Pre-Lab Quiz
 When you’re done:
 Make sure to make your DAY 2 observations for the
Egg-tastic Class Inquiry
https://www.youtube.com/watch?v=kxPwbhFeZSw
4.3 Homework Questions
4. What is the pH of an aqueous solution containing
0.00200 M barium hydroxide, Ba(OH)2
7. A 2.67g sample of hydrogen fluoride gas (HF) is
dissolved in sufficient water to make 1.05L of
solution at 25C to form an acidic solution. Hydrogen
fluoride is a weak acid with Ka= 6.6x10-4
a. Calculate the pH of the solution
b. Calculate the pOH of the solution
Indicators, Neutralizations &
Titrations Revisited
Indicators
 Indicators are dyes that change colour under varying
conditions of acidity.
 Although not as accurate as instruments such as pH meters
in determining acidity, indicators can be used to give less
precise measure of acidity.
 Litmus is a very commonly used indicator which is red in
acids and blue in bases.
 Litmus is an indictor that changes
colour from red to blue in the pH
range of 5.5 to 8.0.
Indicators
 Indicators are coloured compounds that exist in both acidic
and basic forms.
 A general formula may be used for indictors - HIn - and a
reaction written:
HIn
acidic form
colour 1
↔
H+ + Inbasic form
colour 2
 Indicators may be in solution form or paper form. pH paper is
prepared by treating the paper with the indictor solution.
When the paper is then dipped into the solution you are
testing, it will change colour depending on the acidity of the
solution.
Find my pH Activity
 Indicator used: Purple cabbage
 Purple cabbage contains colored pigments (one of which is
anthocyanin) that change colors when they meet an acid or a
base. Acids make purple cabbage juice turn pink. Bases make
the juice turn blue or greenish.
Indicator Examples
Indicator
pH range
Colour change
methyl orange
3.2 - 4.4
red to yellow
litmus
5.8 - 8.0
red to blue
phenolphthalein
8.2 - 10.0
colourless to pink
 1. A given solution turns methyl orange yellow, litmus
blue, and phenolphthalein pink. What is the
approximate pH of the solution?


Methyl orange in yellow when pH is above 4.4
Litmus is blue when pH is above 8.0, and
Phenolphthalein is red when pH is above 10.0.
Therefore the solution would have to have a pH above 10.0
Indicator Examples
Indicator
pH range
Colour change
methyl orange
3.2 - 4.4
red to yellow
litmus
5.8 - 8.0
red to blue
phenolphthalein
8.2 - 10.0
colourless to pink
 2. What color would methyl orange, litmus, and phenolphthalein turn when
testing:
 a. vinegar (pH = 3)



Methyl orange: red
Litmus: red
Phenolphthalein: colorless
 b. sea water (pH = 8)



Methyl orange: yellow
Litmus: blue
Phenolphthalein: colorless
Neutralization Reactions
 What happens when an acid such as HCl is mixed with a base such as NaOH:
HCl(aq) + NaOH(aq) → NaCl(aq) + H2O(l)
 When an acid and a base are combined, water and a salt are the
products.
 Salts are ionic compounds containing a positive ion other than H+ and a
negative ion other than the hydroxide ion, OH-.
 Double displacement reactions of this type are called neutralization
reactions.
 We can write an expanded version of this equation, with aqueous substances
written in their longer form:
H+(aq) + Cl-(aq) + Na+(aq) + OH-(aq) → Na+(aq) + Cl-(aq) + H2O(l)
 Removing the spectator ions we get the net ionic equation:
H+(aq) + OH-(aq) → H2O(l)
Titrations
 A titration is a laboratory procedure used to determine the
concentration of a solution.
 During an acid-base titration an acid with a known
concentration (standard solution) is added to a base with an
unknown concentration (or vice-versa).
 An indicator is also added to the solution which will signal
(by a color change) when the base has been neutralized.
Titrations
 Note that as soon as you see a color change the titration is
complete and the [H+] and [OH-] are equal.
 At this equivalency point the titration is stopped. By
knowing: the initial volume of the base, the volume of acid
added, and the initial concentration of the acid we can
calculate the concentration of the base.
Titration Examples
1) During a titration 75.8 mL of a 0.100 M
standard solution of HCl is titrated to
end point with 100.0 mL of a NaOH
solution with an unknown
concentration. What is the
concentration of the NaOH solution?
2) A 20.0 mL solution of strontium hydroxide,
Sr(OH)2, is placed in a flask and a drop of indicator
is added. The solution turns colour after 25.0 mL
of a standard 0.0500 M HCl solution is added.
What was the original concentration of the
Sr(OH)2 solution?