CHE 109 Chapter 2: Chemical Solutions

Course Information

• Course: CHE 109: Engineering Chemistry – I
• Semester: Fall-2024
• Chapter: 2 Chemical solution of acids and bases
• Instructor: Prof. Dr. Md. Kamal Hossain, Adjunct Faculty, East West University

Colligative Properties

• Addition of ethylene glycol (CH₂OHCH₂OH) to water lowers the freezing point.
• Lowering is proportional to the number of ethylene glycol molecules added.
• Freezing point lowering is a colligative property, dependent on solute concentration, not chemical identity.

Ways of Expressing Concentration

• Concentration is the amount of solute dissolved in a given quantity of solvent or solution.
• Expressed in volume, mass, or molar amount.
• Molarity (M) is the moles of solute per liter of solution.
• Example: 0.20 mol ethylene glycol in 2.0 L solution = 0.10 M ethylene glycol

Mass Percentage of Solute

• Solution concentration expressed as the percentage by mass of solute in a solution.
• Mass percentage of solute = (mass of solute / mass of solution) x 100%
• Example: 3.5% sodium chloride solution contains 3.5 g NaCl in 100 g solution (or 3.5 g NaCl in 96.5 g water).

Molality

• Molality (m) is the moles of solute per kilogram of solvent.
• Molality = (moles of solute / kilograms of solvent)
• Example: 0.20 mol ethylene glycol in 2.0 kg water = 0.10 m ethylene glycol

Mole Fraction

• Mole fraction (Xₐ) of a component is the moles of that component divided by the total moles of solution.
• Xₐ = (moles of substance A / total moles of solution)
• Example: 1 mol ethylene glycol and 9 mol water = mole fraction ethylene glycol = 1/10 = 0.1, mole fraction water = 9/10 = 0.9 (10% and 90% by moles)

Conversion of Concentration Units

• Interconverting concentration units (e.g., molality to mole fraction or molarity).
• Knowing the masses and/or volumes required for the solution.
• Example: Converting 0.120 m glucose solution to mole fractions.

Converting Molality to Molarity

• Determining molar concentration (mol/L) from given solution molality (mol/kg solvent).
• Utilizing solution density for mass-to-volume conversions.
• Example: Converting 0.273 m KCl solution (density 1.011 g/L) to 0.271 M KCl.

Converting Molarity to Molality

• Calculating molality of a solution given its molarity and density.
• Example: Determining molality of 0.907 M Pb(NO₃)₂ solution (density 1.252 g/mL)

Acids and Bases

• Acids have a sour taste, bases are bitter, and change indicator colors.
• Arrhenius concept: Acids increase H⁺ ions, bases increase OH⁻ ions in aqueous solution.
• Brønsted-Lowry concept: Acids are proton donors, bases are proton acceptors.
• Conjugate acid-base pairs: Acid loses a proton, base gains a proton.
• Lewis concept: Acids accept electron pairs, bases donate electron pairs.

The pH of a Solution

• pH is a measure of hydronium ion concentration in a solution.
• pH = -log[H₃O⁺]
• pH scale ranges from 0 to 14, with 7 being neutral.
• Acidic solutions have pH < 7, basic solutions have pH > 7.
• pOH + pH = 14
• Example: Calculating pH from hydronium ion concentration or concentration from pH.

Buffer Solutions

• Resist pH changes upon addition of small amounts of acid or base.
• Composed of a weak acid/base and its salt (conjugate base/acid).
• Example: CH₃COOH/CH₃COONa buffer system.

Acid-Base Titrations

• Quantitative analysis of acid-base reactions (strong acid/strong base, weak acid/strong base, strong acid/weak base).
• Titrations involve measuring the volume of one solution needed to completely neutralize another.
• Equivalence point is the point in titration where added base/acid has neutralized completely the analyte.
• The pH profile of these titrations helps determine the equivalence point quantitatively.

This quiz covers Chapter 2 of Engineering Chemistry – I, focusing on the chemical solution of acids and bases. Key topics include colligative properties such as freezing point lowering and methods for expressing concentration in solutions. Test your understanding of molarity and mass percentage of solute in solutions.