C3 - Quantitative Chemistry

• According to the law of conservation of mass, what happens to atoms during a chemical reaction?

  No atoms are lost or made
• What does the law of conservation of mass imply about the mass of reactants and products in a chemical reaction?

  Mass of products equals mass of reactants
• What is the balanced equation for magnesium reacting with hydrochloric acid?

  Mg(s) + 2 HCl(aq) → MgCl2(aq) + H2(g)
• What does RAM stand for?

  Relative atomic mass
• What does RFM stand for?

  Relative formula mass
• What is relative atomic mass defined as?

  Average mass of atoms relative to 12C
• How is relative formula mass calculated?

  Sum of RAM’s of all atoms
• What is the relative formula mass of CaF2 (Ar values: Ca = 40, F = 19)?

  78
• What is the relative formula mass of C6H12O6 (Ar values: C = 12, H = 1, O = 16)?

  180
• In the reaction 4 MgO(s) + CH4(g) → 4 Mg(s) + 2 H2O(g) + CO2(g), why do carbon dioxide and water escape from the test tube?

  They are both gases
• In an experiment, the mass of magnesium produced was 3.3g, 3.5g and 3.2g, what is the mean mass of magnesium produced?

  3.3 g
• How could the precision of the results in the experiment be increased?

  Measure to more decimal places
• What does Avogadro's constant represent?

  Atoms, molecules or ions in a mole
• What formula links mass, Mr, and moles?

  Mass = Mr x Moles
• What is the value of Avogadro's constant?

  6.02 x 10^23
• What is the mass of 20 moles of calcium carbonate, CaCO3 (Mr = 100)?

  2000 g
• In the reaction N2(g) + 3 H2(g) ⇌ 2 NH3(g), what mass of nitrogen is needed to form 6.8 tonnes of ammonia (Mr of ammonia = 17, Mr of N2 = 28)?

  5.6 tonnes
• How many moles are in 0.32 g of carbon dioxide (Mr = 44)?

  0.007 moles
• What is a limiting reactant?

  Reactant completely used up
• Why is it common to use an excess of one reactant in a chemical reaction?

  To ensure all other reactant is used
• In the decomposition of hydrogen peroxide, 2 H2O2(l) → 2 H2O + O2(g), how many grams of oxygen gas will be given off from 40.8 g of hydrogen peroxide (Mr of H2O2 = 34, Mr of O2 = 32)?

  19.2 g
• What is the formula that links concentration, mass and volume together, when concentration is in g/dm³?

  Concentration = Mass/Volume
• What is the formula that links concentration, moles and volume together, when concentration is in mol/dm³?

  Concentration = Moles/Volume
• What is the relative formula mass (Mr) of $H_2O_2$?


  34
• If 40.8 g of hydrogen peroxide decomposes, what mass of oxygen is produced, given the balanced equation $2 H_2O_2(l) \rightarrow 2 H_2O +$$O_2(g)$?


  19.2 g
• How many moles are present in 40.8 g of a substance with a molar mass of 34 g/mol?

  1.2 moles
• In the reaction $2 H_2O_2(l) \rightarrow 2 H_2O +$$O_2(g)$, what is the stoichiometric ratio of $H_2O_2$ to $O_2$?


  2:1
• If 1.2 moles of $H_2O_2$ decompose according to the equation $2 H_2O_2(l) \rightarrow 2 H_2O +$$O_2(g)$, how many moles of $O_2$ are produced?


  0.6 moles
• What is the relative formula mass (Mr) of $O_2$?


  32
• What is the mass of 0.6 moles of oxygen ($O_2$), given that the Mr of $O_2$ is 32?


  19.2g
• What formula relates concentration (in g per $dm^3$), mass (in g), and volume (in $dm^3$)?


  Concentration = Mass / Volume
• Which formula links concentration (mol per $dm^3$), number of moles, and volume (in $dm^3$)?


  Concentration = Moles / Volume
• If 0.05 moles of $HNO_3$ are used in the reaction $HNO_3 +$$KOH \rightarrow KNO_3 +$$H_2O$, how many moles of KOH are required?


  0.05 moles
• Calculate the concentration of KOH if 0.05 moles are dissolved in 0.031 $dm^3$.


  1.61 mol $dm^{-3}$
• What volume does one mole of any gas occupy at room temperature and pressure (RTP)?

  24 $dm^3$
• What is the process of titration used for?

  Finding the concentration of a solution
• What are the key steps in performing a titration to determine the concentration of an unknown solution?

  • Rinse pipette with solution of unknown concentration and measure known volume.
  • Add an indicator.
  • Rinse burette with solution of known concentration and add gradually.
  • Record volume when indicator changes colour (end point).
  • Ensure concordant volume results.
  • Perform calculations to find concentration.
• Why might the actual yield of a product in a chemical reaction be less than the theoretical yield?

  • Reaction may not go to completion due to reversibility.
  • Product lost during separation.
  • Reactants may participate in side reactions.
• How is percentage yield calculated in a chemical reaction?

  % Yield = (Actual mass / Theoretical mass) x 100%
• What is the percentage yield if 40.5 g of $NH_3$ is produced from 20.0 mol $H_2$ and excess $N_2$, given the balanced equation $N_2 +$$3 H_2 \rightarrow 2 NH_3$?


  17.8%