2025 Unit 8 Part 2- Strong and Weak Acids

• Describe the methods of calculating the strength of an acid or a base

• Describe and solve a titration

​The Self-ionization of Water

• Pure water can ionize to form hydronium and hydroxide ion

  • H₂O + H₂O ↔​ H₃O⁺ + OH^-

  • A reversible reaction

• In pure water at a temperature of 25°C, the concentration of H₃O and OH are both equal to 1.0 x 10^-7

Measuring Ions in water

• Because the concentrations of hydronium and hydroxide are equal in water, it is considered a neutral solution

  • neither acidic nor basic

• The product of the ion concentrations of water is a constant that can be used to measure other pH

  • K_w= [H₃O⁺] x [OH^-] =1.0 x 10¹⁴

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The Meaning of pH

• pH means the power of hydrogen

• The pH scale is a means of measuring acidity and basicity by looking specifically at hydronium ions

  • The scale goes from 0 to 14

    • 0-6 acidic concentrations

    • 7 is neutral

    • 8-14 basic concentrations

  • You can find the pH value by looking at the concentration of H₃O

Every solution falls somewhere on the pH scale

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Calculating pH from [H₃O⁺]

• pH can be calculated with the following equation:

  • pH= - log [H₃O]

    • Because of the negative sign, pH decreases as you add more hydronium ions

    • can be rearranged into [H₃O]= 10^-pH

​Finding the log button on the calculator

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Sample problem

• What is the pH of a 0.00010 M solution of HNO₃, a strong acid

  • Step one: identify concentration: 0.00010 M =​ 1.0 x 10^-4

  • Step two: plug into your calculator and solve

    • pH= - log [H₃O] = -log (1.0 x 10^-4)

Measuring pH

• It is often necessary to measure pH of a substance, and there are many methods to do so

• Indicators: Dyes that turn different colors in the presence of different pH levels

  • can be liquid or paper form

  • some are natural like red cabbage juice, but others are synthetic

    • In general, synthetic indicators can read more specific values

  • pH paper changes red for acids and blue for bases

Measuring pH- pH Meters

• pH meters

  • electronic instruments with an electrical probe that measures the conductivity of the solution, and infers the ion concentration​

  • Are much more precise than indicators

  • better for solutions that are colored because they are easier to read than a color change

pOH

• pH measures the concentration of hydronium ions, pOH measures the concentration of Hydroxide ions

  • Not used as often as pH

• pH+pOH=14

​Strong Acids and Bases

• The strength of an acid or a bases is based on the degree to which they ionize in water​

  • ​Strong acids completely ionize in aqueous solutions to form hydronium ions

    • Examples: HCl, HI, HNO₃

  • Strong Bases completely ionize in solution to produce hydroxide ions

    • Examples: NaOH, LiOH, Ba(OH)₂

• Each Acid has its own ionization constant like water, K_a

  • Weak acids have a small K_a value

    • Diprotic and triprotic acids have multiple K_a values, based on how many ions there are​​

​Weak Acids

• A weak acid only partially ionizes in solution

  • Has an unequal equilibrium arrow to show that the reaction favors the reactants more than the products

  • Each ionizes to a different degree

• Weak acids are more common than strong acids

​Weak Bases

• A weak base produces a small amount of hydroxide ions in water because the formation of reactants is favored

• ​Ionization constant is K_b

  • Weak bases have weak ionization constants​

​Strength vs Concentration

• Two factors contribute to the pH of a solution

  • Strength-how much the acid ionizes

    • Stronger acids will have a lower pH than weak acids in the same amounts

  • Concentration: How many moles there are of the acid

    • A high concentration will have a low pH​

Neutralization Reactions

• Neutralization Reactions occur when an acid and a base combine to form a salt and water

  • Acid + Base → Salt + Water

  • Continues until both reactants are used up

• Amounts needed can be calculated using the mole ratios in a balanced equation

  • Can also calculate the concentrations of acids and bases used

Neutralization Reaction Sample Problem 1

Titrations

• Titration: The General addition of one solution to another to reach an equivalence point

  • It uses an acid or a base of a known concentration to determine the concentration of an unknown

  • As a strong base is added to a strong acid, the two react until they neutralize

    • Equivalence Point: the point at which a neutralization reaction is complete and the pH is equal to 7

Parts of a Titration

• Burette: a graduated glass tube with a tap at the end

• Titrand: a solution with an unknown concentration

• Titrant: a solution with a known concentration used in a titration

  • Can be either an acid or a base​

• Standard solution: a solution of known concentration

• Indicator Solution: a chemical that changes color in the presence of an acid or base

Setting up a Titration

• Setting up Burets: Set up your burette

  • Rinse and acid buret with the titrant buret with the base three times each

  • Fill the acid buret to a point above the 0mL mark with the acid of unknown concentration

    • release some acid from the buret into the waste beaker to release air bubbles

    • Record the volume of the acid in the buret as the initial volume

  • Add three drops of the indicator solution to the flask

Setting up a Titration

• Record the initial volume of your titrant to the closest 0.01 mL

• Place your Flask with your unknown concentration under the buret so that the tip of the buret rests inside the flask

• For the first measurement: Quickly release the titrant until the equivalence point is reached

  • The equivalence point is reached when a very light color remains after 30 seconds of swirling

• Subtract the initial volume reading on the buret from the final volume to find the exact amount added​

• For every other measurement: SLOWLY release the titrant into the flask while gently swirling the Flask until the equivalence point is reached

Titration Calculations

• Once you have the volume of your solution, you can solve for the concentration of the unknown

  • Concentration would be measured Using Molarity

• Steps

  • ​Identify Balanced Chemical Equation of the reaction taking place

  • Determine the moles of your known concentration substance used

  • Convert your Moles of your known substance to Moles of your unknown substance

  • Use Your moles of unknown and amount used to determine the Molarity of the unknown

Titration Calculation Sample Problem

In a titration, 27.4 mL of 0.0154 M Ba(OH)₂ is added to a 20.0 mL sample of a HCl solution​ whose concentration is unknown until the equivalence point is reached. What is the molarity of the acid solution?
Ba(OH)₂ + 2HCl →​ BaCl₂ + 2H₂O

• Step 1: Convert the volume of your known solution into moles

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Titration Calculation Sample Problem

In a titration, 27.4 mL of 0.0154 M Ba(OH)₂ is added to a 20.0 mL sample of an HCl solution​ whose concentration is unknown until the equivalence point is reached. What is the molarity of the acid solution?

• Step 1: Identify the Balanced Equation​

  • Ba(OH)₂ + 2HCl →​ BaCl₂ + 2H₂O

• Step 2: Convert the volume of your known solution into moles=4.22E-4 mol Ba(OH)₂

• Step Three: Use the mole ratio to convert from Ba(OH)​

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Titration Calculation Sample Problem

In a titration, 27.4 mL of 0.0154 M Ba(OH)₂ is added to a 20.0 mL sample of a HCl solution​ whose concentration is unknown until the equivalence point is reached. What is the molarity of the acid solution?

• Step 1: Identify the Balanced Equation​

  • Ba(OH)₂ + 2HCl →​ BaCl₂ + 2H₂O

• Step 2: Convert the volume of your known solution into moles=4.22E-4 mol Ba(OH)₂

• Step 3: Use the mole ration to convert from Ba(OH)​ to HCl= 8.44E-4 mol HCl

• Step 4: Solve for your molarity​

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