topic 8.3 and 18.3 pH

Question 1

Buffer solution

Answer 1

Resists changes in pH upon the addition of small amounts of a strong acid or base or upon the dilution through the addition of water

Question 2

Buffer solution composition

Answer 2

weak acid/base in excess with a strong base/acid

Causes the pH to be more on the side with excess

Question 3

Indicator

Answer 3

a solution that has a different colour in acid or alkaline conditions so it can be used to detect the end point of a titration.

Using too much indicator can be bad - adding too much weak acid will change the pH.

Question 4

Indicator composition

Answer 4

Weak acid that has two colours in equilibrium

HIn ⇌ H⁺ + In⁻

Question 5

pKa and pH with indicators

Answer 5

pKa = pH (roughly)
if an indicator has a pKa (ie 5.1) an increase of pH (ie 7) will mean fewer H⁺ ions in the solution and therefore the equilibrium will shift towards the other side, showing the colour of that side of the reaction.

Alternatively, if there is a decrease in pH (ie 3) there will be more H⁺ ions in the solutions so the equilibrium will shift towards the left side, producing the colour of the indicator.

Question 6

pH curves

Answer 6

look at pH curves

equivalence point:
when moles of alkali = acid (not always 7)
2 x neutralisation point - can determine Kb (pKb = pOH)
1/2 x equivalence point - used to determine Ka (pKa = pH)

End point - when colour changes - good indicator would usually cause this to be during the equivalence point

Buffer region - point where there are small changes in pH

Question 7

Phenolphthalein

Answer 7

pKa - 9.6
pH - 8.3 - 10
Colour in acid - colourless
Colour in alkali - pink
Useful for - strong base in titration

Question 8

Methyl orange

Answer 8

pKa - 3.7
pH - 3.1 - 4.4
Colour in acid - Red
Colour in alkali - Yellow
Useful for - strong acid in titration