Reacting Masses, Limiting Reactants and Yield · seed 1 · A4, ink-friendly. The answer key prints on its own page for grown-ups.
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Reacting masses, limiting reactants and yield

Science · Matter & Materials · ages 15-16
Name ______________________   Date ____________

  1. Why is a real percentage yield almost never 100 per cent?

    • some was lost on the way, or not everything reacted
    • atoms are destroyed as the reaction runs
    • the balance always reads low on a warm sample tray here
    • the theoretical mass was worked out incorrectly

  2. In 2Mg + O₂ → 2MgO, the 2 in front of Mg means that 2 g of magnesium react with 1 g of oxygen.

    Circle one:   True   False

  3. In N₂ + 3H₂ → 2NH₃, how many moles of ammonia come from 1 mole of nitrogen, with hydrogen in excess?

    • 1 mole
    • 2 moles
    • 3 moles
    • half a mole

  4. A reaction could give 50 g of product at most, and 35 g is collected. Which calculation gives the percentage yield?

    • 50 divided by 35, times 100
    • 50 minus 35, times 100
    • 35 added to 50, times 100
    • 35 divided by 50, times 100

  5. You are given 8 g of a substance with an Mr of 16 and asked for a mass of product. What is the first line of working?

    • multiply 8 by 16 to get 128
    • read the ratio off the equation first
    • divide 8 by 16 to get 0.5 moles
    • multiply 8 by the ratio in the equation

  6. In 2Mg + O₂ → 2MgO a flask holds 0.2 moles of magnesium and 0.05 moles of oxygen. Which reactant is limiting?

    • neither, since the ratio in the flask is already correct
    • the magnesium, since there is more of it
    • they run out at the same moment
    • the oxygen, since 0.2 of magnesium would need 0.1

  7. A reaction that should give 8 g gives 6 g. Which of these could NOT explain the shortfall?

    • atoms were destroyed as the reaction ran its course
    • some product was lost while it was transferred
    • the reaction did not run all the way to completion
    • some reactant went a different way and made something else

  8. In 2H₂ + O₂ → 2H₂O, how many moles of water come from 3 moles of hydrogen, with oxygen in excess?

    Answer: ______________

  9. In CaCO₃ → CaO + CO₂, 50 g of calcium carbonate (Mr 100) is heated. What mass of calcium oxide (Mr 56) can form?

    • 50 g, since the ratio is 1 to 1
    • 100 g, since the Mr values add
    • 22 g, from using the Mr of carbon dioxide instead
    • 28 g, from 0.5 moles at 56 g each

  10. Asked for the magnesium oxide from 12 g of magnesium, Priya writes: the equation is 2 to 2, so 12 g of magnesium gives 12 g of magnesium oxide. Where is the slip?

    • the ratio should have been 1 to 2 rather than 2 to 2
    • the ratio counts moles, and moles are not grams
    • magnesium and oxygen do not react at all
    • she should have divided the 12 g by two before starting
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Answer key

For grown-ups. Fold this page away before handing over the rest.

Reacting masses, limiting reactants and yield W1-mt_C2K6kp5Chv-s1

  1. some was lost on the way, or not everything reacted · The maximum assumes everything reacts and everything is collected. In practice some sticks to the glassware, some is lost on transfer, and some reactant may go a different way.
  2. False · The 2 counts particles, so it means 2 moles of magnesium to 1 mole of oxygen. In grams that is 48 g to 32 g, because a mole of each weighs a different amount.
  3. 2 moles · The ratio of N₂ to NH₃ is 1 to 2, so each mole of nitrogen gives two moles of ammonia.
  4. 35 divided by 50, times 100 · Yield is the share of the maximum that you actually got, so the collected mass goes on top and the theoretical mass underneath. Here that comes to 70 per cent.
  5. divide 8 by 16 to get 0.5 moles · Step one is always mass divided by Mr, because the ratio cannot be applied until the amount is in moles.
  6. the oxygen, since 0.2 of magnesium would need 0.1 · Divide each by its front number: magnesium gives 0.2 / 2 = 0.1, oxygen gives 0.05 / 1 = 0.05. Oxygen has the smaller answer, so it runs out first.
  7. atoms were destroyed as the reaction ran its course · Mass is conserved, so no atom is ever destroyed. The missing 2 g is somewhere in the room: on the glassware, in a side product, or still unreacted.
  8. 3 · The ratio of H₂ to H₂O is 2 to 2, which is 1 to 1, so 3 moles of hydrogen give 3 moles of water.
  9. 28 g, from 0.5 moles at 56 g each · 50 / 100 = 0.5 moles of calcium carbonate. The ratio is 1 to 1, so 0.5 moles of calcium oxide, and 0.5 x 56 = 28 g.
  10. the ratio counts moles, and moles are not grams · The ratio really is 1 to 1 by moles, but a mole of magnesium is 24 g and a mole of magnesium oxide is 40 g. 12 / 24 = 0.5 moles, so 0.5 x 40 = 20 g.
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