Calculating Moles from Molarity and Volume

Understanding Molarity and Moles

Molarity (M) is a key concept in chemistry that expresses the concentration of a solution. It is defined as the number of moles of solute per liter of solution. Calculating moles from molarity and volume is crucial for various laboratory applications, including preparing solutions and conducting experiments. Below are three practical examples that demonstrate how to perform these calculations.

Example 1: Preparing a Sodium Chloride Solution

In a laboratory setting, you may need to prepare a saline solution for various experiments. If you require a 0.9 M sodium chloride (NaCl) solution, and you want to prepare 500 mL of it, calculating the number of moles of NaCl needed is essential.

To find the moles of NaCl:

1. Identify the molarity (M): 0.9 M
2. Convert volume to liters: 500 mL = 0.500 L
3. Use the formula: Moles = Molarity × Volume

Calculating:

• Moles of NaCl = 0.9 M × 0.500 L = 0.45 moles

This means you need 0.45 moles of NaCl to create a 500 mL saline solution. Remember that the molar mass of NaCl is approximately 58.44 g/mol, so you would need about 26.3 grams of NaCl.

Example 2: Diluting a Concentrated Solution

Suppose you have a concentrated hydrochloric acid (HCl) solution with a molarity of 12 M, and you need to dilute it to a 2 M solution for a laboratory procedure. If you want to prepare 1 L of the diluted solution, you will first need to calculate the moles of HCl in the concentrated solution.

Using the formula:

1. Identify the molarity (M): 12 M
2. Convert volume to liters: 1 L of the final diluted solution
3. Calculate the moles of HCl in the diluted solution:

Calculating:

• Moles of HCl in 1 L at 12 M = 12 M × 1 L = 12 moles

If you want to dilute it to 2 M:

1. Calculate the required moles for 1 L of 2 M:

• Moles of HCl in 1 L at 2 M = 2 M × 1 L = 2 moles

To achieve the 2 M solution, you must take 0.33 L (or 333.33 mL) of the 12 M solution and dilute it with water to a total volume of 1 L.

Example 3: Analyzing a Chemical Reaction

In a chemical reaction, you may need to determine the number of moles of a reactant based on the volume and molarity of the solution used. For instance, if you are reacting magnesium sulfate (MgSO4) in a reaction and you have a 0.5 M solution, you need to know how many moles are present in 250 mL of that solution.

To find the moles:

1. Identify the molarity (M): 0.5 M
2. Convert volume to liters: 250 mL = 0.250 L
3. Use the formula: Moles = Molarity × Volume

Calculating:

• Moles of MgSO4 = 0.5 M × 0.250 L = 0.125 moles

This means you have 0.125 moles of magnesium sulfate in 250 mL of the 0.5 M solution. Understanding this is crucial for stoichiometric calculations in reactions involving MgSO4.

Conclusion

These examples illustrate how to calculate moles based on molarity and volume, which is an essential skill in various chemical applications. By mastering these calculations, you can ensure accuracy in preparing solutions and conducting experiments.