Lesson 7.03: pH
Concepts: slides 1-16
Lab: Slides 17-39
Lesson Standard & Objectives
• SC.912.P.8.11—Relate acidity and basicity to
hydronium and hydroxyl ion concentration and
ph.
• SC.912.P.8.8—Characterize types of chemical
reactions, for example: redox, acid-base,
synthesis, and single and double replacement
reactions.
After completing this lesson, you will be able to:
– Determine the relationships between pH,
hydronium, and hydroxide ions in a given
solution.
– Describe the process of titration and apply it to
a laboratory investigation.
Mastery of Previous Material Required
• Stoichiometry
– Chemicals react based upon moles (review lesson
4.06)
– Molar mass (review lesson 2.04, 3.09, 4.06)
• Molar mass is the mass in grams of one mole of your
chemical, according to the periodic table
• Molarity
– Lesson 5.06/5.07
• Molarity formula: M = mole solute
liter solution
• Remember: the brackets around a formula indicates
concentration in Molarity: [H2O]
pH and Water
•Water is amphoteric
•Water has the ability to behave as both an acid and a
base, depending on the properties of the other
reactant.
• Water molecules can react with each other.
• One water molecule, acting as an acid, donates a
hydrogen ion to another water molecule, acting as a base.
H 2O + H 2O
OH- + H3O+
pH and water
H 2 O + H 2O
OH- + H3O+
• Oftentimes, we say “hydrogen” ions in solution. However,
it really is the hydronium ion, H3O+
• In pure water, the concentrations of hydroxide and
hydronium ions are equal.
• Pure water behaves as a very weak acid or base.
• Pure water is a very poor electrolyte. Do you know why?
• Pure water is neutral because it contains equal amounts of
hydroxide and hydronium ions.
Acidic Solutions
H 2O + H 2O
OH- + H3O+
In an acidic solutions, the concentration of hydronium ions
is greater than the concentration of the hydroxide ions
present.
Acidic Solution = [H3O+] > [OH-]
Basic Solutions
H2O + NH3
OH- + NH4+
In a basic solutions, the concentration of hydroxide ions is
greater than the concentration of hydronium ions.
Basic Solution = [H3O+] < [OH-]
pH and Water
Acidic Solution = [H3O+] > [OH-]
There are more hydronium (hydrogen) ions in solution
Neutral Solution = [H3O+] = [OH-]
Basic Solution = [H3O+] < [OH-]
There are more hydroxide ions in solution
pH of a Solution
The pH scale is a numeric scale used to indicate the
hydronium ion concentration of a solution.
pH = - log [H3O+]
Recall from module 5: Brackets are used to represent
concentration in molarity
Review Molarity:
M = moles solute
liter solution
Using Concentration to Solve for pH
What is the pH of a 1.2 x 10-3 molar nitric acid, HNO3 solution?
pH = - log [H3O+]
pH =
pH =
Using Concentration to Solve for pH
When you are given the [OH-] in a basic solution, you
can use that value to find the [H3O+] present using the
following relationship:
[H3O+] x [OH-] = 1.0 x 10-14 M
The value 1.0 x 10-14 is called the ionization constant of
water.
Ionization Constant of Water – Formula Sheet
• We can rearrange the formula to solve for
any of the variables
[H3O+] x [OH-] = 1.0 x 10-14 M
[OH-] = 1.0 x 10-14 M
[H3O+]
[H3O+] = 1.0 x 10-14 M
[OH-]
Using Concentration to Solve for pH – Practice #1
What is the pH of a 2.3 x 10-2 molar NaOH solution?
Practice #2
What is the pH of a solution with a concentration of
6.8 x 10-4 molar H3O+?
Practice #3
What is the pH of a solution with a concentration of
4.2 x 10-5 molar OH-?
Using pH to solve for Concentration
Rearrange the pH formula: pH = - log [H3O+]
[H3O+] = 10-pH
We can determine the concentration of hydronium and
hydroxide ions in a solution that has a pH of 2.3 by
plugging into the formula above.
[H3O+] = 10-2.3
[H3O+] = 0.0050 M
Titration Lab - Vocabulary
•
•
•
•
Titration
Buret
Stopcock
Stoichiometrically
Equivalent
• Endpoint
• Acid-base indicator
• Titrant
What is a Titration?
• Titration is a laboratory method that allows
you to determine the concentration of an
acid or a base by reacting it with a known
concentration of the other.
• Imagine you have a beaker of HCl, but you
don’t know the concentration. Well, if
have some NaOH with a KNOWN
concentration, we can react them! Once,
they are neutralized, we will know how
many moles of acid we have! Then, we can
calculate its molarity.
• Confused?
• Don’t worry…let’s talk about this one step
at a time…
Titration Apparatus
Titration is a laboratory method
that allows you to determine the
concentration of an acid or a
base by reacting it with a known
concentration of the other.
Buret – (burette) narrow piece
of glassware that is used to
deliver small amounts of one
reactant to a flask containing the
other reactant.
Stopcock – The bottom of the buret can
be adjusted, using the stopcock , to release a
small stream of liquid or add the reactant drop
by drop. Think of turning on/off a faucet.
