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PH Scale In this presentation you will: explore how to measure the strength of acids and bases Next >

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Presentation on theme: "PH Scale In this presentation you will: explore how to measure the strength of acids and bases Next >"— Presentation transcript:

1 pH Scale In this presentation you will: explore how to measure the strength of acids and bases Next >

2 Introduction Acids and bases can be measured with the pH scale which specifies the concentration of hydrogen ions, H +, in a solution. To show the concentration, the ion is enclosed in square brackets, [H + ].

3 pH Scale Pure (distilled) water has a pH of 7. The strongest acids are around pH 0, and the stronger bases around pH 14. Next > The pH scale is shown for some common substances. Household Lye Bleach Ammonia Borax Baking Soda Sea Water Blood Distilled Water Milk Corn Boric Acid Orange Juice Vinegar Lemon Juice Battery Acid pH values of some common substances Milk of Magnesia

4 pH Scale As the pH scale is logarithmic, change in the acidity or basicity of a solution of one whole pH unit actually represents a change of ten times the concentration of hydrogen ions in the solution. Next > Household Lye Bleach Ammonia Borax Baking Soda Sea Water Blood Distilled Water Milk Corn Boric Acid Orange Juice Vinegar Lemon Juice Battery Acid pH values of some common substances H + concentration, M in moles per liter Milk of Magnesia

5 The pH value is the negative logarithm of the H+ H+ ion concentration: pH Scale pH = - log 10 [H + ] Next > The pH of an acidic or basic solution can be calculated from the concentration of H+ H+ ions in the solution. Household Lye Bleach Ammonia Borax Baking Soda Sea Water Blood Distilled Water Milk Corn Boric Acid Orange Juice Vinegar Lemon Juice Battery Acid pH values of some common substances H + concentration, M in moles per liter Milk of Magnesia

6 pH Scale Next > Household Lye Bleach Ammonia Borax Baking Soda Sea Water Blood Distilled Water Milk Corn Boric Acid Orange Juice Vinegar Lemon Juice Battery Acid pH values of some common substances H + concentration, M in moles per liter Milk of Magnesia The concentration of hydrogen ions in most aqueous solutions is between 1×10 -14 M and 1 M.

7 pH Scale Next > Concentration M in moles per liter is sometimes expressed as mol/dm 3. These are equivalent terms. Household Lye Bleach Ammonia Borax Baking Soda Sea Water Blood Distilled Water Milk Corn Boric Acid Orange Juice Vinegar Lemon Juice Battery Acid pH values of some common substances H + concentration, M in moles per liter Milk of Magnesia From pH = - log 10 (10 -14 ) = 14 to pH = - log 10 (1) = 0 Therefore, the range of pH values for these solutions is between 14 and 0:

8 Ionized pH Calculations – Strong Acids A strong acid is fully dissociated into H+H+ ions and its associated base ions (known as its conjugate base) in aqueous solution. Next > A strong acid is one that fully ionizes, or dissociates into its ions, in an aqueous solution. Hydrogen Ion Chlorine Ion (Conjugate base)

9 Ionized pH Calculations – Strong Acids Next > This makes pH calculations of strong acids, of known concentration, simple to work out. Hydrogen Ion Chlorine Ion (Conjugate base) The molar concentration of H+ H+ will be equal to the concentration of the acid.

10 pH Calculations – Strong Acids In a 0.25 M solution of HCl, the concentration of H+H+ ions is also 0.25 M. HCl(aq) → H + (aq) + Cl - (aq) pH = 0.6 pH = - log 10 [0.25] pH = - log 10 [H + ] Next > [HCl] = [H + ] = [Cl - ] = 0.25 mol/L Hence:

11 pH Calculations – Strong Bases The ionic product of water (K w ) is used to calculate the pH of bases. H 2 O(l) ⇌ H + (aq) + OH - (aq) However, in alkaline solutions, the concentration of H+ H+ ions is extremely low. Next > Like strong acids, strong bases are fully dissociated into ions. K w = [H + ] × [OH - ] = 1 × 10 −14 mol/L

12 pH Calculations – Strong Bases H 2 O(l) ⇌ H + (aq) + OH - (aq) Next > To calculate the concentration of H+H+ ions in a basic solution, we have to rearrange this equation. Water naturally dissociates into H+ H+ and OH − ions with equal concentrations of 1×10 −7 mol/L at 298 K. K w = [H + ] × [OH - ] = 1 × 10 −14 mol/L [H + ] = K w / [OH - ]

13 Consider a solution of NaOH of 0.25 M concentration at 298 K: [H + ] = 1 × 10 −14 / 0.25 = 4 × 10 -14 mol/L To calculate the ionic product of water: [NaOH] = [Na + ] = [OH - ] = 0.25 Next > pH Calculations – Strong Bases pH = - log 10 (4 × 10 -14 ) = 13.4 K w = [H + ] × [OH - ] = 1 × 10 −14 mol/L pH = - log 10 [H + ] Therefore, the pH of NaOH is:

14 It is also possible to use the concentration of the hydroxide ion (OH - ) to measure the strength of an acid or base, as pOH. Next > Relationship between pH and pOH K w = [H + ] × [OH - ] = 1 × 10 −14 mol/L pH = - log 10 [H + ]

15 For a given concentration of hydrogen, say [H + ] = 1 × 10 −7 mol/L In this case, pH = 7 and pOH = 7 Next > Relationship between pH and pOH then, [OH - ] = K w / [H + ] = 1 × 10 −14 / 1 × 10 −7 = 1 × mol/L pOH = - log 10 [OH - ] Since, pH = - log 10 [H + ], then pOH = - log 10 [OH - ] = - log 10 (1 × 10 −7 ) = 7 Example

16 Next > Relationship between pH and pOH In all cases, the sum of pOH and pH will equal 14 because of the relationship: Given either pOH or pH, then for any acid or base the other can be calculated. K w = [H + ] × [OH - ] = 1 × 10 −14 mol/L pOH + pH = 14

17 Measuring pH It is often necessary to know the pH of a solution, in order to identify it as acidic or basic. Next > Additionally, pH changes during a reaction must often be continuously monitored, for example in a titration experiment.

18 Measuring pH The most common experimental techniques used for finding pH are: Indicators, or chemical substances that react to different pH levels by changing color. Next >

19 Measuring pH Next > The most common experimental techniques used for finding pH are: pH meter, an instrument which measures the pH directly. Universal pH indicator paper, which changes color with pH. A chart is used to identify the pH range shown by each color.

20 The concentration of hydrogen ions in a solution with a pH of 1 is...? Question 1 A) twice the concentration of a solution with a pH 2. B) half the concentration of a solution with a pH 2. C) ten times the concentration of a solution with a pH 2. D) a tenth of the concentration of a solution with a pH 2. Next >

21 Question 1 The concentration of hydrogen ions in a solution with a pH of 1 is...? A) twice the concentration of a solution with a pH 2. B) half the concentration of a solution with a pH 2. C) ten times the concentration of a solution with a pH 2. D) a tenth of the concentration of a solution with a pH 2.

22 What is the hydroxide ion concentration for a 1 × 10 -4 M HCl solution? Question 2 A) 1 × 10 -4 M B) 1 × 10 -6 M C) 1 × 10 -8 M D) 1 × 10 -10 M Next >

23 Question 2 What is the hydroxide ion concentration for a 1 × 10 -4 M HCl solution? A) 1 × 10 -4 M B) 1 × 10 -6 M C) 1 × 10 -8 M D) 1 × 10 -10 M

24 In this presentation you have seen: Summary an explanation of the pH scale End > pH calculation of an acid and a base the relationship between pH and pOH


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