Determine the Limiting reagent.
Using the balanced equation, determine the limiting reagent. The limiting reagent will be used to calculate the concentration of the solution.
You can locate the mass of citric acid and sodium bicarbonate on the Alka-Seltzer package
Reagents
Mass (mg)
Molar Mass
Sodium Bicarbonate
1916
Citric Acid
1000
The Correct Answer and Explanation is:
To determine the limiting reagent, we need to:
- Convert the mass of each reagent to moles using their molar masses.
- Use the balanced chemical equation to find the stoichiometric ratio.
- Determine which reagent is limiting — the one that gets used up first in the reaction.
Step 1: Balanced Chemical Equation
The reaction between citric acid (H₃C₆H₅O₇) and sodium bicarbonate (NaHCO₃) is: H₃C₆H₅O₇+3NaHCO₃→Na₃C₆H₅O₇+3CO₂+3H₂O\text{H₃C₆H₅O₇} + 3\text{NaHCO₃} \rightarrow \text{Na₃C₆H₅O₇} + 3\text{CO₂} + 3\text{H₂O}
This tells us that 1 mole of citric acid reacts with 3 moles of sodium bicarbonate.
Step 2: Moles of Each Reagent
- Molar Mass of Sodium Bicarbonate (NaHCO₃) = 84.01 g/mol
- Molar Mass of Citric Acid (H₃C₆H₅O₇) = 192.13 g/mol
Convert mass to grams:
- Sodium bicarbonate: 1916 mg = 1.916 g
- Citric acid: 1000 mg = 1.000 g
Now calculate moles:
- Moles of NaHCO₃ = 1.916 g / 84.01 g/mol ≈ 0.02281 mol
- Moles of H₃C₆H₅O₇ = 1.000 g / 192.13 g/mol ≈ 0.00521 mol
Step 3: Determine Limiting Reagent
According to the balanced equation, we need 3 moles of NaHCO₃ for every mole of citric acid:
- Required moles of NaHCO₃ for 0.00521 mol citric acid = 0.00521 × 3 = 0.01563 mol
We have 0.02281 mol of NaHCO₃ available, which is more than enough.
Thus, citric acid is the limiting reagent, as it will be completely consumed first.
Conclusion (Explanation)
In chemical reactions, the limiting reagent is the substance that is completely consumed when the reaction is complete. The amount of product formed depends on this reagent. In this case, by using the masses given on the Alka-Seltzer package and converting them to moles, we find that citric acid provides only 0.00521 mol. Given the 1:3 reaction ratio, it would require only 0.01563 mol of sodium bicarbonate, which is less than the 0.02281 mol available. Hence, citric acid will run out first and limits the extent of the reaction. This makes it the limiting reagent, and any calculations for products (like CO₂ production or final concentrations) must be based on the amount of citric acid present.
