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Lesson 14.4 The pH of a Solution Suggested Reading Zumdahl Chapter 14 Section 14.3?Essential QuestionWhat is the pH scale?Learning Objectives:.Distinguish between aqueous solutions that acre acidic, neutral, or alkaline using the pH scale.?Identify which aqueous solution is more acidic using the pH scale.?State that each change of one pH unit represents a 10-fold change in the hydrogen ion concentration [H+].Deduce changes in [H+] when the pH of a solution changes by more than one pH unit.?IntroductionIn the previous lesson you learned that whether an aqueous solution is acidic, neutral, or basic depends on the hydrogen ion concentration. Because these values are often very small, it is usually more convenient to describe acidity in terms of pH, which is defined as the negative of the logarithm of the molar hydrogen-ion concentrationpH = - log[H+]For a solution in which the hydrogen ion concentration is 1.0 x 10-3 M, the pH is?pH = - log[1.0 x 10-3] = 3.00The rule for significant figures when working with logs is that the number of decimal places after the decimal points in the pH equals the number of significant figures reported in the hydrogen ion concentration.?The pH ScaleExpanding on the table from lesson 14.3 gives:?In acidic solution?[H+] >?1.0 x 10-7??pH < 7?In neutral solution?[H+] =?1.0 x 10-7??pH = 7?In basic (alkaline) solution?[H+] <?1.0 x 10-7??pH > 7?This gives the pH scaleGreat Explanation of the pH Scale:Watch the following YouTube Video: explanation:Watch the following YouTube Video: pH from the Hydrogen Ion ConcentrationThis is pretty straight-forward. Lets look at an example.A sample of orange juice has a hydrogen ion concentration of 2.9 x 10-4 M. What is the pH? Is the solution acidic?Solution:Plug the hydrogen ion concentration into the equation for pH given above, pH = - log (2.9 x 10-4?) = 3.54. The pH is less than 7.00, so the solution is acidic. Note the use of significant figure; there are two significant figures in the concentration, so the value for pH has two decimal places.This video shows why each change of one pH unit represents a 10-fold change in hydrogen ion concentration. This is one of the AP objectives, so don't skip it if you are taking the AP exam!Watch the following YouTube Video: pH from the Hydroxide Ion ConcentrationYou can find the pH of a solution of known hydroxide ion concentration in one of two ways:Solve the ion product constant Kw = [H+][OH-] for??[H+]?and then use pH = -log?[H+], at 25 deg C.Use pOH where pOH =?-log?[OH-] and pH + pOH = 14.00For example, suppose you wish to find the pH of and ammonia solution whose hydroxide ion concentration is 1.9 x 10-3 M. First, calculate pOHpOH =?-log?[1.9 x 10-3?M] = 2.72since pH + pOH = 14.00 we can solve for pH, where pH = 14.00 - 2.72 = 11.28.?YouTube VideoInterconverting pH and Hydrogen Ion ConcentrationIt is useful to be able to interconvert between?pH and Hydrogen Ion Concentration. Lets look at an example.Calculating the Hydrogen Ion Concentration for the pHThe pH of human blood is 7.40. What is the hydrogen ion concentration?Solution:?[H+] = antilog(-pH) = 10-pH?This gives?[H+] = antilog(-7.40) = 10-7.40 = 4.0 x 10-8 M.?Because pH was given to two decimal places we write the hydrogen ion concentration with two significant figures.HOMEWORK: Practice exercises 16.6 – 16.9 ................
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