Balancing Chemical Equations: Quiz

Balancing Chemical Equations: Quiz

7 questions

1. Question 1EasyWhy must chemical equations be balanced?

   AAtoms are not created or destroyed in a reactionBReactions always produce the same number of moleculesCThe masses of reactants and products are always 1 : 1DCoefficients must always add up to an even number
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   Answer: Atoms are not created or destroyed in a reaction

   This is the law of conservation of mass. The number of molecules can change (2H₂ + O₂ → 2H₂O: 3 molecules become 2), but the atoms cannot.

2. Question 2MediumHow many oxygen atoms are shown by $\ce{2Al2(SO4)3}$?

   A12B14C24D7
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   Answer: 24

   2 × 3 × 4 = 24. The coefficient multiplies the whole formula, and the subscript 3 multiplies everything in the bracket.

3. Question 3EasyWhich is the correct balanced equation for $\ce{Na + Cl2 -> NaCl}$?

   A$\ce{Na + Cl2 -> NaCl2}$B$\ce{2Na + Cl2 -> 2NaCl}$C$\ce{Na + Cl2 -> 2NaCl}$D$\ce{2Na + 2Cl2 -> 2NaCl}$
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   Answer: $\ce{2Na + Cl2 -> 2NaCl}$

   Cl: 2 on the left, so 2NaCl; then Na: 2 on the right, so 2Na. Changing NaCl to NaCl₂ changes a subscript, which is not allowed.

4. Question 4MediumWhen $\ce{C3H8 + O2 -> CO2 + H2O}$ is balanced, what is the coefficient of $\ce{O2}$?

   A3B4C5D10
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   Answer: 5

   C₃H₈ + 5O₂ → 3CO₂ + 4H₂O. The right side has 3 × 2 + 4 × 1 = 10 O atoms, which need 5 O₂.

5. Question 5MediumWhich equation is balanced?

   A$\ce{CaCO3 + HCl -> CaCl2 + H2O + CO2}$B$\ce{Zn + 2HCl -> ZnCl2 + H2}$C$\ce{Mg + O2 -> MgO}$D$\ce{H2O2 -> H2O + O2}$
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   Answer: $\ce{Zn + 2HCl -> ZnCl2 + H2}$

   Zn 1 = 1, H 2 = 2, Cl 2 = 2. The others need coefficients: 2HCl, 2Mg + O₂ → 2MgO, and 2H₂O₂ → 2H₂O + O₂.

6. Question 6HardBalance $\ce{Al + O2 -> Al2O3}$. What are the coefficients, in order?

   A2, 3, 1B4, 3, 2C2, 1, 1D4, 6, 2
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   Answer: 4, 3, 2

   4Al + 3O₂ → 2Al₂O₃: Al 4 = 4 and O 6 = 6. Using 2, 3, 1 leaves O at 6 on the left but only 3 on the right.

7. Question 7EasyA student balances $\ce{H2 + O2 -> H2O}$ by writing $\ce{H2 + O2 -> H2O2}$. What is wrong?

   ANothing: the atoms now balanceBThey changed a subscript, so the product is now a different substanceCHydrogen must always have a coefficient of 2DOxygen should be written as O, not O₂
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   Answer: They changed a subscript, so the product is now a different substance

   H₂O₂ is hydrogen peroxide, not water. The correct answer changes coefficients only: 2H₂ + O₂ → 2H₂O.