A 7.0 mass % solution of H3PO4 in water has a density of 1.0353 g/mL. Calculate the pH and the molar concentrations of all species present (H3PO4, H2PO4-, PO43-, H3O+ , and OH-) in the solution. Values of equilibrium constants are listed in Appendix C.

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Identify the given information: The solution is 7.0 mass % H3PO4 with a density of 1.0353 g/mL.

Calculate the mass of H3PO4 in 1 liter of solution: Use the density to find the total mass of the solution, then apply the mass percentage to find the mass of H3PO4.

Convert the mass of H3PO4 to moles: Use the molar mass of H3PO4 to convert the mass to moles.

Set up the equilibrium expressions: Use the equilibrium constants (Ka1, Ka2, and Ka3) for the dissociation of H3PO4 to find the concentrations of H2PO4^-, PO4^3-, H3O^+, and OH^-.

Calculate the pH: Use the concentration of H3O^+ to find the pH of the solution.

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Key Concepts

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

Concentration and Molarity

Concentration refers to the amount of solute present in a given volume of solution. Molarity, a common unit of concentration, is defined as moles of solute per liter of solution (mol/L). To calculate molarity from mass percent, one must first determine the mass of solute in a specific volume of solution, which can be derived from the solution's density.

Acid-Base Equilibria

Acid-base equilibria involve the dissociation of acids and bases in solution, which can be described using equilibrium constants (Ka for acids and Kb for bases). For phosphoric acid (H3PO4), it can lose protons in a stepwise manner, leading to the formation of various species such as H2PO4- and PO43-. Understanding these equilibria is essential for calculating the concentrations of all species in the solution.

pH and pOH Calculations

pH is a measure of the hydrogen ion concentration in a solution, calculated as pH = -log[H3O+]. The pOH, which measures hydroxide ion concentration, is related to pH by the equation pH + pOH = 14 at 25°C. To find the pH of the solution, one must first determine the concentration of H3O+ ions, which can be derived from the dissociation of H3PO4 and the equilibrium constants provided.

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pH and pOH Calculations

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