Problem 90 - Chemistry: Atoms First

Question

What volume of a 0.00945-M solution of potassium hydroxide would be required to titrate 50.00 mL of a sample of acid rain with a \(\mathrm{H}_{2} \mathrm{SO}_{4}\) concentration of \(1.23 \times 10^{-4} \mathrm{M}\) \(\mathrm{H}_{2} \mathrm{SO}_{4}(a q)+2 \mathrm{KOH}(a q) \longrightarrow \mathrm{K}_{2} \mathrm{SO}_{4}(a q)+2 \mathrm{H}_{2} \mathrm{O}(l)\)

Step-by-Step Solution

Answer

1.30 mL of the KOH solution would be required to titrate the acid rain sample.

Step 1: Write down the balanced chemical equation

The balanced chemical equation for the reaction is already provided: \[\mathrm{H}_{2}\mathrm{SO}_{4}(aq) + 2\mathrm{KOH}(aq) \longrightarrow \mathrm{K}_{2}\mathrm{SO}_{4}(aq) + 2\mathrm{H}_{2}\mathrm{O}(l)\].This tells us that 1 mole of sulfuric acid requires 2 moles of potassium hydroxide for complete neutralization.

Step 2: Calculate the moles of \(\mathrm{H}_{2}\mathrm{SO}_{4}\)

Use the concentration and volume to calculate the moles of \(\mathrm{H}_{2}\mathrm{SO}_{4}\).\[\text{Moles of }\mathrm{H}_{2}\mathrm{SO}_{4} = (1.23 \times 10^{-4}M) \times (0.05000L) = 6.15 \times 10^{-6} \text{ moles} \].

Step 3: Determine the moles of KOH needed

Using the stoichiometry from the balanced equation, calculate the moles of KOH needed to react completely with the \(\mathrm{H}_{2}\mathrm{SO}_{4}\).\[\text{Moles of KOH} = 2 \times \text{moles of } \mathrm{H}_{2}\mathrm{SO}_{4} = 2 \times 6.15 \times 10^{-6} = 1.23 \times 10^{-5} \text{ moles}\].

Step 4: Calculate the volume of KOH solution required

Use the concentration of the KOH solution to find the volume needed.\[\text{Volume of KOH} = \frac{\text{Moles of KOH}}{\text{Concentration of KOH}} = \frac{1.23 \times 10^{-5} \text{ moles}}{0.00945M} = 1.30 \times 10^{-3} \text{ liters or } 1.30 \text{ mL} \].

Key Concepts

Stoichiometry

Stoichiometry is the cornerstone of chemical reactions, where the relationship between reactants and products is quantitatively examined. In the context of acid-base titrations, stoichiometry allows us to calculate the precise amount of a titrant needed to react completely with an analyte. It is based on the principle of conservation of mass, which states that matter is neither created nor destroyed during a chemical reaction.

Using the balanced chemical equation \(\mathrm{H}_2\mathrm{SO}_4(aq) + 2\mathrm{KOH}(aq) \longrightarrow \mathrm{K}_2\mathrm{SO}_4(aq) + 2\mathrm{H}_2\mathrm{O}(l)\), we can deduce that sulfuric acid (\(\mathrm{H}_2\mathrm{SO}_4\)) reacts with potassium hydroxide (\(\mathrm{KOH}\)) in a 1:2 mole ratio. This means for every mole of \(\mathrm{H}_2\mathrm{SO}_4\), two moles of \(\mathrm{KOH}\) are required for a complete neutralization. When performing the titration, we leverage this stoichiometric relationship to determine the necessary volume of the KOH solution.

Molarity Calculation

Molarity, denoted as M, is a measure of concentration that expresses the number of moles of a solute in one liter of solution. It's critical for solving titration problems as it helps quantify the relationship between the volume of a solution and the amount of substance it contains.

To calculate the molarity of a solution, you can use the formula: \( \text{Molarity (M)} = \frac{\text{Moles of solute}}{\text{Volume of solution in liters}} \). This equation is essential for step 4 of the titration problem, where we are aiming to find the volume of KOH needed. Once we know the amount of KOH in moles (obtained from stoichiometry), and the molarity of the KOH solution (provided), the calculation of the required volume becomes straightforward. Thus, molarity serves as a bridge between the amount of substance and the volume of its solution.

Chemical Equation Balancing

The balance of a chemical equation ensures that the Law of Conservation of Mass is respected, meaning the number of atoms for each element in the reactants side is equal to the number of atoms in the products side. For a successful titration calculation, it's imperative that we start with a balanced chemical equation.

In our given equation \(\mathrm{H}_2\mathrm{SO}_4(aq) + 2\mathrm{KOH}(aq) \longrightarrow \mathrm{K}_2\mathrm{SO}_4(aq) + 2\mathrm{H}_2\mathrm{O}(l)\), we already have it balanced: one sulfate ion (SO4) is on each side, four hydrogen atoms are on each side, and two potassium atoms are on each side. The coefficients (1 for \(\mathrm{H}_2\mathrm{SO}_4\) and 2 for \(\mathrm{KOH}\)) tell us the ratio in which substances react or are produced. Without the correct balance, we could not have performed the stoichiometry accurately, as the mole ratio derived from the equation is key for further calculations.