[20.5] pH and strong bases

Question 1

descrbe the acid-base equilibrium of water

Answer 1

• H₂O (l) + H₂O (l) ⇌ H₃O⁺ (aq) + OH⁻ (aq)
• H₂O (l) ⇌ H⁺ (aq) + OH⁻ (aq)
• acid 1 + base 2 ⇌ acid 2 + base 1

Question 2

effect of increasing temperature on the acid-base equilibrium of water

Answer 2

• equilibrium moves right in endothermic direction to oppose temperature increase
• Kw increases
• [H+] increases so pH decreases
• still neutral as [H+] = [OH-]

Question 3

Ka equation treating water as a weak acid and rearranged

Answer 3

• Ka = [H⁺ (aq)] [OH⁻ (aq)] / [H₂O (l)]
• Ka × [H₂O (l)] = [H⁺ (aq)] [OH⁻ (aq)]

Question 4

what is Kw?

Answer 4

ionic product of water

Question 5

how do you calculate Kw?

Answer 5

• Kw = [H⁺] [OH⁻]
• Kw = Ka × [H₂O (l)]

Question 6

what is the value of the constant Kw at 298K?

Answer 6

1.00 × 10⁻¹⁴ mol² dm⁻⁶

Question 7

what is the importance of Kw having a value of 1.00 × 10⁻¹⁴ mol² dm⁻⁶ at 298K?

Answer 7

• sets up the neutral point in the pH scale
• pH of pure water is 7 if you calculate it

Question 8

how do you calculate the pH of solutions of acids and alkalis?

Answer 8

• calculate moles H+
• calculate moles OH-
• calculate moles of excess H+ of OH-
• if excesss OH-, calculate excess [OH-] then calculate excess [H+]
• if excess [H+] caluclate excess [H+]
• calculate pH

Question 9

what can the pH of a strong base be caluclated from?

Answer 9

• concentration of the base
• ionic product of water, Kw