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Understanding pH: Measuring Acidity and Basicity in Solutions

This resource provides a comprehensive overview of pH, the scale that measures the acidity or alkalinity of a solution. It explains how pH is determined by the concentration of hydrogen ions ([H+]) and how this relates to hydroxide ions ([OH-]). The scale ranges from 0 to 14, with lower values indicating acidity and higher values indicating basicity. The document includes sample problems for calculating pH and pOH, demonstrating concepts such as the relationship between pH and [H+] as well as real-world applications of acid-base chemistry.

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Understanding pH: Measuring Acidity and Basicity in Solutions

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  1. Created by Seth Floyd - What's on Your Shelf? Acid/Base Chemistry • The pH scale and what does it mean?

  2. Created by Seth Floyd - What's on Your Shelf? pH • What is it? • What does it mean? • How do you measure it? • Can you determine other things from pH?

  3. pH an Overview pH is the measure of the concentration of hydrogen ions is a solution. pH is measured on a log scale. pH = - log[H+] pH is used to indicate the degree of acidity or alkalinity (basic) of a solution. A solution is of a water solvent (polar) Created by Seth Floyd - What's on Your Shelf?

  4. When there is a high [H+], the solution is acidic. On the other hand when the [H+] concentration is low, the solution is basic. So if there is a high [H+], there must be a low [OH-] If there is a low [H+], there must be a high [OH-] So what does H+ and OH- create? H2O Created by Seth Floyd - What's on Your Shelf?

  5. Water Pure water should have a neutral pH, due to have equal amounts of [H+] & [OH-]. Acid and bases are created by adding polar molecules to water to increase [H+], or increase [OH-] The pH scale is used to measure the acidity or alkalinity of a solution. Created by Seth Floyd - What's on Your Shelf?

  6. Created by Seth Floyd - What's on Your Shelf? pH Scale The pH scale ranges from 0 to 14. 0 being the most acidic and 14 being the most basic. So what does the pH tell us about [H+] of a solution? If something has a pH of: pH 1 = [H+] = 0.1 M pH 2 = [H+] = 0.01 M pH 3 = [H+] = 0.001M pH 4 = [H+] = 0.0001M pH 5 = [H+] = 0.00001M pH 6 = [H+] = 1X 10-6M pH 7 = [H+] = 1X 10-7M pH 8 = [H+] = 1X 10-8M pH 9 = [H+] = 1X 10-9M pH 10=[H+] = 1X 10-10M pH 11=[H+] = 1X 10-11M pH 12=[H+] = 1X 10-12M pH 13=[H+] = 1X 10-13M pH 14=[H+] = 1X 10-14M

  7. Created by Seth Floyd - What's on Your Shelf? How to Figure it out? • If you were given HCl with a concentration of 0.003M, what would be the pH? • pH = -log [H+] • pH = -log[0.003M] • pH = 2.52 • pH will not have a unit!!!

  8. Created by Seth Floyd - What's on Your Shelf? How does pH and pOH relate? • pH and pOH relate in the following way • pH + pOH = 14 • pOH is the measure of the [OH-], in a solution. • What would be the pOH of an solution with a pH of 5.75? • pOH = 14 - 5.75 • pOH = 8.25

  9. Created by Seth Floyd - What's on Your Shelf? If given pH • To determine the concentration from pH, one must do the following: • [H+] = 10 (-pH) • So what is the [H+] of a solution with a pH of 12.05? • [H+] = 10 (-12.05) • [H+] = 8.91 X 10 -13 M antilog

  10. Created by Seth Floyd - What's on Your Shelf? Sample Problem 1 • What is the pH and pOH of an solution with [H+] of 3.33 X 10-9 M? • 1st solve for pH pH = -log [H+] • pH = - log[3.33 X 10-9 M] • pH = 8.48 • 2nd  solve for pOH pOH = 14 – pH • pOH = 14 - 8.48 • pOH = 5.52 (acidic)

  11. Created by Seth Floyd - What's on Your Shelf? Sample Problem 2 • What is the [H+] of a solution with a pOH of 4.14? • 1st determine pH pH = 14 – pOH • pH = 14 – 4.14 • pH = 9.86 (basic) • 2nd determine [H+] [H+] = 10 (-pH) • [H+] = 10 (-9.86) • [H+] = 1.38 X 10 -10 M

  12. Created by Seth Floyd - What's on Your Shelf? Sample Problem 3 • What is the pH of a solution with a [OH-] = 2.35 X 10-9? • 1stdetermine pOH pOH = -log [OH-] • pOH = -log[2.35X10-9 M] • pOH = 8.63 • 2nd determine pH pH = 14 – pOH • pH = 14- 8.63 • pH = 5.37 (acidic)

  13. Created by Seth Floyd - What's on Your Shelf? Sample Problem 4 • What is the pH of a solution that was made from 4.57g of HCl dissolved in 750 mL of water? • 1st determine moles of HCl 4.57g HCl 1 mol HCl = 0.125 mol HCl 36.46 g HCl

  14. Created by Seth Floyd - What's on Your Shelf? Sample 4 …. • 2nddetermine concentration of [H+], since this is an acid the [H+] would be the same as the concentration of the whole molecule of HCl. We will get [H+] & [Cl-], both with the total of 0.125 moles each. • So concentration will be moles / volume (L) • [H+] = 0.125 moles/ 0.750 L • [H+] = 0.333M

  15. Created by Seth Floyd - What's on Your Shelf? Problem 4… • Next determine pH • pH = -log [H+] • pH = -log [0.333M] • pH = 0.48 (very acidic)

  16. Created by Seth Floyd - What's on Your Shelf? Your Turn.. • What is the [H+] of an acid with a pH of 5.56? • Determine the pH of a 0.0045 M HCl? • Determine pOH of a solution with a pH of 8.65? • What is the pH of a base with an [OH-] of 3.44X10-5 M? • Ans: 1. 1.35 • 2. 2.75X 10-6M • 3. 5.35 • 4. 9.54

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