The pH scale

Question 1

What is the pH scale?

Answer 1

• Determines how acidic or basic an aqueous solution is
• It measures the concentration of hydrogen ions in a solution
• pH = −log[H+(aq)]

Question 2

Explain the pH scale.

Answer 2

• The higher the concentration of H+ ions the lower the pH value and vice versa
• A higher concentration of OH- ions corresponds to a higher pH value
• Scale ranges from 0-14 (values below and above are possible)

Question 3

Explain the logarithmic scale.

Answer 3

• Base 10
• Change of 1 pH unit corresponds to 10 times the change in H+ ion concentration
• pH of 3 is 10 times more acidic than pH of 4
• To the power of 10

Question 4

Why is a logarithmic notation used?

Answer 4

• Instead of using scientific notation, use log10
• Easier to compare concentration of H+ ions

Question 5

How do you calculate the pH value of a solution?

Answer 5

pH = −log[H+(aq)]

[H+(aq)] = 10^−pH

Question 6

What is an acid-base indicator?

Answer 6

• A weak acid or a weak base in which the dissociated and undissociated forms have different colours
• Depending on the concentration of H+ ions in the solution, one colour is shown

Question 7

What are the two acid-base indicators?

Answer 7

• Litmus paper and universal indicator

Question 8

Explain the role of litmus paper.

Answer 8

• Made from lichens, it can be either red or blue
• Blue litmus changes red in acidic solution, red litmus changes blue in alkaline solution
• Qualitative indicator, cannot be used to determine a pH value

Question 9

Explain the role of universal indicator?

Answer 9

• A mixture of indicators that produced different colours in solutions of different pH
• Colours correspond to a rainbow sequence
• Visual comparison of the colour with a standard colour chart allows to measure the pH accurate to the nearest whole number

Question 10

What do pH meters do?

Answer 10

• More accurate method of measuring pH
• Electrode of pH meter is placed in the solution to be tested, voltage is generated, pH meter reading displayed on screen
• pH meter is measured using buffers of known pH (4.0, 7.0, 10.0)

Question 11

Why is water a poor electrical conductor?

Answer 11

• It has a low concentration of mobile ions which are responsible for conductivity in solutions
• Water molecules do dissociate, but only a few

Question 12

What is the reaction called in which water molecules dissociate? Provide the formula.

Answer 12

• Auto-ionisation of water
  H2O (l) ⇌ H+ (aq) + OH– (aq)

Question 13

What is the position of equilibrium?

Answer 13

• Lies very far to the left (towards H2O), concentration of water is constant

Question 14

What is Kw?

Answer 14

• Since the concentration of water is constant, the ionic product of water (Kw) is formed
• The ionic product of water is the product of the [H+] and [OH−] in water at a particular temperature
• Kw=[H+][OH−]
• Kw = 1.00 x 10^-14 mol^2 dm^-6 which is at 298K

Question 15

Explain the concentrations of [OH–] and [H+] in relation to acidic and basic solutions.

Answer 15

• Kw is temperature-dependent, at a certain temperature the product of [H+] and [OH –] is constant
• As H+ increases, OH- decreases
• Neutral solution when H+ is equal to OH-
• Acidic solutions: [H+] > [OH–]
• Basic solution: [OH–] > [H+]

Question 15

How do you calculate [OH–] and [H+]?

Answer 15

• This means the concentrations of them are calculated using the formula [H+] = 10^-pH and the value of Kw at 298K
• Use the formula Kw=[H+][OH−] and make [OH−] subject of the formula