Chemistry Chapter on Water Ionization and pH

Water Auto-Ionization

• Water molecules undergo a reversible reaction to form hydronium (H3O+H_3O^+H3​O+) and hydroxide (OH−OH^-OH−) ions.
• The equilibrium expression for this process is Kw=[H3O+][OH−]K_w = [H_3O^+][OH-]Kw​=[H3​O+][OH−], where KwK_wKw​ is the ion-product constant for water.
• At 25°C, Kw=1.0×10−14K_w = 1.0 × 10^{-14}Kw​=1.0×10−14.
• In pure water, [H3O+]=[OH−]=1.0×10−7[H_3O^+]=[OH^-]=1.0 × 10^{-7}[H3​O+]=[OH−]=1.0×10−7 M.

pH, pOH, and pKw

• pH is the negative logarithm of the hydronium ion concentration: pH=−log[H3O+]pH = -log[H_3O^+]pH=−log[H3​O+].
• Similarly, pOH is the negative logarithm of the hydroxide ion concentration: pOH=−log[OH−]pOH = -log[OH-]pOH=−log[OH−].
• The relationship between pH, pOH, and pKw is: pH+pOH=pKw=14.00pH + pOH = pK_w = 14.00pH+pOH=pKw​=14.00.

Sample Exercise: Calculating [OH-] and pH

• A solution with [H3O+]=3.4×10−4M[H_3O^+]=3.4 × 10^{-4} M[H3​O+]=3.4×10−4M is acidic because its pH is less than 7.
• The [OH−][OH^-][OH−] can be calculated using the KwK_wKw​ expression: [OH−]=1.0×10−14[H3O+][OH-]=\frac{1.0 × 10^{-14}}{[H_3O^+]}[OH−]=[H3​O+]1.0×10−14​.

Titration of Strong Acids and Strong Bases

• The reaction between a strong acid and a strong base is a neutralization reaction: H3O+(aq)+OH−(aq)→2H2O(l)H_3O^+(aq) + OH^-(aq) \rightarrow 2H_2O(l)H3​O+(aq)+OH−(aq)→2H2​O(l).
• The equivalence point is reached when the moles of acid and base are equal.
• The volume of titrant needed to reach the equivalence point can be calculated using the equation: Ma×Va=Mb×VbM_a \times V_a = M_b \times V_bMa​×Va​=Mb​×Vb​.
• Before the equivalence point, the pH is determined by the excess acid concentration.
• At the equivalence point, the pH is 7.00.
• After the equivalence point, the pH is determined by the excess base concentration.

Titration Curve of a Strong Acid with a Strong Base

• The titration curve of a strong acid with a strong base shows a steep rise near the equivalence point.
• The pH at the equivalence point is 7.00.

Titration of a Weak Acid with a Strong Base

• The reaction between a weak acid and a strong base produces the conjugate base of the weak acid.
• The pH is calculated using the KaK_aKa​ expression for the weak acid.
• The equivalence point is reached when the moles of acid and base are equal.
• The pH at the equivalence point is greater than 7.00.

Experiment 3: Determination of Acetic Acid Content in Vinegar

• The concentration of acetic acid in vinegar can be determined by titrating it with a standardized NaOH solution.
• The standardization of NaOH solution involves titrating it with a primary standard (KHP).
• The data from the titration is used to calculate the concentration of acetic acid in the vinegar sample using stoichiometry and dimensional analysis.
• The volume of NaOH needed to reach the equivalence point is calculated using the equation: molCH3COOH=molNaOHmol CH_3COOH = mol NaOHmolCH3​COOH=molNaOH.
• The titration curve of a weak acid with a strong base shows a gradual rise near the equivalence point.

Precipitation Titration

• A precipitation titration is based on the reaction between two ionic compounds to form an insoluble precipitate.
• The equivalence point is reached when the stoichiometric amount of titrant has been added to react with the analyte.
• The Mohr method is used to determine the chloride concentration using a standard solution of silver nitrate.
• The concentration of chloride in the sample can be calculated in terms of mg/L and % NaCl using dimensional analysis.

Solubility

• The solubility of a substance is the amount of the substance that can be dissolved in a given quantity of solvent at a given temperature.
• The solubility guidelines for common ionic compounds can be used to predict which compounds are soluble or insoluble in water.
• The solubility of a compound is affected by several factors, including temperature and the presence of other ions in the solution.