Introduction to Titration | College Board AP Chemistry Revision Notes 2022

Determine the Concentration of an Analyte

One of the most important analytical techniques is titration
Titrations include
- Acid-base titrations
- Redox titrations
The most important glass equipment used in a titration is a burette
Titrations are used to determine the concentration of a chemical compound in an unknown sample
- The unknown sample is called the analyte
The concentration of analyte is determined using a solution of known concentration that is able to react with the analyte
- The solution of known concentration is called titrant
The burette is filled with the titrant, while the analyte is placed in a conical flask under the burette
The endpoint or equivalence point occurs when the moles of titrant have reacted completely with the moles of analyte
- In most titrations indicator is placed in the analyte solution
- The indicator is a substance that will change color near the endpoint of the titration

Common titration setup

common-setup-for-a-titration

Titration setup showing the titrant and the analyte. It is a good practice to fill the burette to 0.0 cm³as shown in the diagram

How to perform a titration?

Measure a known volume of analyte and place it into a conical flask
Fill the burette with titrant. Remember that the titrant must be a solution of known concentration
If necessary, add few drops of indicator in the conical flask
Open the tap on the burette carefully and add the analyte portion by portion to the conical flask

After each portion the conical flask must be swirled

As you start getting near the endpoint, the portions should be added dropwise
Close the burette tap when one drop has caused a permanent color change
Record the volume of titrant used for the equivalence point. This volume will be used to determine the concentration of analyte

Determining the concentration of analyte

One of the most assessed calculations in AP Chemistry regarding titrations is determining the amount of analyte
The steps for this calculation are summarized in the following diagram

Steps for determining the concentration of analyte

steps-for-determinig-the-concentration-of-analyte

General steps that must be followed to determine the concentration of analyte using the volume recorded from the equivalence point of the titration

Worked example

A 30.0 mL HCl sample of unknown concentration was titrated using NaOH 0.150 M. A permanent color change was reached when adding 25.0 mL of titrant. Determine the concentration of the analyte. The balanced chemical equation is given below

HCl + NaOH → NaCl + H₂O

Answer:

Step 1: Analyze the statement and set up the steps that you are going to use to solve the problem. The initial quantity is always given by the statement. Remember to start with the reactant or product that you have the most information about

Since, volume and molarity are given for sodium hydroxide (titrant), it must be the starting point

steps-for-determining-the-concentration-of-analyte-worked-example

Step 2: Calculate the moles of NaOH by transforming the volume of the equivalence point into L and using the molarity equation

$25.0 mL of solution \times \frac{1 L}{1000 mL} = 0.0250 L$

$M = \frac{n of solute}{V of solution in liters}$

Rearranging the equation,

n of solute = M × V of solution in liters

n of solute = 0.150 M × 0.0250 L

n of solute = 0.00375 mol of NaOH

Step 3: Calculate the moles of HCl that reacted using the ratio from the chemical equation

$0.00375 mol of NaOH \times \frac{1 mol of HCl}{1 mol of NaOH} = 0.00375 mol of HCl$

Since the ratio is 1:1 the moles of HCl must be the same

Step 4: Calculate the molarity of the HCl solution using the volume of the sample in L

$30.0 mL of solution \times \frac{1 L}{1000 mL} = 0.0300 L$

$M = \frac{n of solute}{V of solution in liters}$

$M = \frac{0.00375 mol}{0.0300 L}$

M = 0.125 M

Step 5: Write the answer with the appropriate number of significant figures

Since all the quantities from the statement are given with 3 significant figures. The answer must be written with 3 significant figures

The concentration of the unknown HCl sample is 0.125 M

Introduction to Titration (College Board AP Chemistry)

Revision Note

Determine the Concentration of an Analyte

Common titration setup

How to perform a titration?

Determining the concentration of analyte

Steps for determining the concentration of analyte

Worked example

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Author: Martín