BMST - Basic Math and Science Chemical Reactions and Stoichiometry Questions and Answers — Questions and Answers
Question 1: When the following chemical equation is balanced, what is the correct coefficient for sodium hydroxide (NaOH)? ___ FeCl₃ + ___ NaOH → ___ Fe(OH)₃ + ___ NaCl
- 2
- 6
- 3 (Correct answer)
- 1
Correct answer: 3
To balance the equation, we must have the same number of each type of atom on both sides. The unbalanced equation has 1 Fe, 3 Cl, 1 Na, 1 O, and 1 H on the left, and 1 Fe, 1 Cl, 1 Na, 3 O, and 3 H on the right. To balance the Cl atoms, a coefficient of 3 is needed for NaCl. To balance the Na atoms, a coefficient of 3 is then needed for NaOH. This also balances the O and H atoms. The final balanced equation is FeCl₃ + 3NaOH → Fe(OH)₃ + 3NaCl.
Question 2: A chemist observes a reaction where a single, complex compound breaks down into two or more simpler substances when heat is applied. Which of the following reaction types does this observation describe?
- Synthesis
- Decomposition (Correct answer)
- Single Replacement
- Double Replacement
Correct answer: Decomposition
A decomposition reaction is a type of chemical reaction in which one compound breaks down into two or more simpler substances. The general form is AB → A + B. Synthesis is the opposite (A + B → AB), while single and double replacement reactions involve the exchange of elements between two or more compounds.
Question 3: What is the approximate molar mass of magnesium nitrate, Mg(NO₃)₂?
- 148.3 g/mol (Correct answer)
- 74.3 g/mol
- 124.3 g/mol
- 86.3 g/mol
Correct answer: 148.3 g/mol
To calculate the molar mass, sum the atomic masses of all atoms in the formula. Mg: 24.3 g/mol. N: 14.0 g/mol. O: 16.0 g/mol. The formula Mg(NO₃)₂ has 1 Mg, 2 N, and 6 O atoms. Molar Mass = (1 × 24.3) + (2 × 14.0) + (6 × 16.0) = 24.3 + 28.0 + 96.0 = 148.3 g/mol.
Question 4: In the complete combustion of propane (C₃H₈), the balanced equation is: C₃H₈ + 5O₂ → 3CO₂ + 4H₂O. If 44.1 g of propane is burned, how many grams of carbon dioxide (CO₂) are produced?
- 44.1 g
- 88.2 g
- 176.4 g
- 132.3 g (Correct answer)
Correct answer: 132.3 g
First, find the moles of propane (C₃H₈, molar mass ≈ 44.1 g/mol): 44.1 g / 44.1 g/mol = 1.0 mole C₃H₈. According to the balanced equation, the mole ratio of C₃H₈ to CO₂ is 1:3. So, 1.0 mole of C₃H₈ will produce 3.0 moles of CO₂. Finally, convert moles of CO₂ to grams (molar mass of CO₂ ≈ 44.0 g/mol): 3.0 moles × 44.0 g/mol = 132.0 g. The closest answer is 132.3 g.
Question 5: A technician is performing a reaction that requires 1 mole of reactant X for every 2 moles of reactant Y. The technician mixes 3 moles of X with 5 moles of Y. Which of the following statements is true?
- Reactant X is the limiting reactant, and some Y will be left over.
- Reactant Y is the limiting reactant, and some X will be left over. (Correct answer)
- Both reactants will be completely consumed.
- The reaction will not proceed as there is no limiting reactant.
Correct answer: Reactant Y is the limiting reactant, and some X will be left over.
To completely use 3 moles of X, you would need 3 moles X * (2 moles Y / 1 mole X) = 6 moles of Y. Since only 5 moles of Y are available, Y is the limiting reactant. To completely use 5 moles of Y, you would need 5 moles Y * (1 mole X / 2 moles Y) = 2.5 moles of X. Since 3 moles of X are available, there is an excess of X. Therefore, reactant Y is the limiting reactant and will be completely consumed, leaving some reactant X.
Question 6: In an experiment, the calculated maximum amount of product that could be formed (theoretical yield) was 50.0 grams. After the reaction, the actual amount of product collected was 42.5 grams. What is the percent yield of this reaction?
- 117.6%
- 85.0% (Correct answer)
- 92.5%
- 7.5%
Correct answer: 85.0%
The percent yield is calculated using the formula: (Actual Yield / Theoretical Yield) × 100%. In this case, (42.5 g / 50.0 g) × 100% = 0.85 × 100% = 85.0%.
BMST - Basic Math and Science Chemical Reactions and Stoichiometry Questions and Answers
When the following chemical equation is balanced, what is the correct coefficient for sodium hydroxide (NaOH)? ___ FeCl₃ + ___ NaOH → ___ Fe(OH)₃ + ___ NaCl