Calculate the pH of a formic acid solution that contains 1.35% formic acid by mass. (Assume a density of 1.01 g/mL for the solution.)

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Calculate the mass of formic acid in 100 g of solution: Since the solution is 1.35% formic acid by mass, there are 1.35 g of formic acid in 100 g of solution.

Determine the volume of the solution: Using the density (1.01 g/mL), convert the mass of the solution (100 g) to volume in mL.

Calculate the molarity of formic acid: Use the mass of formic acid (1.35 g) and its molar mass (46.03 g/mol) to find the number of moles, then divide by the volume of the solution in liters to find the molarity.

Write the expression for the ionization of formic acid (HCOOH) in water and set up the equilibrium expression using the acid dissociation constant (Ka) for formic acid.

Use the ICE table (Initial, Change, Equilibrium) to determine the concentration of H+ ions at equilibrium, and then calculate the pH using the formula pH = -log[H+].

Key Concepts

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

pH and its Calculation

pH is a measure of the acidity or basicity of a solution, defined as the negative logarithm of the hydrogen ion concentration. To calculate pH, one must first determine the concentration of hydrogen ions in the solution, which can be derived from the dissociation of acids in water. For weak acids like formic acid, the dissociation constant (Ka) is also needed to find the equilibrium concentrations.

Formic Acid and its Properties

Formic acid (HCOOH) is a weak acid that partially dissociates in solution. Its dissociation can be represented by the equation HCOOH ⇌ H⁺ + HCOO⁻. The extent of this dissociation is characterized by its acid dissociation constant (Ka), which indicates how much of the acid ionizes in solution. Understanding its properties is crucial for calculating the pH of its solution.

Concentration Calculations

To calculate the concentration of formic acid in the solution, one must convert the mass percentage to molarity. This involves determining the mass of formic acid in a given volume of solution, using the density to find the volume. The molarity is then calculated by dividing the number of moles of formic acid by the volume of the solution in liters, which is essential for further pH calculations.

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