The pH scale and what does it mean? Acid/Base Chemistry The pH scale and what does it mean?

pH What is it? What does it mean? How do you measure it? Can you determine other things from pH?

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)

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

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.

Representative pH Values 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 Representative pH Values Substance pH battery acid 0.5 gastric acid 1.5-2.0 lemon juice 2.4 cola 2.5 vinegar 2.9 orange juice 3.5 acid rain 4.5-5.0 tea 5.5 milk 6.5 pure water 7.0 saliva 7.0-7.4 blood 7.3-7.5 sea water 8.0 soap 9.0-10.0 ammonia 11.5 household bleach 12.5 lye (sodium hydroxide) 13.5

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!!!

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

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

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)

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

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)

Sample Problem 4 4.57g HCl 1 mol HCl = 0.125 mol HCl 36.46 g HCl 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

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

Problem 4… Next determine pH pH = -log [H+] pH = -log [0.333M] pH = 0.48 (very acidic)

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