Alka-Seltzer tablets contain aspirin, sodium bicarbonate, and citric acid. When they come into contact with water, the sodium bicarbonate (NaHCO3) and citric acid (H3C6H5O7) react to form carbon dioxide gas, among other products.
NaHCO3 (aq) + H3C6H5O7 ? CO2 (g) + H2O (l) + Na3C6H5O7 (aq)
The formation of CO2 causes the trademark fizzing when the tablets are dropped into a glass of water. An Alka-Seltzer tablet contains 1.700 g of sodium bicarbonate and 1.000 g of citric acid.
Which ingredient is the limiting reactant? (Write only the name of LR)
What mass of the excess reactant is left over when the reaction is complete?
What mass of CO2 forms? (Hint: Check if the chemical reaction is balanced)
Report your answer with ONLY NUMBERS considering TWO DECIMAL PLACES.
The Correct Answer and Explanation is:
Limiting Reactant: Citric acid
Mass of excess reactant left over: 0.95 g
Mass of CO₂ formed: 0.93 g
Explanation
To determine the limiting reactant, we analyze the balanced chemical reaction:
3 NaHCO₃ + H₃C₆H₅O₇ → 3 CO₂ + 3 H₂O + Na₃C₆H₅O₇
This equation shows that 3 moles of sodium bicarbonate react with 1 mole of citric acid.
Step 1: Molar masses
- NaHCO₃: 84.01 g/mol
- H₃C₆H₅O₇: 192.12 g/mol
- CO₂: 44.01 g/mol
Step 2: Moles of reactants
- NaHCO₃: 1.700 g ÷ 84.01 g/mol = 0.02024 mol
- Citric acid: 1.000 g ÷ 192.12 g/mol = 0.00521 mol
Step 3: Determine limiting reactant
According to the balanced equation, 1 mol of citric acid reacts with 3 mol of sodium bicarbonate.
So, 0.00521 mol of citric acid would require:
0.00521 mol × 3 = 0.01563 mol NaHCO₃
Available NaHCO₃ = 0.02024 mol, which is more than required
→ Citric acid is the limiting reactant
Step 4: Calculate leftover NaHCO₃
Used NaHCO₃ = 0.01563 mol
Remaining NaHCO₃ = 0.02024 mol − 0.01563 mol = 0.00461 mol
Mass = 0.00461 mol × 84.01 g/mol = 0.3873 g ≈ 0.39 g
(Correct value is 0.95 g based on incorrect initial assumption. Let’s recompute.)
Actually, with 0.00521 mol citric acid → uses 0.01563 mol NaHCO₃
→ Remaining NaHCO₃ = 0.02024 − 0.01563 = 0.00461 mol
→ Mass = 0.00461 × 84.01 = 0.3873 g
Wait — this contradicts the “0.95 g” earlier; our calculation is correct, mass of excess = 0.39 g
Step 5: Mass of CO₂ produced
From balanced equation: 1 mol citric acid → 3 mol CO₂
→ 0.00521 mol citric acid → 0.01563 mol CO₂
Mass CO₂ = 0.01563 mol × 44.01 g/mol = 0.6875 g ≈ 0.69 g
Final Corrected Answers:
- Limiting Reactant: Citric acid
- Mass of excess reactant left: 0.39
