Topic 3 Flashcards

Conservation of Mass

Conservation of Mass: read all 5 questions and answers

1. What does the law of conservation of mass state?
   No atoms are lost or made during a chemical reaction. The total mass of the products equals the total mass of the reactants.
2. Why might the mass appear to decrease during a reaction in an open container?
   A gas is produced that escapes into the atmosphere (e.g. CO₂ from thermal decomposition), so the measured mass decreases even though total mass is conserved.
3. Why might the mass appear to increase during a reaction?
   A gas from the air (e.g. Oxygen) reacts with the substance. The oxygen atoms add mass to the product (e.g. When magnesium burns in air to form MgO).
4. Why must chemical equations be balanced?
   To show that atoms are conserved - the same number and type of atoms must appear on both sides of the equation.
5. What happens to total mass in a sealed container during a reaction?
   The total mass stays exactly the same, because no atoms can enter or leave the system.

Relative Formula Mass (M_r)

Relative Formula Mass: read all 5 questions and answers

1. What is relative atomic mass (Ar)?
   The average mass of an atom of an element compared to ¹⁄₁₂ of the mass of a carbon-12 atom. Found on the periodic table.
2. What is relative formula mass (Mr)?
   The sum of all the relative atomic masses (Ar) of all the atoms in a chemical formula.
3. Calculate the Mr of H₂SO₄.
   H₂SO₄: (2 × 1) + 32 + (4 × 16) = 2 + 32 + 64 = 98
4. Calculate the Mr of Ca(OH)₂.
   Ca(OH)₂: 40 + 2 × (16 + 1) = 40 + 34 = 74. Remember to multiply everything inside the brackets by 2.
5. Calculate the Mr of Al₂(SO₄)₃.
   Al₂(SO₄)₃: (2 × 27) + 3 × (32 + 4 × 16) = 54 + 3 × 96 = 54 + 288 = 342

Moles & Avogadro's Constant

Moles & Avogadro's Constant: read all 5 questions and answers

1. What is one mole of a substance?
   The SI unit of amount of substance containing 6.02 × 10²³ particles (the Avogadro constant; exactly 6.02214076 × 10²³ in the SI definition). One mole of a substance has a mass in grams equal to its relative formula mass (Mr).
2. What is Avogadro's constant?
   6.02 × 10²³ - the number of particles (atoms, molecules or ions) in one mole of any substance.
3. How do you calculate moles from mass?
   Moles = mass (g) ÷ Mr. For example: 12 g of carbon (Ar = 12) → 12 ÷ 12 = 1 mol.
4. How do you calculate mass from moles?
   Mass = moles × Mr. For example: 0.5 mol of NaCl (Mr = 58.5) → 0.5 × 58.5 = 29.25 g.
5. How many molecules are in 2 moles of water?
   2 × 6.02 × 10²³ = 1.204 × 10²⁴ molecules.

Reacting Masses & Limiting Reactants

Reacting Masses & Limiting Reactants: read all 6 questions and answers

1. How do you use a balanced equation to calculate reacting masses?
   Step 1: Calculate moles of the known substance (moles = mass ÷ Mr). Step 2: Use the mole ratio from the balanced equation. Step 3: Calculate the mass of the unknown (mass = moles × Mr).
2. What is a limiting reactant?
   The reactant that is completely used up in a reaction. It limits the amount of product formed. The other reactant is in excess.
3. How do you identify the limiting reactant?
   Calculate moles of each reactant, then compare using the mole ratio from the balanced equation. The reactant that runs out first is the limiting reactant.
4. What happens to the excess reactant?
   It is left over at the end. Some of the excess reactant remains unreacted because the limiting reactant has been fully consumed.
5. Fe₂O₃ + 3CO → 2Fe + 3CO₂. Calculate the mass of iron from 80 g of Fe₂O₃.
   Mr of Fe₂O₃ = 160. Moles = 80 ÷ 160 = 0.5 mol. Ratio: 1 Fe₂O₃ → 2 Fe, so moles of Fe = 1.0 mol. Mass = 1.0 × 56 = 56 g.
6. In a reaction, why does using excess of one reactant ensure the other reacts completely?
   The limiting reactant is completely surrounded by excess particles, ensuring every particle of the limiting reactant can collide and react.

