Chapter 9 Exercise 14 Calculate Molarity

Chapter 9 Exercise 14: Calculate Molarity

Introduction

This video explains how to calculate the molarity of a solution, a fundamental concept in chemistry that describes the concentration of a solute in a given volume of solution. The presenter walks through several examples to demonstrate the process of calculating molarity.

Main Points

1. Molarity Formula

- Definition: Molarity (M) is defined as the number of moles of solute per liter of solution.

- Formula:

     \[
     M = \frac{\text{moles of solute}}{\text{liters of solution}}
     \]

- Units: Moles per liter (mol/L).

2. Example A: Glucose Solution

- Given:

     - 2 moles of glucose
     - 4 liters of solution

- Calculation:

     \[
     M = \frac{2 \text{ moles}}{4 \text{ liters}} = 0.5 \text{ M}
     \]

- Result: 0.5 molar glucose solution.

3. Example B: Potassium Hydroxide (KOH) Solution

- Given:

     - 4 grams of KOH
     - 2 liters of solution

- Steps:

     - **Convert grams to moles:**
       - Use the periodic table to find the molar mass of KOH.
       - Molar mass of KOH = 39 (K) + 16 (O) + 1 (H) = 56 grams/mole.
       - Convert: 
         \[
         \frac{4 \text{ grams}}{56 \text{ grams/mole}} = 0.07143 \text{ moles}
         \]
     - **Calculate molarity:**
       \[
       M = \frac{0.07143 \text{ moles}}{2 \text{ liters}} = 0.036 \text{ M}
       \]

- Result: 0.036 molar potassium hydroxide solution.

4. Example C: Sodium Chloride (NaCl) Solution

- Given:

     - 5.85 grams of NaCl
     - 400 milliliters (0.4 liters) of solution

- Steps:

     - **Convert grams to moles:**
       - Use the periodic table to find the molar mass of NaCl.
       - Molar mass of NaCl = 23 (Na) + 35.5 (Cl) = 58.5 grams/mole.
       - Convert:
         \[
         \frac{5.85 \text{ grams}}{58.5 \text{ grams/mole}} = 0.1 \text{ moles}
         \]
     - **Convert milliliters to liters:**
       \[
       400 \text{ mL} = 0.4 \text{ L}
       \]
     - **Calculate molarity:**
       \[
       M = \frac{0.1 \text{ moles}}{0.4 \text{ liters}} = 0.25 \text{ M}
       \]

- Result: 0.25 molar sodium chloride solution.

Conclusion

The video provides clear examples of how to calculate the molarity of various solutions by converting grams to moles, milliliters to liters, and using the molarity formula. Understanding these calculations is essential for working with solutions in chemistry.