020525 CHEM 102 SP25 Unit 04 pre

Unit 4: Describing Concentration

Concentration Units

• Concentration calculations require only one missing variable in a problem.

• Types of concentration questions include:

  • What is the concentration when ...?

  • How many grams or moles are needed to...?

  • How many liters or kg of solvent are needed to...?

Concentration Unit Calculation

• Different ways to express concentration:

  • Molarity (M):

    • Formula: M = mol solute / L solution

  • Mass Percent (%m/m & w/w):

    • Formula: %mass = (mass component / mass solution) × 100%

  • Volume Percent (%v/v):

    • Formula: %vol = (volume component / volume solution) × 100%

  • Mass/Volume Percent (%m/v or w/v):

    • Formula: m/v % = (mass component / volume solution) × 100%

  • Parts per million (ppm):

    • Formula: ppm = (mass solute / mass solution) × 10^6

  • Parts per billion (ppb):

    • Formula: ppb = (mass solute / mass solution) × 10^9

  • Mole Fraction (Χi):

    • Formula: Χ𝑖 = 𝑛𝑛𝑖𝑖 / 𝑛𝑛𝑡𝑡𝑡𝑡

  • Molality (m):

    • Formula: m = moles of solute / kg of solvent

Same Solution, Multiple Representations

• Solutions can be represented in various ways, yielding the same concentration values across different units.

Example Calculations: Finding Molality

• Example 1: Dissolving 0.790 mol KCl in 0.4975 kg of water:

  • Calculation: m = 0.790 mol / 0.4975 kg = 1.59 m

• Example 2: Dissolving 0.790 mol KCl in 500.0 mL of water (density = 0.995 g/cm³):

  • Calculation involves converting volume to mass and then to kg.

• Example 3: Dissolving 58.89 g KCl in 0.4975 kg of water:

  • Calculation: m = (58.89 g / 74.55 g/mol) / 0.4975 kg = 1.59 m

• Example 4: Dissolving 58.89 g KCl in 500.0 mL of water Requires conversion of volume to mass.

Solute Calculations

• How many moles of KCl to prepare 1.59 m solution in:

  • 500.0 mL water (density = 0.995 g/cm³): 0.790 moles required.

• How many grams of KCl to prepare 1.59 m solution in:

  • Calculation derived from moles needed and molecular weight.

Example: Finding Solvent

• Calculation to find water required for desired molality:

  • Given m = 1.59 m, KCl mass conversions determine solvent mass.

Converting Between Concentration Units

1. Convert the numerator and denominator using appropriate conversion factors.

2. Divide to get the new concentration unit.

Example: Calculate Molality of KCl

• Given:

  • 6.08 M KCl with density of solution = 1.104 g/mL.

• Procedure:

  • Calculate mass of solvent from volume of solution.

  • Convert concentration to molality.

• The moles remain constant, conversion of units considers density.