Problem 78 - Chemistry: Atoms First

Question

What volume of 0.0105-M HBr solution is required to titrate 125 mL of a 0.0100-M Ca(OH)_solution? \(\mathrm{Ca}(\mathrm{OH})_{2}(a q)+2 \mathrm{HBr}(a q) \rightarrow \mathrm{CaBr}_{2}(a q)+2 \mathrm{H}_{2} \mathrm{O}(l)\)

Step-by-Step Solution

Answer

0.2381 L (or 238.1 mL) of 0.0105-M HBr solution is required to titrate 125 mL of a 0.0100-M Ca(OH)_2 solution.

Step 1: Write down the balanced chemical equation

The balanced chemical equation for the reaction of calcium hydroxide \(\mathrm{Ca(OH)_2}\) with hydrobromic acid \(\mathrm{HBr}\) is already provided: \(\mathrm{Ca(OH)_2}(aq) + 2\mathrm{HBr}(aq) \rightarrow \mathrm{CaBr}_2(aq) + 2\mathrm{H}_2O(l)\). Each mole of \(\mathrm{Ca(OH)_2}\) reacts with 2 moles of \(\mathrm{HBr}\).

Step 2: Calculate moles of \(\mathrm{Ca(OH)_2}\)

Calculate the moles of \(\mathrm{Ca(OH)_2}\) using its concentration and volume. The molarity (M) is 0.0100 M and the volume (V) is 125 mL or 0.125 L. \[\text{moles of } \mathrm{Ca(OH)_2} = M \times V = 0.0100\ M \times 0.125\ L\]

Step 3: Determine moles of \(\mathrm{HBr}\) needed

From the balanced chemical equation, 2 moles of \(\mathrm{HBr}\) are needed per mole of \(\mathrm{Ca(OH)_2}\). Multiply the moles of \(\mathrm{Ca(OH)_2}\) calculated in Step 2 by 2 to find the moles of \(\mathrm{HBr}\) required.

Step 4: Calculate the volume of \(\mathrm{HBr}\) solution

Using the moles of \(\mathrm{HBr}\) required and its molarity (0.0105 M), calculate the volume of \(\mathrm{HBr}\) solution needed to titrate the \(\mathrm{Ca(OH)_2}\) solution. \(V = \frac{\text{moles of } \mathrm{HBr}}{\text{Molarity of } \mathrm{HBr}}\).

Key Concepts

Chemical Titration

Chemical titration is a laboratory technique used to determine the concentration of a solute in a solution. It involves the gradual addition of a solution of known concentration (the titrant) to a solution whose concentration we want to determine (the analyte). The point at which the reaction is complete is known as the endpoint, which can often be determined visually with an indicator or through the use of instruments.

In the exercise, the titration process requires adding a 0.0105-M hydrobromic acid (HBr) solution to calcium hydroxide \( \text{Ca(OH)_2} \) until the reaction between them is complete. To do this successfully, we have to understand some underlying concepts a student may struggle with, which include molarity and stoichiometry, as well as the importance of a balanced chemical equation.

Molarity and Concentration

Molarity is a measure of the concentration of a solution and is defined as the number of moles of solute per liter of solution. It's denoted as 'M' and is crucial in titrations as it allows for the calculation of how much titrant to use. In our problem, we're working with 0.0105-M HBr and 0.0100-M Ca(OH)_2 solutions.

The calculation of the necessary volume of HBr starts with the known volume and molarity of Ca(OH)_2. To determine the moles of Ca(OH)_2, multiply molarity (M) by volume (V). The resulting value gives us the footing we need to find out how much of our titrant (HBr) is necessary to reach the endpoint of the titration.

Stoichiometry of Reactions

Stoichiometry is the part of chemistry that involves calculating the amounts of reactants and products in a chemical reaction. It relies on the balanced chemical equation, which provides the ratio of the molecules involved. In the given balanced equation, \( \text{Ca(OH)_2} + 2HBr \rightarrow \text{CaBr}_2 + 2H_2O \) signifies that two moles of HBr react with one mole of Ca(OH)_2.

With the moles of Ca(OH)_2 calculated, we use stoichiometry to deduce that twice the amount of HBr is needed because of the 1:2 ratio from the equation. This aspect of chemical reactions can be tough for students to grasp, but it's essential for solving titration problems.

Balancing Chemical Equations

Balancing chemical equations is fundamental in chemistry because it ensures that the Law of Conservation of Mass is followed—matter cannot be created nor destroyed. To balance a chemical equation, one must make sure that the number of atoms of each element is the same on both the reactants and products side of the equation.

In our exercise, the equation \( \text{Ca(OH)_2} + 2HBr \rightarrow \text{CaBr}_2 + 2H_2O \) is already balanced. Understanding how to balance equations is necessary for stoichiometry and thus for solving titration problems. A common mistake is neglecting to balance chemical equations, which leads to incorrect stoichiometric calculations and can make the task of titration seem more difficult than it is. It's important for students not only to learn to balance equations but also to comprehend the relationship between the balanced equation and the quantities involved in the chemical reaction.