Problem 86 - Chemistry The Molecular Science

Question

You need \(1.00 \mathrm{~L}\) of \(0.125-\mathrm{M} \mathrm{H}_{2} \mathrm{SO}_{4}\). Which method is best to prepare this solution? Explain your choice. (a) Dilute \(36.0 \mathrm{~mL}\) of \(1.25-\mathrm{M} \mathrm{H}_{2} \mathrm{SO}_{4}\) to a volume of \(1.00 \mathrm{~L}\). (b) Dilute \(20.8 \mathrm{~mL}\) of \(6.00-\mathrm{M} \mathrm{H}_{2} \mathrm{SO}_{4}\) to a volume of \(1.00 \mathrm{~L}\). (c) Add \(50.0 \mathrm{~mL}\) of \(3.00-\mathrm{M} \mathrm{H}_{2} \mathrm{SO}_{4}\) to \(950 . \mathrm{mL}\) water. (d) Add 500. mL of \(0.500-\mathrm{M} \mathrm{H}_{2} \mathrm{SO}_{4}\) to \(500 . \mathrm{mL}\) water.

Step-by-Step Solution

Answer

Option B is best, as it results in 0.1248 moles, near the required 0.125 moles.

Step 1: Understand the Target Solution

The goal is to prepare 1.00 L of a 0.125 M H₂SO₄ solution. This means the final solution must contain 0.125 moles of H₂SO₄ per liter of solution.

Step 2: Calculate Moles Required

Using the concentration formula, calculate the moles of solute needed: \[ ext{Moles of } ext{H}_2 ext{SO}_4 = C imes V = 0.125 ext{ M} imes 1.00 ext{ L} = 0.125 ext{ moles} \] This means we need 0.125 moles of H₂SO₄ in the final solution.

Step 3: Evaluate Option A

Calculate moles of H₂SO₄ using 36.0 mL of 1.25 M solution:\[ ext{Moles} = C imes V = 1.25 ext{ M} imes 0.036 ext{ L} = 0.045 ext{ moles} \]This is insufficient as it provides only 0.045 moles.

Step 4: Evaluate Option B

Calculate moles from 20.8 mL of 6.00 M solution:\[ ext{Moles} = C imes V = 6.00 ext{ M} imes 0.0208 ext{ L} = 0.1248 ext{ moles} \]This is almost 0.125 moles, close enough for practical laboratory purposes.

Step 5: Evaluate Option C

Determine moles from 50.0 mL of 3.00 M solution:\[ ext{Moles} = C imes V = 3.00 ext{ M} imes 0.050 ext{ L} = 0.150 ext{ moles} \]This is more than needed, making the solution too concentrated.

Step 6: Evaluate Option D

Calculate moles from 500 mL of 0.500 M solution:\[ ext{Moles} = C imes V = 0.500 ext{ M} imes 0.500 ext{ L} = 0.250 ext{ moles} \]This results in twice the moles required, making this solution too concentrated as well.

Step 7: Choose the Best Method

Option B, which dilutes 20.8 mL of 6.00 M to 1.00 L, gives 0.1248 moles, which is closest to the required 0.125 moles, and thus is the best choice considering practical precision limits.

Key Concepts

Molarity

Molarity is a way to express the concentration of a solution. It tells us how many moles of a solute are present in one liter of a solution. The formula to calculate molarity is simple: \[\text{Molarity (M)} = \frac{\text{moles of solute}}{\text{liters of solution}}\]In simpler terms, if you have a bottle with 1 liter of liquid and it contains 0.125 moles of sulfuric acid (H₂SO₄), then the molarity of this solution is 0.125 M. Remember, when preparing chemical solutions in a lab, always take note of the molarity as it determines how concentrated the solution is. To ensure precise results, it's essential to calculate the correct molarity before starting your experiment.

Dilution Method

The dilution method involves combining a concentrated solution with more solvent to achieve a desired concentration. This process is ideal when you have a higher concentration solution but need something more dilute. To calculate the dilution, you can use the equation: \[M_1 \times V_1 = M_2 \times V_2\]where \(M_1\) and \(V_1\) are the molarity and volume of the initial concentrated solution, and \(M_2\) and \(V_2\) the molarity and volume of the diluted solution. Choosing the right method for dilution is crucial to achieving accurate results. The goal is to match or come as close as possible to the desired product concentration, while maintaining the integrity and accuracy of the solution.

Acid Solution

Acid solutions can be found in many applications including cleaning products and batteries. Understanding how to prepare them in a laboratory setting is important for safety and effectiveness. Sulfuric acid (H₂SO₄) is commonly used in labs due to its strong acidic nature. When creating an acid solution, safety is paramount. Always add the concentrated acid to the water, never the other way around. This minimizes the risk of a violent reaction. Proper handling and storage are key in maintaining both safety and the stability of the solution. Use appropriate protective equipment such as gloves and goggles and work in a well-ventilated area.

Laboratory Techniques

Mastering basic laboratory techniques is essential for creating accurate chemical solutions. Here are some tips that can make your lab work more precise:

• **Measuring Accurately:** Use calibrated measuring devices such as pipettes or graduated cylinders. Precision is critical when determining volumes, especially when they are as small as a few milliliters.
• **Mixing Properly:** When combining solutions, use magnetic stirrers or vortex mixers to ensure thorough mixing. This guarantees even concentration throughout the solution.
• **Recording Observations:** Document every step and observation. Keeping detailed records can help you trace any errors in your method and adjust future preparations.

Practicing these techniques often will help you become more proficient in preparing solutions accurately, leading to reliable experimental results every time. By following these guidelines, you can also enhance safety and increase the success rate of your experiments.