The pH Scale The power or potential of Hydrogen Topic 8.3

The Nature Of Science (NOS) Occam’s razor –a scientific and philosophic rule requiring that the simplest of competing theories be preferred to the more complex –the pH scale was proposed to simplify the comparison of very, very low concentrations of H + ions in solution relative to water

pH scale the pH scale is a way of expressing the strength of acids and basesthe pH scale is a way of expressing the strength of acids and bases instead of using very small numbers for the concentration of H +, we just use the negative power of 10 on the Molarity of the H + ioninstead of using very small numbers for the concentration of H +, we just use the negative power of 10 on the Molarity of the H + ion no unitsno units

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pH of 1 is 100x more acidic than pH of 3 pH of 4 is 1,000x more basic than pH of 1 pH of 6 is 10,000 less concentrated [H + ] than pH of 2 pH[H + ] ions in scientific notation [H + ] ions traditionally 01x x x x x x x x x x x x x x x

Properties of Water

7 water will self-ionize to a very small extent or in pure water at 25°C, the concentration of H + (or H 3 O + ) and OH - ions are equal – [H + ] = [OH - ] – both have a concentration of 1.0 x M

K c ? the ionic product (the [H 2 O] is considered a constant so it is left out) (K w ) for water at 298 K is: K w = [H + ] [OH - ] = 1.0 x x 1.0 x = 1.0 x acid solutions have a greater concentration of H + than OH - basic solutions are opposite and have greater concentration of OH -

pH [H + ] [OH - ] pOH Solving problems involving H +, OH -, pH, and pOH

pH = - log [H+] Example: If [H + ] = 1 X , what is the pH? pH = - log 1 X pH = - (- 10) pH = 10 Example: If [H + ] = 1.8 X pH = - log 1.8 X pH = - (- 4.74) pH = 4.74 Solving for pH when given [H+]

11 Try These! Find the pH of these: 1)A 0.15 M solution of hydrochloric (HCl) acid pH =.82pH =.82 2) A 3.00 X M solution of nitric acid (H 2 NO 3 ) pH = 6.5pH = 6.5 3) A 1.7 X M solution of nitric acid (H 2 NO 3 ) pH = 2.77pH = 2.77

If the pH of Coke is 3.12, what is the [H + ]? [H + ] = 10 -pH [H + ] = = 7.6 x M [H+] = 10 -pH Solving for [H+] when given pH

A solution has a pH of 8.5. What is the Molarity of hydrogen ions in the solution?A solution has a pH of 8.5. What is the Molarity of hydrogen ions in the solution? pH = - log [H + ] = [H + ] 3.16 X = [H + ] pH = - log [H + ] = [H + ] 3.16 X = [H + ]

14 since acids and bases are opposites, pH and pOH are oppositessince acids and bases are opposites, pH and pOH are opposites pOH looks at the perspective of a basepOH looks at the perspective of a base pOH = - log [OH - ] ince pH and pOH are on opposite ends,since pH and pOH are on opposite ends, pH + pOH = 14 Solving for pOH when given [OH-]

What is the pH of the M NaOH solution? [OH-] = M pOH = - log pOH = 2.82 pH = 14 – 2.82 = 11.18

Solving for [OH - ] when given pOH [OH - ] = 10 -pOH If the pOH of a solution is 11.5, what is the [OH - ]? [OH - ] = 10 -pOH [OH - ] = = 3.16 x M –On a side note- remember: [H + ] [OH - ] = 1.0 x »(x) 3.16 x = 1.0 x »therefore [H + ] would be 3.2 x 10 -3

Yet another problem A solution of fresh milk has a pH of 6.7. Calculate [H + ] and [OH - ]. [H + ] = [H + ] = 1.99 x mol dm -3 pOH is 7.3 ( = 7.3) [OH - ] = [OH - ] = 5.01 x mol dm -3

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19 pH testing There are several ways to test pHThere are several ways to test pH –Blue litmus paper (turns red = acid) –Red litmus paper (turns blue = basic) –pH (Hydrion) paper (multi-colored) –pH meter –Universal indicator (multi-colored) –Indicators like phenolphthalein –Natural indicators like red cabbage, radishes

20 pH meter tests the voltage of the electrolytetests the voltage of the electrolyte converts the voltage to pHconverts the voltage to pH must be calibrated with a buffer solution and stored in a solutionmust be calibrated with a buffer solution and stored in a solution

21 pH indicators indicators are dyes that can be added that will change color in the presence of an acid or base. some indicators only work in a specific range of pH once the drops are added, the sample is ruined some dyes are natural, like radish skin or red cabbage