Calculating Molarity of a Solution

Key Concept

• Molarity: Defined as the moles of solute divided by the liters of solution.

Practical Measurement

• Measuring the volume of a solution is practical.
• Counting the number of molecules is impractical.
• We calculate moles from the mass of the solute.

Example: Sugar in Water

Objective

• Calculate the concentration of a sugar solution.

Given:

• Substance: Sugar (Sucrose, ( C_{12}H_{22}O_{11} ))
• Mass of Sugar: 50 grams
• Volume of Water: 200 milliliters (0.2 liters)_

Steps to Calculate Molarity

1. Convert Mass to Moles

   • Molar Mass of Sucrose: 342 grams/mole
   • Calculation: [ \text{Moles of Sucrose} = \frac{50 \text{ grams}}{342 \text{ grams/mole}} = 0.146 \text{ moles} ]

2. Calculate Molarity

   • Use formula: [ \text{Molarity (M)} = \frac{\text{Moles of solute}}{\text{Liters of solution}} ]
   • Calculation: [ \text{Molarity} = \frac{0.146 \text{ moles}}{0.2 \text{ liters}} = 0.731 \text{ M} ]

Conclusion

• To find the concentration of a solution:
  1. Convert mass of solute to moles using molar mass.
  2. Divide moles by volume of solution in liters to find molarity.