Limiting Reagent and Percent Yield
Calculate the limiting reagent, theoretical yield, and percent yield for a precipitation reaction. Essential skills for quantitative chemistry.
Problem Scenario
When 25.0 g of calcium chloride (CaCl2) is mixed with 35.0 g of sodium phosphate (Na3PO4), calcium phosphate precipitates. The actual yield of calcium phosphite obtained is 18.5 g. 3CaCl2 + 2Na3PO4 -> Ca3(PO4)2 + 6NaCl
Given Data
Requirements
- Calculate moles of each reactant
- Determine the limiting reagent
- Calculate theoretical yield
- Calculate percent yield
Solution
Step 1:
Calculate moles of CaCl2: 25.0 g / 110.98 g/mol = 0.225 mol CaCl2
Step 2:
Calculate moles of Na3PO4: 35.0 g / 163.94 g/mol = 0.213 mol Na3PO4
Step 3:
Determine limiting reagent using mole ratios. From the equation: 3 mol CaCl2 : 2 mol Na3PO4. For 0.225 mol CaCl2, we need: 0.225 x (2/3) = 0.150 mol Na3PO4. We have 0.213 mol, which is more than enough. For 0.213 mol Na3PO4, we need: 0.213 x (3/2) = 0.320 mol CaCl2. We only have 0.225 mol. CaCl2 is the limiting reagent.
Step 4:
Calculate theoretical yield from limiting reagent. Moles of Ca3(PO4)2 = 0.225 mol CaCl2 x (1 mol Ca3(PO4)2 / 3 mol CaCl2) = 0.0750 mol. Mass = 0.0750 mol x 310.18 g/mol = 23.3 g
Step 5:
Calculate percent yield: (Actual yield / Theoretical yield) x 100% = (18.5 g / 23.3 g) x 100% = 79.4%
Final Answer
CaCl2 is the limiting reagent. Theoretical yield = 23.3 g Ca3(PO4)2. Percent yield = 79.4%. The reaction did not go to completion, possibly due to incomplete precipitation or loss during filtration.
Key Takeaways
- ✓The limiting reagent determines the maximum product possible
- ✓Compare mole ratios, not masses, to find limiting reagent
- ✓Theoretical yield is calculated from the limiting reagent
- ✓Percent yield is always less than or equal to 100%
Common Errors to Avoid
- ✗Using masses instead of moles to determine limiting reagent
- ✗Using the wrong stoichiometric coefficients
- ✗Forgetting to convert moles to grams for final answer
- ✗Confusing actual and theoretical yield in the percent yield formula
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Common questions about this problem type
Calculate how much of each reactant you would need to completely react with the other. The reactant that runs out first (you have less than needed) is limiting.
Real reactions rarely achieve 100% yield due to incomplete reactions, side reactions, loss during transfer or purification, and equilibrium limitations.