Ch.17 - Acids and Bases

Tro - Chemistry: A Molecular Approach 6th Edition

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Tro 6th Edition

Ch.17 - Acids and Bases

Problem 109a

Chapter 17, Problem 109a

Determine the pH of each solution. a. 0.10 M NH₄Cl

Verified step by step guidance

Step 1: Identify that NH4Cl is a salt and will dissociate completely in water to form NH4+ and Cl-. The NH4+ ion is the conjugate acid of the weak base NH3, and will therefore react with water to form NH3 and H3O+.

Step 2: Write the chemical equation for the reaction of NH4+ with water: NH4+ + H2O -> NH3 + H3O+.

Step 3: Set up an ICE (Initial, Change, Equilibrium) table to keep track of the concentrations of each species. Initially, the concentration of NH4+ is 0.10 M, and the concentrations of NH3 and H3O+ are 0. At equilibrium, the concentration of NH4+ will have decreased by x, and the concentrations of NH3 and H3O+ will have increased by x.

Step 4: Write the expression for the equilibrium constant of the reaction, Ka, which is equal to [NH3][H3O+] / [NH4+]. Since NH4+ is the conjugate acid of NH3, Ka can be found by taking the inverse of the Kb for NH3 and using the relationship Kw = Ka * Kb.

Step 5: Substitute the equilibrium concentrations from the ICE table into the Ka expression and solve for x, which represents the concentration of H3O+ at equilibrium. The pH of the solution can then be found by taking the negative logarithm of the H3O+ concentration.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

pH Scale

The pH scale measures the acidity or basicity of a solution, ranging from 0 to 14. A pH of 7 is considered neutral, while values below 7 indicate acidity and values above 7 indicate basicity. The scale is logarithmic, meaning each whole number change represents a tenfold change in hydrogen ion concentration.

Ammonium Chloride (NH4Cl) as a Weak Acid

Ammonium chloride is a salt formed from the weak base ammonia (NH3) and the strong acid hydrochloric acid (HCl). In solution, NH4Cl dissociates into NH4+ and Cl-. The NH4+ ion can donate a proton to water, forming NH3 and H3O+, which contributes to the acidity of the solution, thus lowering the pH.

Henderson-Hasselbalch Equation

The Henderson-Hasselbalch equation relates the pH of a solution to the concentration of an acid and its conjugate base. For weak acids, it is expressed as pH = pKa + log([A-]/[HA]). This equation is useful for calculating the pH of buffer solutions and can be adapted to find the pH of solutions containing weak acids or their salts, like NH4Cl.

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Henderson-Hasselbalch Equation

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