pH & Indicators Flashcards
(22 cards)
Limitations of the pH scale
Limited to 0-14 range, even though pH values outside this are possible
Doesn’t work with very concentrated solutions
Limited to aqueous soln.
Strong acid
HCl
Strong base
NaOH
KOH (potassium hydroxide)
Weak acid
Ethanoic acid
Weak base
NH3
To find the pH of a weak acid (ethanoic acid)
[H+]= (Ka x M(acid))
Phenolphthalein colour change
Colourless—> pink
Methyl orange colour change
Red—>yellow
Litmus colour change
Red—>blue
Acid—>base
Acid-base indicator
A substance that changes colour according to the pH of the soln. It’s placed in
Range of an indicator
The pH interval over which there’s a clear change in colour for that indicator
What is Kw
The ionic product of water [H+][OH-]
pH of a neutral soln. is 7 at…
25 degrees
The stronger the acid…
The weaker the conjugate base
The weaker the acid…
The stringer the conjugate base
Indicators for titrating a strong acid(HCl) against a strong base(NaOH)?
Phenolphthalein (8-10)
methyl orange (3-5)
litmus (5-8)
Indicator for titrating a strong acid(HCl) against a weak base(NH3) ?
Methyl orange (3-7)
The indicator has one colour at 3, and another at 7 this will identify the end-point accurately
Its range lies within the vertical line of the graph
Indicator for a weak acid (ethanoic) against a strong base(NaOH)?
Phenolphthalein (8-10)
Its range lies iwthin the steep vertical lart of the graph
Weak acid(ethanoic) vs. Weak base(NH3)
No suitable indicator—> no sudden increase in pH at any stage of titration
Not possobke to find the end point
Self-ionisation of water
Water dissociates slightly to form ions H+, OH-
What happens when an acid is added to water?
Imcreases conc. of H+ ions, value of OH- decreases
What happens when a base is added to water?
Increases conc. of OH- ions, value of H+ decreases
