Problem 47 - Chemistry and Chemical Reactivity

Question

An experiment in your laboratory requires \(500 .\) mL of a \(0.0200 \mathrm{M}\) solution of \(\mathrm{Na}_{2} \mathrm{CO}_{3} .\) You are given solid \(\mathrm{Na}_{2} \mathrm{CO}_{3},\) distilled water, and a \(500 .\) -mL volumetric flask. Describe how to prepare the required solution.

Step-by-Step Solution

Answer

Weigh 1.060 g of Na₂CO₃, dissolve it in some distilled water, and fill up to 500 mL in a volumetric flask.

Step 1: Calculate Moles of Na₂CO₃ Needed

To prepare a 500 mL solution of 0.0200 M Na₂CO₃, we first need to calculate the number of moles of Na₂CO₃ required. Molarity is defined as moles per liter, so we use the formula:\[ ext{Moles of Na₂CO₃} = ext{Volume (L)} imes ext{Molarity (M)}\]The volume must be in liters: 500 mL = 0.500 L.\[ ext{Moles of Na₂CO₃} = 0.500 ext{ L} imes 0.0200 ext{ M} = 0.0100 ext{ moles}\]

Step 2: Calculate Mass of Na₂CO₃ Needed

Next, we calculate the mass of Na₂CO₃ required using its molar mass. The molar mass of Na₂CO₃ is approximately 105.99 g/mol.\[ ext{Mass} = ext{Moles} imes ext{Molar Mass} = 0.0100 ext{ moles} imes 105.99 ext{ g/mol} = 1.0599 ext{ g}\]We need approximately 1.060 g of Na₂CO₃.

Step 3: Weigh the Na₂CO₃

Accurately weigh out 1.060 g of solid Na₂CO₃ using a balance. Ensure the balance is calibrated and that you use a clean container or weighing paper to hold the substance.

Step 4: Dissolve Na₂CO₃ in Water

Transfer the weighed Na₂CO₃ into a 500 mL volumetric flask. Add some distilled water to the flask to dissolve the solid. Swirl the flask gently until all the Na₂CO₃ is completely dissolved before adding more water.

Step 5: Fill to the Mark

After the Na₂CO₃ is dissolved, carefully add distilled water to the volumetric flask. Fill up to the 500 mL mark on the flask. Ensure the bottom of the meniscus is at the mark when viewed at eye level.

Step 6: Mix the Solution

Cap the flask and invert it several times to ensure a thorough mixing of the solution so that the Na₂CO₃ is evenly distributed throughout the 500 mL of solution. The solution is now ready for use.

Key Concepts

Molarity Calculations

Molarity is a key concept in chemistry that defines how concentrated a solution is. It is expressed as moles of a solute per liter of solution. To calculate molarity, you need to know two things: the number of moles of solute and the volume of the solution in liters. Here’s a simple breakdown:

• Number of Moles of Solute: This can be found using the formula \( \text{Moles} = \text{Volume (L)} \times \text{Molarity (M)} \).
• Volume of Solution: This must always be in liters for molarity calculations, even if the problem gives you milliliters. For example, 500 mL can be converted to 0.500 L.

By substituting the values, you can easily calculate the moles of the substance you need, making the rest of the solution preparation steps straightforward. For instance, to make a 0.0200 M solution, if you know the total volume, you can find out exactly how many moles are needed, using the formula above. This understanding is crucial for precisely preparing solutions in the lab.

Mass Calculations

Once you know the number of moles required, the next step is to convert this value into mass. This involves using the molar mass of the compound. The molar mass is the weight of one mole of a substance and is typically measured in grams per mole (g/mol).

• Molar Mass of Compound: For \( \text{Na}_2 \text{CO}_3 \), the molar mass is approximately 105.99 g/mol. This piece of information is necessary for finding the mass needed.
• Calculating Mass: Use the formula \( \text{Mass} = \text{Moles} \times \text{Molar Mass} \) to find the precise amount to weigh out. For instance, if you calculate that you need 0.0100 moles, the mass will be \( 0.0100 \text{ moles} \times 105.99 \text{ g/mol} = 1.0599 \text{ g} \).

Knowing how to perform these calculations correctly ensures that you can accurately measure and prepare a solution with the desired concentration, minimizing errors in experiments.

Lab Procedures

Proper lab procedures ensure safety and accuracy when preparing chemical solutions. Here's a simple guide to follow:

• Weighing the Chemical: Use a calibrated balance to measure the precise amount of \( \text{Na}_2 \text{CO}_3 \). Clean the container or paper you use for weighing to avoid contaminants.


• Dissolution Process: Transfer the weighed \( \text{Na}_2 \text{CO}_3 \) into a volumetric flask. Add some distilled water to begin dissolving it. Swirl or gently shake the flask until the compound is fully dissolved.


• Final Dilution: Add distilled water gradually, nearly to the 500 mL mark but stopping just shy to prevent overfilling. Then, adjust carefully until the bottom of the meniscus aligns with the mark. Always view the meniscus at eye level to ensure accuracy.


• Mixing: After reaching the desired volume, cap the flask and invert it several times. This step fully mixes the solution, ensuring uniform concentration.

Following these procedures not only ensures the precision of your solution but also helps maintain a safe and efficient laboratory environment.