Chapter 9 Exercise 16 Using Molarity as a Conversion Factor

Chapter 9 Exercise 16: Using Molarity as a Conversion Factor

Introduction

This video covers how to use molarity as a conversion factor to determine the mass of solute needed to prepare a specific volume of solution. Molarity (M) is a unit of concentration representing the number of moles of solute per liter of solution.

Main Points

1. Molarity and Conversion Factors

- Definition: Molarity (M) = moles of solute per liter of solution.

- Unit Breakdown: Moles of solute/Liter of solution.

- Example: 1.5 M NaOH means 1.5 moles of NaOH per liter of solution.

2. Example A: Sodium Hydroxide (NaOH) Solution

- Given:

     - 2 liters of 1.5 M NaOH solution.
     - Calculate grams of NaOH needed.

- Steps:

     - Convert molarity into moles per liter: 1.5 moles NaOH/1 liter.
     - Find molar mass of NaOH using periodic table:
       - Na: 23 g/mol
       - O: 16 g/mol
       - H: 1 g/mol
       - Total = 40 g/mol
     - Use conversion factors:
       - Convert liters to moles: \(2 \text{ L} \times \frac{1.5 \text{ moles}}{1 \text{ L}} = 3 \text{ moles NaOH}\)
       - Convert moles to grams: \(3 \text{ moles} \times 40 \text{ g/mole} = 120 \text{ g NaOH}\)
     - **Result:** 120 grams of NaOH.

3. Example B: Potassium Chloride (KCl) Solution

- Given:

     - 4 liters of 0.2 M KCl solution.
     - Calculate grams of KCl needed.

- Steps:

     - Convert molarity into moles per liter: 0.2 moles KCl/1 liter.
     - Find molar mass of KCl using periodic table:
       - K: 39 g/mol
       - Cl: 35.45 g/mol
       - Total = 74.45 g/mol
     - Use conversion factors:
       - Convert liters to moles: \(4 \text{ L} \times \frac{0.2 \text{ moles}}{1 \text{ L}} = 0.8 \text{ moles KCl}\)
       - Convert moles to grams: \(0.8 \text{ moles} \times 74.45 \text{ g/mole} = 59.56 \text{ g KCl}\)
     - **Result:** 59.56 grams of KCl.

4. Example C: Hydrochloric Acid (HCl) Solution

- Given:

     - 25 milliliters (0.025 liters) of 6 M HCl solution.
     - Calculate grams of HCl needed.

- Steps:

     - Convert molarity into moles per liter: 6 moles HCl/1 liter.
     - Find molar mass of HCl using periodic table:
       - H: 1 g/mol
       - Cl: 35.45 g/mol
       - Total = 36.45 g/mol
     - Use conversion factors:
       - Convert milliliters to liters: 25 mL = 0.025 L.
       - Convert liters to moles: \(0.025 \text{ L} \times \frac{6 \text{ moles}}{1 \text{ L}} = 0.15 \text{ moles HCl}\)
       - Convert moles to grams: \(0.15 \text{ moles} \times 36.45 \text{ g/mole} = 5.4675 \text{ g HCl}\)
     - **Result:** 5.4675 grams of HCl.

Conclusion

Using molarity as a conversion factor involves breaking down molarity into moles per liter and using it alongside molar mass to convert between liters of solution and grams of solute. This method is essential for accurately preparing solutions in chemistry.