Question

You need to make 150.0 mL of a 0.10-M NaCl solution. You have solid NaCl, and your lab partner has a 2.5-M NaCl solution. Explain how you each make the 0.10-M NaCl solution

Short answer

To prepare 150.0 mL of 0.10-M NaCl solution using solid NaCl, first calculate the moles of NaCl required: Moles of NaCl = 0.10 M × (150.0 mL × (1 L / 1000 mL)). Then find the mass of solid NaCl needed: Mass of solid NaCl = Moles of NaCl × 58.44 g/mol. Dissolve the calculated mass of solid NaCl in 100 mL of water, then add water until the total volume reaches 150.0 mL. To prepare the 0.10-M NaCl solution using a 2.5-M NaCl solution, use the dilution formula (M1 × V1 = M2 × V2) to determine the volume of the 2.5-M NaCl solution needed: V1 = (M2 × V2) / M1. Measure the calculated volume of the 2.5-M NaCl solution and add water until the total volume reaches 150.0 mL.

Step-by-step solution

Calculate the moles of NaCl required for desired solution

We'll first determine how many moles of NaCl are needed for the desired 0.10-M NaCl solution. Since Molarity (M) = moles of solute/liters of solution, we can rearrange the formula to find the moles of NaCl. Moles of NaCl = Molarity × Volume of Solution (in liters) Moles of NaCl = 0.10 M × (150.0 mL × (1 L / 1000 mL))

Calculate mass of solid NaCl needed

Next, we need to convert moles of NaCl to grams. The formula needed is: Mass (g) = moles × molar mass With the molar mass of NaCl = 58.44 g/mol, we can find the mass of solid NaCl required as follows: Mass of solid NaCl = Moles of NaCl × 58.44 g/mol

Prepare the 0.10-M NaCl solution using solid NaCl and water

To prepare the desired 0.10-M NaCl solution, weigh the calculated mass of solid NaCl and dissolve it in approximately 100 mL of water. Stir well to ensure the solid completely dissolves. Once the NaCl is dissolved, add water until the total volume reaches 150.0 mL. The final concentration of the solute in the resulting solution will be 0.10 M.

Calculate volume of the 2.5-M NaCl solution needed

For the second part of the problem, we need to determine the volume of the 2.5-M NaCl solution required for dilution to obtain 0.10 M NaCl concentration. To do this, we will use the dilution formula: M1 × V1 = M2 × V2 Where M1 is the initial molarity, V1 is the initial volume, M2 is the final molarity, and V2 is the final volume. We need to find V1. Rearrange the formula to calculate V1: V1 = (M2 × V2) / M1

Prepare the 0.10-M NaCl solution using the 2.5-M NaCl solution

Using the calculated volume of the 2.5-M NaCl solution, carefully measure this volume using a graduated cylinder or pipette. Add the measured volume to a volumetric flask or beaker and add water until the total volume reaches 150.0 mL. Stir well to ensure thorough mixing. The final concentration of the solute in the resulting solution will be 0.10 M.

Key concepts

Understanding Molarity

Molarity, often symbolized by "M," is a measure of the concentration of a solute in a solution. It indicates how many moles of a substance are present in one liter of solution. In simple terms, molarity is about knowing how strong or weak a solution is. For example, a 0.10 M NaCl solution means there are 0.10 moles of sodium chloride (NaCl) dissolved in every liter of the solution. To calculate molarity, use the formula:

• Molarity (M) = moles of solute / liters of solution

To utilize this formula, you should know the amount of solute in moles and the total volume of the solution in liters. This calculation is essential for preparing solutions with desired concentrations.
Understanding molarity is crucial in laboratories, as it helps in performing consistent experiments and obtaining reliable results.

The Dilution Formula Explained

Sometimes, you might have a concentrated stock solution, and you need to make a diluted solution with a lower molarity. This is where the dilution formula comes in handy. The formula is:

• M1 × V1 = M2 × V2

Here, M1 is the molarity of the starting solution, V1 is the volume of the starting solution you will use, M2 is the molarity of the diluted solution, and V2 is the volume of the diluted solution you want to prepare. This equation helps you find out how much of the concentrated solution is needed to achieve a particular concentration in a new volume. Using the dilution formula ensures that you maintain the precise concentration needed for your experiments or tasks, which is vital in both academic and professional settings.

Moles Calculation Made Simple

Calculating moles is a fundamental aspect of preparing solutions. A mole measures the quantity of a substance and is based on Avogadro's number, which is approximately 6.022 × 10^23. To find the number of moles in a solution, you can rearrange the molarity formula:

• Moles of solute = Molarity (M) × Volume of solution (L)

For instance, if you have a solution that needs to be prepared with 0.10 M concentration and a volume of 150.0 mL (or 0.150 L), multiply these values to calculate the moles of solute required. Understanding how to calculate moles accurately is crucial because it dictates the exact amount of solute you need, influencing the success of the solution preparation.

Mass Calculation in Solution Preparation

Once you know the moles of a compound, you can easily find its mass, which is often required when weighing out solutes like solid NaCl for solution preparation. The formula to convert moles to mass is:

• Mass (g) = moles × molar mass (g/mol)

The molar mass is the mass of one mole of a substance and is given in grams per mole (g/mol). For NaCl, it's 58.44 g/mol. So, if you calculate the moles of NaCl needed, multiply it by the molar mass to get the mass in grams. This step ensures you measure the correct amount of solid NaCl to achieve the desired solution concentration. Calculating mass is essential in ensuring precision and consistency in scientific experiments and processes.