Formula Mass

  Relative Formula Mass (RFM) - the sum of all RAM's of all atoms in a formula.
• What is the relative formula mass of

  1. CaF₂
  2. C(6)H(12)O(6)

  Number 1 : (Ca = 40, F = 19)

  40 + 19 + 19 = 78

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  Number 2: (C = 12, H = 1, O = 16)

  
  

  (12 x 6) + (1 x 12) + (16 x 6) = 180
• What is Avogadro's constant?

  The number of atoms, molecules, or ions in a mole of a particular substance.
• What is the value of Avogardo's constant?

  $6.02 × 10²³$
• What is the formula that links Mass, Molecular Mass and moles together?

  Mass = Mr (Molecular Mass) × Moles
• What is the mass of 20 moles of Calcium Carbonate, CaCO₃

  Mass = Mr × Moles

  Mr = 100

  100 × 20 = 2000 g
• Calculate the amount of carbon dioxide in moles in 0.32g of carbon dioxide

  Relative atomic Masses: carbon = 12, oxygen = 16

  Moles = Mass/Mr

  0.32/44 = 0.007
• What is Molecular Mass?

  The sum of the atomic masses of all atoms in a molecule.
• Nitrogen and Hydrogen form Ammonia by the following equation:

  N₂(g) + 3H₂(g) ⇌ 2NH₃(g)

  Calculate the Molecular Mass of Nitrogen needed to form 6.8 tonnes of ammonia.

  Relative atomic masses: H = 1, N =14

  Step 1 - Work out the number of moles of ammonia Hyrodgen (H₃) = 3 and Nitrogen = 14, 3+14=17

  68 000 / 17 = 400 000 moles of ammonia

  Step 2 - Use the balanced equation to calculate the ratio of Nitrogen to Ammonia = 1:2

  Therefore the number of moles of Nitrogen is 400 000 / 2 = 200 000

  Step 3 - Work out the mass of Nitrogen (Mr of N₂ = 28)

  200 000 x 28 = 5600000g = 5.6 tonnes
• What is a limiting reactant?

  The reactant that is all used up is called the limiting reactant. - it sets a limit on how much product. can form. the reactant that is left over is described as being in excess.
• Hydrogen Peroxide decomposes in water to form oxygen. How many grams of Oxygen will be given off from 40.8 g of Hydrogen peroxide?

  Step 1: Write a balanced equation: 2 H₂O₂ (l) → 2 H₂O + O₂(g). Mr H₂O₂ = 34

  Step 2: Write in the number of moles in 40.8g : 40.8/34 = 1.2 moles

  The ratio in the balanced equation H₂O₂ : O₂ = 2:1

  Step 3: Therefore the number of moles O₂ = 0.6

  Step 4: Mass of oxygen = 0.6 x 32 (Mr O₂ = 32) = 19.2 g
• What two formulas links concentration, mass, and volume together?

  Concentration (g/dm^3) = Mass (g) / Volume (dm^3)

  Concentration (Mol/dm^3) = nr of moles/volume (dm^3)
• 31.0cm^3 of Potassium Hydroxide Solution is neutralised by adding 25.0cm^3 of moldm^-3 nitric acid

  HNO₃ + KOH ⟶ KNO₃ + H₂O

  Calculate the concentration of the potassium hydroxide solution in moldm^-3

  Step 1: Calculate the number of moles HNO₃ used = concentration x volume

  Step 2: 2 x 0.025 dm^3 (25/1000 to convert the units) = 0.05 moles

  Ratio = 1:1 therefore number of moles of KOH must equal 0.05

  Step 3: Calculate the concentration of KOH

  Volume = Moles/concentration; 0.05/0.031 = 1.61
• What is the molar volume of a gas at room temperature and pressure?

  1 mole of gas at room temperature and pressure occupies 24dm^3
• How is the percentage yield of a product in a product in a chemical reaction

  %yield = Actual mass of a product x 100% ÷ Maximum theoretical mass of a product
• What is atom economy?

  A measure of the amount of starting materials that end up as useful products, effectiveness in essence.

  It is a ratio of the relative formula mass of the desired product to the sum of relative formula masses of reactants
• Look at these two equations which produce CuCl₂

  Reaction 1: CuCO₃(s) + 2HCl (aq) → CuCl₂(aq) + H₂O(l) + CO₂(g)

  Reaction 2: CuO (s) + 2HCl (aq) → CuCl₂(aq) + H₂O(l)

  Relative formula mass: CuO = 79.5; HCl = 36.5; CuCl₂ = 134.5; H₂O = 18

  Which reaction has a better atom economy

  Reaction 2: Look at the reactants

  Total formula mass of reactants: 152.5

  Formula Mass of CuCl₂ = 134.5

  134.5/152.5 x 100 = 88.2%