Practice a College Board-style short-answer question on Chemical Reactions. Write your response, then reveal the model answer to see exactly what earns each point.
Short Answer Question · Unit 4 · Stoichiometry & Net Ionic Equations
A student combines 50.0 mL of 0.200 M Pb(NO₃)₂(aq) with 50.0 mL of 0.300 M NaCl(aq). A white precipitate forms according to the reaction:
Pb(NO₃)₂(aq) + 2NaCl(aq) → PbCl₂(s) + 2NaNO₃(aq)
A
Write the net ionic equation for this reaction, and identify the spectator ion(s).
✓ Model answer (earns the point)
The net ionic equation is Pb²⁺(aq) + 2Cl⁻(aq) → PbCl₂(s). The spectator ions are Na⁺ and NO₃⁻, since they remain dissolved and unchanged on both sides of the full ionic equation.
Why it scores: Writes a correctly balanced net ionic equation with proper states AND correctly names both spectator ions.
B
Determine the limiting reactant in this reaction, showing your reasoning.
✓ Model answer (earns the point)
Moles of Pb(NO₃)₂ = 0.0500 L × 0.200 M = 0.0100 mol. Moles of NaCl = 0.0500 L × 0.300 M = 0.0150 mol. According to the 1:2 mole ratio, 0.0100 mol Pb(NO₃)₂ would require 0.0200 mol NaCl to fully react, but only 0.0150 mol NaCl is available. Since NaCl runs out first, NaCl is the limiting reactant.
Why it scores: Calculates moles of both reactants AND correctly applies the stoichiometric mole ratio to determine which one is insufficient, rather than just comparing raw concentrations or volumes.
C
Calculate the theoretical yield of PbCl₂ in grams (molar mass of PbCl₂ = 278.1 g/mol).
✓ Model answer (earns the point)
Using the limiting reactant (NaCl, 0.0150 mol) and the 2:1 mole ratio of NaCl to PbCl₂: moles of PbCl₂ = 0.0150 mol NaCl × (1 mol PbCl₂ ÷ 2 mol NaCl) = 0.00750 mol PbCl₂. Mass of PbCl₂ = 0.00750 mol × 278.1 g/mol = 2.09 g PbCl₂.
Why it scores: Correctly uses the limiting reactant (not the excess reactant) to calculate moles of product via the correct mole ratio, AND converts to mass using molar mass with the correct number of significant figures.
How to score points on SAQs
Always show moles, not just grams or volumes. Stoichiometry calculations must pass through moles — graders want to see that step explicitly.
State which reactant is limiting and why. "NaCl is limiting because only 0.0150 mol is available, less than the 0.0200 mol needed" scores higher than just stating the answer.
Use the limiting reactant for all yield calculations. A common error is accidentally calculating yield from the excess reactant.
Double-check states of matter in net ionic equations. (aq) and (s) labels matter and are often explicitly checked by graders.
Keep it tight. Show the calculation steps clearly, but don't over-explain — clear work earns the point faster than a long paragraph.