Transcript Setup for Titrating an Acid with a Base

pH TESTING
• There are several ways to test pH
– Blue litmus paper
• red = acid
– Red litmus paper
• blue = basic
– pH paper
• multi-colored
– pH meter
• 7 is neutral, <7 acid, >7 base
– Universal indicator
• multi-colored
– Indicators
• Phenolphthalein
– Natural indicators like red cabbage, radishes
pH Indicators
• Indicators are dyes that can be
added that will change color in
the presence of an acid or base
• Some indicators only work in a
specific range of pH
• Once the drops are added, the
sample is ruined
• Some dyes are natural, like
radish skin or red cabbage
Paper Testing
• Paper tests like litmus paper and pH paper
– Put a stirring rod into the solution and stir
– Take the stirring rod out
– Place a drop of the solution from the end
of the stirring rod onto a piece of the
paper
– Read and record the color change
– Note what the color indicates
– You should only use a small portion of the
paper
• You can use one piece of paper for
several tests
pH Meter
• Remember that acids and bases
are electrolytes
– Strong acids/bases are STRONG
electrolytes
– Weak acids/bases are WEAK
electrolytes
• Tests the voltage of the electrolyte
• Converts the voltage to pH
• Very cheap, accurate
• Must be calibrated with a buffer
solution
ACID-BASE REACTIONS
TITRATIONS
• To analyze the acid or base content of a solution, chemists often
perform a TITRATION
– A titration involves the delivery of a measured volume of a
solution of known concentration (TITRANT) from a buret into
the solution being analyzed (ANALYTE)
– Titrant is added slowly to the analyte until exactly enough has
been added to just react with all of the analyte
• EQUIVALENCE POINT
– [H+] = [OH-]
– Indicator or pH meter is used to determine equivalence point
– Neutralization is said to occur when the acid and base have
same numbers of moles present in the combined solution
Setup for Titrating an Acid with a Base
HOW TO SOLVE A TITRATION-TYPE
PROBLEM
• Based on stoichiometry
– Write a balanced chemical equation
– Calculate the number of moles of the standard solution present in the solution
• Standard solution is the solution in the buret and has a known concentration
• Will always be given a volume and concentration of the standard solution
– Use the molar relationship from the equation (STOICHIOMETRY) to convert from
moles of standard solution into moles of unknown solution
– Calculate the unknown quantity (molarity or volume) of the unknown solution by
using the relationship M = n/V using the number of moles found in step 3
– Sometimes, you will need to convert from moles to other quantities such as
volume of a gas (L) and grams
• Remember your stoichiometry road map!
NEUTRALIZATION (TITRATION)
EXAMPLE PROBLEM 1
• Boric acid (H3BO3) is a solid acid that
is good for controlling ants. It also
neutralizes bases. How many grams
of boric acid would be needed to
neutralize 35.00 grams of calcium
hydroxide?
NEUTRALIZATION (TITRATION)
EXAMPLE PROBLEM 2
• 35.62 mL of NaOH is neutralized with
25.2 mL of 0.0998 M HCl by titration
to an equivalence point. What is the
concentration of the NaOH?
NEUTRALIZATION (TITRATION)
EXAMPLE PROBLEM 3
• 25 drops of citric acid are titrated with .112 M
KOH until the indicator changes from yellow to
blue. 48 drops of KOH are added before a
color change occurs.
What is the
concentration of the citric acid? What is the pH
of the citric acid?
**20 drops = 1 mL**