Calculate The Molarity of The Following Solutions Worksheet Answers

Molarity is a fundamental concept in chemistry that measures the concentration of a solute in a solution. This guide provides worksheet answers for calculating molarity, explains the formula, and includes a calculator for quick results.

What is Molarity?

Molarity (M) is defined as the number of moles of solute dissolved in one liter of solution. It is one of the most common ways to express solution concentration in chemistry. Molarity is particularly useful when dealing with reactions and dilutions where precise concentrations are required.

The unit for molarity is moles per liter (mol/L). For example, a 1.0 M solution contains 1 mole of solute in every liter of solution.

Molarity Formula

M = n / V

Where:

M = Molarity (mol/L)
n = Number of moles of solute (mol)
V = Volume of solution (L)

The formula shows that molarity is calculated by dividing the number of moles of solute by the volume of the solution in liters. This gives the concentration of the solute in the solution.

Worksheet Answers

Below are answers to common molarity calculation problems. Each problem includes the given values and the calculated molarity.

Problem 1

Calculate the molarity of a solution prepared by dissolving 5.00 grams of sodium chloride (NaCl) in enough water to make 250.0 mL of solution.

Solution

1. Calculate the moles of NaCl: n = mass / molar mass = 5.00 g / 58.44 g/mol ≈ 0.0856 mol

2. Convert volume to liters: V = 250.0 mL × (1 L/1000 mL) = 0.2500 L

3. Calculate molarity: M = n / V = 0.0856 mol / 0.2500 L ≈ 0.342 M

Problem 2

What is the molarity of a solution containing 0.250 moles of potassium nitrate (KNO₃) in 500.0 mL of solution?

Solution

1. Convert volume to liters: V = 500.0 mL × (1 L/1000 mL) = 0.5000 L

2. Calculate molarity: M = n / V = 0.250 mol / 0.5000 L = 0.500 M

Problem 3

How many moles of solute are present in 750.0 mL of a 0.750 M solution?

Solution

1. Convert volume to liters: V = 750.0 mL × (1 L/1000 mL) = 0.7500 L

2. Calculate moles of solute: n = M × V = 0.750 M × 0.7500 L ≈ 0.5625 mol

Common Mistakes

When calculating molarity, several common mistakes can lead to incorrect results. Here are some pitfalls to avoid:

Incorrect Units: Ensure that the volume is in liters. If given in milliliters, convert to liters first.
Molar Mass Errors: Double-check the molar mass of the solute to ensure accurate mole calculations.
Significant Figures: Pay attention to significant figures in the given values and round the final answer appropriately.
Volume Measurement: Ensure that the volume measurement includes the solute and any added solvent.

FAQ

What is the difference between molarity and molality?

Molarity (M) is defined as moles of solute per liter of solution, while molality (m) is defined as moles of solute per kilogram of solvent. Molarity depends on temperature because the volume changes with temperature, whereas molality remains constant.

How do I convert molarity to molality?

To convert molarity to molality, you need to know the density of the solution. The formula is m = M × (density of solution / density of water). This conversion is necessary when the solution's volume changes significantly with temperature.

Can molarity be used for gases?

Molarity is typically used for liquid solutions. For gases, molar concentration (mol/L) is often used instead, as gases are compressible and their volumes can change with pressure and temperature.