Acids and Bases: Definition and Determination of pH

Question 1

What is a pH scale?

Answer 1

• pH scale is a measure of the hydrogen ion concentration in a solution

Question 2

Why is a logarithmic scale, the pH scale, used to measure hydrogen ion concentration?

Answer 2

• The concentration of hydrogen ions in aqueous solution covers a very wide range

Question 3

Give the equation for calculating pH

Answer 3

• pH = -log10[H+]

Question 4

Give the equation for calculating [H+] from pH

Answer 4

• [H+] = 10^-pH

Question 5

Define the term monoprotic acid

Answer 5

• Each molecule of an acid will release one proton when it dissociates
• E.g HCl → H+ + Cl

Question 6

Describe the relationship between [H+] and acid concentration of strong monoprotic acids

Answer 6

• [H+] = concentration of strong monoprotic acid

Question 7

How do you calculate the pH of strong monoprotic acids?

Answer 7

• [H+] = concentration of strong monoprotic acid

- Use the concentration of strong monoprotic acid in pH = -log10[H+]

Question 8

Define the term diprotic acid

Answer 8

• Each molecule of an acid will release two protons when it dissociates
• E.g H2SO4 → 2H+ + SO4^2-

Question 9

Describe the relationship between [H+] and acid concentration of strong diprotic acids

Answer 9

• [H+] is double the concentration of strong diprotic acids

Question 10

How do you calculate the pH of strong diprotic acids?

Answer 10

• [H+] = 2 x [strong diprotic acid]

- pH = -log10[H+]

Question 11

Describe the relationship between [OH-] and the concentration of strong bases

Answer 11

• Strong bases donate one mole of OH- per mole of base

- [OH-] = [strong base]

Question 12

Which equation do you need to calculate [H+] when calculating pH of strong bases?

Answer 12

• Kw = [H+][OH-]

Question 13

How do you calculate the pH of strong bases?

Answer 13

• Find values of Kw and [OH-]
• Rearrange Kw equation into [H+] = Kw / [OH-]
• Use [H+] in pH = -log10[H+]

Question 14

How do you calculate the pH of a mixture of strong acids and alkalis?

Answer 14

• Calculate number of moles of strong acid and strong base
• Figure out whether acid or base is in excess and calculate difference
• If acid in excess, use the difference to calculate [H+]
• If base in excess, use Kw to calculate [H+] ( [H+] = Kw / [OH-] )
• Use [H+] in pH = -log10[H+]

Question 15

How do you calculate the pH of a strong acid after dilution?

Answer 15

• Calculate number of moles of H+
• Calculate [H+] using new volume with added water
• Calculate pH using pH= -log10[H+]