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UNIT TWO

p

Outcome Ugly OK SuperCalculate the pH of a solution containing a strong acidCalculate the concentration of hydronium ions in a solutionwhen given the pHWrite the equation for the ionic product of water

Use the ionic product to find the pH of a basic solution

pH

Tells us the concentration of H 3O+ ions in a solution Ranges from 0-14 Less than 7 = acidic Greater than 7 = basic Exactly 7 = neutral The pH scale is logarithmic, so a solution with pH 5 has 10 times the [H 3O+] as a solution with pH 6.

Calculating pH

pH = - log [H 3O+]

Example:Calculate the pH of a solution which has a H 3O+ concentration of 4 X 10 -5 mol L -1 [H3O+]= 4 X 10 -5 mol L -1 pH = - log [H 3O

+]pH = - log [4 x 10 -5]pH = 4.4

Examples:Calculate the pH of the following solutions with the H + concentrations given (show working).

a. [H+] = 6 X 10 -5 mol L -1 b. [H+] = 7 X 10 -4 mol L -1 c. [H+] = 3.3 X 10 -8 mol L -1 d. [H+] = 8 X 10 -2 mol L -1 e. [H+] = 4 X 10 -10 mol L -1

Calculating [H 3O+] when given pH

[H3O+] = 10 -pH

Examples:Calculate [H 3O+] when given the pH

a. pH = 12 b. pH = 2 c. pH = 7

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UNIT TWO

pH of Polyprotic Acids

MONOPROTIC ACIDS When monoprotic acids dissociate in water one H 3O+ is produced

E.g.HNO3 + H2O

From this equation you can see that the n(H 3O+) = n(HNO 3) so the [H 3O

+] is the same as the concentration

of HNO 3 ExamplesCalculate the concentration of H 3O+ and pH in the following solutions

A. 0.5 mol L -1 HCl

B. 1.32 mol L -1 HNO3

POLYPROTIC ACID When polyprotic acids dissociate in water more than one H 3O+ is producedE.g.H2SO4 + H2O

From this equation you can see that the n(H 3O+) ≠ n(H2SO4 ) so the [H 3O+] is not the same as the

concentration of H 2SO4 The concentration of H 3O+ is proportional to the number of H’s in the acid E.g. n(H 3O+) = 2 X n(H2SO4)

Example

What is the pH of a 0.005 M H 2SO4 solution?Step 1 Write the equation for the ionisation of H 2SO4 in water.Step 2 n(H3O

+) = 2 X n(H2SO4) = =Step 3 Calculate the pH.

ExamplesCalculate the concentration of H 3O+ and pH in the following solutions

C.

0.5 mol L-1

H2SO4

D. 1.32 mol L -1 H3PO4

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UNIT TWO

The Ionic Product of Water Pure water is neutral and has a pH of 7 at 25°C. The concentrations of H 3O+ and OH - in pure water are

equal to each other. They are both 10 -7 mol L -1

IONIC PRODUCT OF WATER

[H3O+]X[ OH-] = 10 -7 X 10-7 = 10 -14

The [H 3O

+

] concentration and [OH-

] concentration are related to each other. If [H3O+] goes up the [OH -] goes down – In other words as a solution gets more acidic ([H 3O+] goes up) theamount of OH - in that solution goes down.

By using the equation above you can work out the [OH -] if you know the [H 3O+]

ExampleWhat is the [OH -] in a solution if the [H 3O+] is 10 -3?

ExampleThe concentration of OH - in a cleaning product was found to be 10 -3. Find the pH of the solution.

Step 1 Write the expression for the ionic product of water.Step 2 Rearrange the expression to find the concentration of hydronium ions.Step 3 Substitute into the expression and find the H 3O+.Step 4 Calculate the pH

PRACTICE1. Calculate the hydronium ion and hydroxide ion concentrations in the following solutions.

a. 0.050 molL -1 HCl

b. 0.050 molL -1 NaOH

c. 1.3 x10 -4 molL -1 HCl

d. 0.025 molL -1 H2SO4

e. 4.3 x10 -5 molL -1 HNO3

2. Calculate the pH of the following solutions.

a. 0.015 molL -1 HCl

b. 7.8 x10 -4 molL -1 HCl

c. 1.5 x10 -6 molL -1 NaOH

d. 2.5 x10 -4 molL -1 NaOH

e. 4.7 x10 -4 molL -1 H2SO4

3. Calculate the [H3O+] and [OH -] concentration in the following solutions.

(a) pH = 12.0 (c) pH = 1.85

(b) pH = 3.0 (d) pH = 8.7