Chemistry 122- Acid Base Unit
Chapter 19.2, Water Equilibrium Notes
Objective: Describe how [H+] and [OH-] are related in an aqueous solution. Classify the solution as neutral, acidic or basic given the [H+] or [OH-]. Convert the [H+] and [OH-] into pH and pOH values.
About 2 molecules out of every billion water molecules have enough energy when they collide to transfer a hydrogen ion from one water molecule to the other.
When water ionizes, or falls apart into ions: it is called the “self ionization” of water
H2O ßàH1+ + OH1-
The proton or hydrogen ion usually attaches to another water molecule, creating the hydronium ion.
H2O + H2O ßà H3O+ + OH-
The self-ionization of water occurs to a very small extent:
[H1+ ] = [OH1-] = 1 x 10-7 M
Since they are equal, a neutral solution results from water
· Kw = [H1+ ] x [OH1-] = 1 x 10-14 M2
Kw is called the “ion product constant” for water. It is essentially Keq applied to an aqueous solution. The water in the reactants is not included because it is a liquid, not an aqueous ion.
In an aqueous solution, when [H+] increases, [OH-] decreases. When [H+] decreases, [OH-] increases. LeChatelier’s principle applies here- if additional ions are added to the solution, equilibrium shifts and the concentration of the other ion decreases.
For all aqueous solutions, the product of the hydrogen-ion concentration and the hydroxide- ion concentration equals 1.0 x 10-14
Kw is constant in every aqueous solution: [H+] x [OH-] = 1 x 10-14 M2
· If [H+] > 10-7 then [OH-] < 10-7
· If [H+] < 10-7 then [OH-] > 10-7
If we know one, other can be determined by inputting the value of the known concentration and using the kw constant value. We can also determine the acidity or basicity of the solution.
· If [H+] > 10-7 , it is acidic and [OH-] < 10-7
· If [H+] < 10-7 , it is basic and [OH-] > 10-7
Example:
Calculate the H+ ion concentration in a solution of 0.32 mol/L Ba(OH)2
Step 1: Write the balanced Ionization equation
Step 2: Calculate the concentration of each entity
Step 3: Use Kw to calculate the [H+] or [OH-]
Try:
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1. What is the hydroxide ion concentration in a solution that has a hydrogen ion concentration of 6.8 x 10 -10M? Is the solution acidic or basic?
2. If the hydroxide ion concentration in a solution is 5.67 x 10-3M, calculate the hydrogen ion concentration. Is the solution acidic or basic.
3. A 0.15M solution of hydrochloric acid at 25oC is found to have a hydrogen ion concentration of 0.15M. Calculate the concentration of hydroxide ions.
4. Calculate the hydrogen ion concentration in a 0.25M solution of barium hydroxide.
5. Determine the hydrogen and hydroxide ion concentrations in 500. mL of an aqueous solution containing 2.6 grams of sodium hydroxide.
6. The hydrogen ion concentration in an industrial effluent is 4.40 mmol/L (4.40 x10-3 mol/L). Determine the concentration of hydroxide ions in the effluent.
7. The hydroxide ion concentration in a household cleaner is 0.299mmol/L. Calculate the hydrogen ion concentration in the cleaner.
8. Calculate the hydroxide ion concentration in a solution prepared by dissolving 0.37g of hydrogen chloride in 250ml of water.
9. Calculate the hydrogen ion concentration in a saturated solution of calcium hydroxide that has a solubility of 6.9 mmol/L.
10. What is the hydrogen ion concentration in a solution made by dissolving 20.0g of potassium hydroxide in water to form 500ml of solution?
Chemistry 122- Acid Base Unit
Chapter 19.2, Water Equilibrium Notes-pH and pOH
Showing how acidic or basic a solution is by expressing the [H+] is cumbersome, so the pH scale is more widely used.
The pH scale relates the [H+] in solution.
pH-> Power of Hydrogen
Runs from 0-14. (Numbers can occasionally be negative example: assuming 100% ionization of the very strong acid HCl, the pH of a 10M solution is -1.00)
Shortcut...
** 1.0 x 10-11 means that the pH is 11.
** a pH 0f 6 is represented by a [H+] of 1.0 x 10-6.
This only works when it is 1.0 x ...
Definition: pH = -log[H+]
· in neutral pH = -log(1 x 10-7) = 7
· in acidic solution [H+] > 10-7
· pH < -log(10-7)
· pH < 7 (from 0 to 7 is the acid range)
· in base, pH > 7 (7 to 14 is base range)
Significant Digits
The number of digits after the decimal in the pH is equal to the number of significant digits in the concentration.
Example:
To convert [H+] to a pH value...
Take the negative log...
ex: 2.7 x 10-11mol/L =[H+]
(Show here what order the buttons need to be pushed in the calculator to do this algorithm)
So the pH is 10.57
We can go backwards from pH to [H+] as well.
To convert a pH value to [H+] ...
Take the negative inverse log...
ex: pH = 9.63
(Show here what order the buttons need to be pushed in the calculator to do this algorithm)
So the [H+] is 2.3 x 10-10 mol/L
pOH
pOH is basically the same but conveys the [OH-] instead.
· pOH = -log [OH-]
· [H+] x [OH-] = 1 x 10-14 M2
· pH + pOH = 14
· Thus, a solution with a pOH less than 7 is basic; with a pOH greater than 7 is an acid
· Not greatly used like pH is.
Practice:
1. Food scientists and dieticians measure pH of foods when they devise recipes and special diets.
Food / [H+] (mol/L) / [OH-] (mol/L) / pH / pOHOranges / 5.5x10-3
Asparagus / 5.6
Olives / 2.0x10-11
Blackberries / 10.60
b) Based on pH only, which foods would taste more sour?
2. To clean a clogged drain, 26 g of sodium hydroxide is added to water to make 150mL of solution. What are the pH and pOH values for the solution?
3. What mass of potassium hydroxide is contained in 500 ml of solution that has a pH of 11.5
4. If the concentration of hydrogen ions is 4.2 x 10-3M, what is the pH?
5. If the concentration of hydrogen ions is 4.4 x 10-7M, what is the pH?
6. IF the pH of the solution is 10.33, what is the hydrogen ion concentration?
7 . If the pH of the solution is 4.56, what is the concentration of hydrogen ions?