Molarity Practice Worksheet Solutions: Boost Your Chemistry Skills

Understanding the intricacies of molarity in chemistry is a fundamental skill for any budding scientist. It's not just about knowing how to dilute solutions or calculate how much of a solute you need; it's about grasping the essence of concentration measurements that are vital for both theoretical chemistry and practical applications like drug formulation or environmental testing. Let's delve into the essentials of molarity through this comprehensive guide and practice worksheet solutions.

What is Molarity?

Before we dive into calculations, let's refresh our definition of molarity:

• Molarity (M) is defined as the number of moles of solute per liter of solution. The formula for calculating molarity is:

M = moles of solute / volume of solution in liters

Here, the moles of solute are calculated based on the substance's molecular mass and its mass, while the volume is usually measured in liters or milliliters (1L = 1000mL).

Step-by-Step Guide to Solving Molarity Problems

Let's go through the steps to solve molarity problems:

1. Identify the Solute: Determine what chemical compound you're dealing with. For example, NaCl (sodium chloride) or glucose (C₆H₁₂O₆).
2. Determine the Moles: Use the compound's molar mass to convert from grams to moles.

   Moles = Mass of Solute / Molar Mass of Solute
   

3. Calculate the Volume of Solution: Ensure the volume is in liters. Remember, 1L = 1000mL.
4. Use the Molarity Formula: Plug the values into the molarity formula.
5. Calculate: Perform the division to find the molarity.

Example 1: Preparing a 0.2 M NaCl Solution

Suppose we need to prepare a 0.2 M solution of sodium chloride (NaCl).

🧪 Note: The molecular weight of NaCl is 58.44 g/mol.

1. Identify the Solute: NaCl
2. Determine Moles:

   Moles needed = (0.2 M) * (1 L) = 0.2 moles
   

3. Convert Moles to Grams:

   Grams of NaCl = 0.2 moles * 58.44 g/mol = 11.688 g
   

4. Calculate Volume: We're using 1L as our volume for this example, but you could use a different volume.

Example 2: Finding the Molarity of a Given Solution

You have a 250 mL solution containing 15 grams of glucose (C₆H₁₂O₆). What is the molarity?

🔬 Note: The molecular weight of glucose is 180.16 g/mol.

1. Identify the Solute: Glucose
2. Determine Moles:

   Moles = 15 g / 180.16 g/mol = 0.0832 moles
   

3. Convert Volume to Liters: 250 mL = 0.250 L
4. Calculate Molarity:

   Molarity = 0.0832 moles / 0.250 L = 0.333 M
   

Molarity Practice Problems

Here are some additional problems for practice:

1. Calculate the molarity of a solution made by dissolving 12.5 grams of sodium bicarbonate (NaHCO₃) in enough water to make 500 mL of solution.
2. Determine the amount of calcium chloride (CaCl₂) required to make 300 mL of a 0.5 M solution.

Feel free to attempt these problems on your own, or follow along with the step-by-step solutions below:

Solution for Problem 1:

• Molecular weight of NaHCO₃: 84.01 g/mol
• Moles of NaHCO₃:

  Moles = 12.5 g / 84.01 g/mol ≈ 0.1489 moles
  

• Volume in Liters: 500 mL = 0.5 L
• Molarity:

  Molarity = 0.1489 moles / 0.5 L ≈ 0.298 M
  

Solution for Problem 2:

• Molecular weight of CaCl₂: 111.0 g/mol
• Volume in Liters: 300 mL = 0.3 L
• Moles of CaCl₂:

  Moles = 0.5 M * 0.3 L = 0.15 moles
  

• Gram of CaCl₂ required:

  Mass = 0.15 moles * 111.0 g/mol = 16.65 g
  

To summarize, mastering molarity involves understanding how to:

• Identify the solute and calculate the number of moles.
• Convert between units of volume (liters to milliliters).
• Use the molarity formula to either prepare or analyze solution concentrations.
• Perform basic stoichiometric calculations involving molecular weights.

With practice, you can become adept at not just solving problems but also in recognizing when molarity calculations are necessary in real-world chemical applications.