Lecture on Concentration Expressions and Calculations

Introduction

• Overview of concentration expressions:
  • Weight per Weight (w/w)
  • Weight per Volume (w/v)
  • Volume per Volume (v/v)

Example Calculation: Molarity from Percentage

• Problem Statement: Calculate the molarity of hydrochloric acid with 70% w/w and density of 1.67 g/ml.
• Objective: Convert percentage to molarity (mol/L).

Conversion Process

1. Convert w/w to Molarity:
   • Start with percentage weight per weight (w/w).
   • Use unit conversion: gram per gram to mol per liter.
   • Use molar mass (g/mol) to convert grams to moles.
   • Density (g/ml) to convert grams to volume (ml).
   • Conversion from ml to liters.
2. Calculation Steps:
   • Percentage: 70% means 70g in 100g solution.
   • Density: 1.67 g/ml helps to find volume.
   • Molar mass: 100.5 g/mol for HCl.
   • Final Molarity: 11.63 mol/L.

Solution Preparation Example

• Task: Prepare 500 ml of 2 molar sulfuric acid from 90% w/w commercial reagent.
• Calculation for Preparation:
  • Determine molarity of concentrated acid.
  • Calculate necessary volume to transfer.
  • Dilute to achieve desired concentration.
• Steps:
  • Measure and transfer 59.2 ml to a 500 ml volumetric flask.
  • Dilute with water to the mark and shake.

Expression of Concentration

• Parts Per Thousand (ppt): Exponent 3
• Parts Per Million (ppm): Exponent 6
• Parts Per Billion (ppb): Exponent 9

Usage Contexts

• Used for dilute concentrations like pollution or water quality.
• Conversion examples:
  • 1 ppm = 1 gram per million grams of solution.

Calculation Examples

• Sodium in Solution: 2.5 mg of sodium in 500 ml results in 5 ppm.
• Cadmium in Water: 1.25 ppb, determine mass in 1L water.

Conversion Techniques

• Molarity from ppm: Example with ferrous iron in solution.
  • Important to know terms (weight per weight, etc.)
  • Use molecular weight to convert.
• Conversion of ppm to molarity:
  • Understand terms and unit conversion (e.g., mg/L to mol/L).

Tips for Lab Preparation

• Understand concentration expressions for correct lab preparation.
• Use volumetric flasks for precision.

Summary

• Techniques for converting between various concentration expressions.
• Practical examples to apply these techniques in real-world chemistry problems.