End Point
In a titration, we always react an acid with a base or vice versa.
When ‘stoichiometrically equivalent’ amounts of each have
reacted, the reaction will be neutral. We call this the End Point.
acid + base = neutral (pH=7)
Recall that acids have a pH < 7, and bases pH > 7
Stoichiometrically equivalent means:
moles of acid = moles of base
Then, we can use stoichiometry to calculate moles of the
unknown. (we’ll see more about this later)
Measuring the pH
• Most of the time, acids and
bases are and form colorless
solution. So, how do we
know when we have
reacted enough?
• We must use something to
measure the pH:
1. *An acid-base indicator
(Chemical)
2. A pH meter (instrument)
Acid Base Indicator in our lab
Acid-Base indicator
– Phenolphthalein (Chemical)
– Turns pink in basic solution
End Point
Part I Titration
Solution = titrant
• The solution that is in a buret is often referred to
as the titrant.
• Be sure to remove all air bubbles from the buret.
Why?
• Be sure the dropper end of the buret is filled with
solution as well.
• It is important to record the initial volume of
solution in the buret in your lab sheet and the
“notebook” before the titration begins.
Unknown concentration in the flask.
The purpose of lab is to determine it!
Part I Titration
http://www.youtube.com/watch?v=g8jdCWC10vQ
In part I of the lab, you will use an acid-base indicator to determine the end point of
the reaction. Add a few drops of the indicator to the flask before titrating.
What would happen if we forgot?
In part II of the lab, you will use a pH meter (electronic) to determine the end point of
the reaction. The pH at the end point =
Part I Titration
When you start the titration, click on the
stopcock (turn the knob) to drop a few
milliliters of titrant to the flask. Go slowly
until you get the hang of it.
You will see the indicator change color in
the bottom flask when the titrant makes
contacts with the solution in the flask.
Be sure to constantly stir the solution
inside the flask by carefully moving the
flask in small circles as the titrant is
being added. You will see the color change
disappear as you stir. As you near the end
point, the color change will begin to linger.
Slow down! Add the titrant drop by drop.
If you add too much, and the flask is too
pink, you will have to start over! Patience is
a virtue for this lab.
Part I Titration
Your endpoint will look like the
picture.
The color change should stay after
the solution has been stirred for 10
seconds, but it should be so light
that you may need to hold a piece of
white paper behind the flask to
notice the color change.
When you have reached the end
point, read the final volume in the
buret and record it in your data table
or notebook.
Part I Titration Calculations (HINTS!)
1. Use subtraction to determine the amount of titrant added
from the buret into the flask.
Final volume of titrant – initial volume of titrant = volume of titrant used
Review:
What is the titrant?
How do we read a buret?
Reading the Buret (Burette)
0 mL
Top: Initial
Bottom: Final
50 mL
The numbers go in reverse order. 0 mL is at the top (when buret
is full, V = 50 mL).
Report 2 decimal places! Remember those sig figs from 1st
semester (lesson 1.07)
Part I Titration Calculations (HINTS!)
2. Starting with the volume of titrant (NaOH) added and the
known molarity of the titrant (given in lab), you can use
stoichiometry to solve for the molarity of the substance in the
flask (in this case, HCl).
NaOH (aq) + HCl (aq)  NaCl (aq) + H2O (l)
Vol. of titrant
used (mL)
x
1 L NaOH
1000 mL titrant
x
0.25 mol NaOH
1 L NaOH
x
1 mol HCl
1 mol NaOH
You will need to put in the volume of your titrant
above. The rest of the math is the same!
Part I Titration Calculations
3. Once you have solved for the moles of solute in the flask,
divide the moles by the measured volume of solution in the
flask to determine the unknown molarity of the solution in the
flask.
M (HCl) =
moles of HCl (found in calc #2 you just did)
Volume (L) of HCl you put into
the flask at the beginning
Part II: Titration with a pH Meter
Part II
• Titration using a pH meter
• Record more data, will graph
Part II Titration Lab using pH Meter
Same procedure as part I, except
the endpoint is detected by a rapid
change in pH, rather than the color
change of an indicator.
If the titration is between a strong
acid and strong base, the endpoint
is reached when the pH is 7.0
Note: NO COLOR CHANGE
OBSERVED FOR THIS PART!!!
Part II pH Graph
Your pH graph should look similar to the graph below.
Match That Term!!!
Vocabulary
Terms
Titration
Stopcock
Buret
Definition
The bottom of the buret that can be adjusted to
release a small stream of liquid or add the reactant
drop by drop.
When stoichiometrically equivalent amounts of
each have reacted and the reaction is neutral.
Moles of acid = Moles of base
The solution in a buret.
End Point
Stoichiometrically
Equivalent
Titrant
A laboratory method that allows you to determine
the concentration of an acid or a base by reacting
it with a known concentration of the other.
Narrow piece of glassware that is used to deliver
small amounts of one reactant to a flask containing
the other reactant.
Label that picture!
Terms
Hint: What’s inside?
Buret
Stopcock
Titrant
Unknown solution