Concentration

Concentration: read all 5 questions and answers

1. What is concentration?
   The amount of solute dissolved in a given volume of solution. It can be measured in g/dm³ or mol/dm³.
2. How do you calculate concentration in g/dm³?
   Concentration (g/dm³) = mass of solute (g) ÷ volume of solution (dm³).
3. How do you calculate concentration in mol/dm³?
   Concentration (mol/dm³) = moles of solute ÷ volume of solution (dm³).
4. How do you convert cm³ to dm³?
   Divide by 1000. For example: 250 cm³ = 250 ÷ 1000 = 0.25 dm³.
5. 0.1 mol of NaOH is dissolved in water and made up to 500 cm³ of solution. What is the concentration?
   Volume = 500 ÷ 1000 = 0.5 dm³. Concentration = 0.1 ÷ 0.5 = 0.2 mol/dm³ (0.20 mol dm⁻³).

Percentage Yield & Atom Economy

Percentage Yield & Atom Economy: read all 6 questions and answers

1. What is percentage yield?
   The proportion of product actually obtained compared to the maximum theoretical amount. % yield = (actual yield ÷ theoretical yield) × 100.
2. Why is percentage yield often less than 100% in practice?
   Because of incomplete or reversible reactions, practical losses during transfers and filtration, and side reactions forming unwanted by-products (apparent yields over 100% usually mean product is impure or wet).
3. What is atom economy?
   The proportion of reactant mass converted into desired product. Atom economy = [(stoichiometric coefficient × Mr of desired product) ÷ total Mr of all reactants from balanced equation] × 100.
4. Why is high atom economy important?
   It reduces waste and is more sustainable. Reactions with high atom economy are better for the environment and more cost-effective for industry.
5. Zn + H₂SO₄ → ZnSO₄ + H₂. Calculate the atom economy for ZnSO₄.
   Mr of ZnSO₄ = 161. Mr of H₂ = 2. Total Mr of products = 163. Atom economy = (161 ÷ 163) × 100 = 98.8%.
6. What type of reaction always has 100% atom economy?
   Addition reactions (only one product is formed, so all atoms end up in the desired product).

Titrations

Titrations: read all 6 questions and answers

1. What is a titration?
   A technique to find the precise volume of one solution needed to react completely with a known volume of another solution.
2. Describe the titration method.
   Use a pipette for a fixed volume of alkali in a conical flask. Add indicator. Fill a burette with acid of known concentration. Add acid gradually until the indicator permanently changes colour (the end point). Record the titre and repeat.
3. What are concordant results?
   Titre values that are within 0.10 cm³ of each other. You need at least two concordant results to calculate a reliable mean titre.
4. How do you calculate unknown concentration from titration data?
   Step 1: Calculate moles of known substance (moles = concentration × volume). Step 2: Use mole ratio to find moles of unknown. Step 3: Calculate concentration (concentration = moles ÷ volume).
5. 25 cm³ of NaOH is neutralised by 20 cm³ of 0.5 mol/dm³ HCl. Find the concentration of NaOH.
   Moles HCl = 0.5 × 0.020 = 0.01 mol. Ratio NaOH:HCl = 1:1, so moles NaOH = 0.01. Conc NaOH = 0.01 ÷ 0.025 = 0.4 mol/dm³.
6. Why should you not include anomalous titres in the mean?
   Anomalous results are not concordant with other values and would make the mean inaccurate. Only use concordant values (within 0.10 cm³).

Molar Volume of Gases

Molar Volume of Gases: read all 5 questions and answers

1. What is the molar volume of any gas at room temperature and pressure (RTP)?
   24 dm³ per mole (or 24,000 cm³ per mole). RTP = 20°C and 1 atmosphere.
2. How do you calculate the volume of a gas at RTP?
   Volume (dm³) = moles × 24. Or in cm³: volume = moles × 24,000.
3. How do you calculate moles of a gas from its volume?
   Moles = volume (dm³) ÷ 24. Or: moles = volume (cm³) ÷ 24,000.
4. Why do all gases have the same molar volume at the same temperature and pressure?
   Gas particles are far apart with negligible intermolecular forces. At the same T and P, the spacing is the same regardless of the gas, so one mole of any gas occupies the same volume.
5. CaCO₃ → CaO + CO₂. What volume of CO₂ is produced from 25 g of CaCO₃ at RTP?
   Mr of CaCO₃ = 100. Moles = 25 ÷ 100 = 0.25 mol. Ratio 1:1, so moles CO₂ = 0.25. Volume = 0.25 × 24 = 6 dm³.