Calculating the pH of Strong Acid Solutions

A/ivoys write the major species present in the soiution. 

The H+ from the strong acid drives the equiiibrium H20 — H+ + OH to the ieft. 

When we deal with acid-base equilibria, we must focus on the solution components and their chemistry. For example, what species are present in a 1.0-M solution of HCl Since hydrochloric acid is a strong acid, we assnme that it is completely dissociated. Thns although the label on the bottle says 1.0 M HCl, the solution contains virtually no HCl molecules. Typically, container labels indicate the substance(s) used to make up the solution but do not necessarily describe the solution components after dissolution. Thns a 1.0-M HCl solution contains H and Cl ions rather than HCl molecules. 

To illnstrate the main ideas involved, let us calculate the pH of 1.0 M HCl. We first list the major species H+, Cl , and H2O. Since we want to calculate the pH, we will focus on those major species that can furnish H. Obviously, we must consider H+ from the dissociation of HCl. However, H2O also furnishes H by autoionization, which is often represented by the simple dissociation reaction 

However, is autoionization an important source of ions In pure water at 25°C, [H+] is 10 M. In I.O M HCl solution, the water will produce even less than 10 M H+, since by Le Chatelier s principle the H from the dissociated HCl will drive the position of the water equilibrium to the left. Thus the amount of H contributed by water is negligible compared with the 1.0 M H from the dissociation of HCl. Therefore, we can say that [H ] in the solution is 1.0 A/. The pH is then 

HNO3 is a strong acid, so the ions in solution are H and NO3. In this case. 

OBJECTIVE To learn to calculate the pH of solutions of strong acids. 

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