Analía Bellizzi – Chemistry Classes

Ronald Reagan Senior High School

## AS- Titration Online

#### Introduction

In chemistry laboratory, it is sometimes necessary to experimentally determine the concentration of an unknown acid or base solution. A procedure for making this kind of determination is called an acid-base titration. In this laboratory process, a solution of known concentration, called the standard solution, is carefully added to a solution of unknown concentration until the mixture becomes neutral. The neutral point of the solution is recognized by an indicator’s color change. If the unknown solution is acidic, then the standard solution will be basic. The opposite would be true if the unknown solution was basic.
We know that the mixing of equal amounts of acid and base ions will create neutral water. At the molecular level, this reaction can be illustrated with the following equation.

#### H+ + OH– → H2O

(acid) + base  →  water

This equation states that one mole of hydrogen ions (acid) will neutralize one mole of hydroxide ions (base). Since we can exactly measure the moles of the standard solution, we can assume that the moles of the solution of unknown concentration will be the same at the neutral point. This is called the end-point of the titration.

#### Objective:

In this experiment the unknown solution will be HCl(aq) and the standard solution will be the base sodium hydroxide. You will know the concentration of the base and the volume of the acid and base used. With this information you can use the titration formula to calculate the concentration of the acid. The diagram below shows the set up

#### Remember how to measure when using a 50 cm3 burette The arrows show two consecutive lines in each burette.

A    18.4 cm3 – 18.3 cm3 = 0.1 cm3              →  0.1 cm3 / 2 = 0.05 cm3  → measurement: 18.75 cm3

B    0.2 cm3 – 0.1 cm3 = 0.1 cm3                  → 0.1 cm3 / 2 = 0.05 cm3    → measurement:   9.25 cm3

C    23.4 cm3 – 23.3 cm3 = 1 cm3                 → 0.1 cm3 / 2 = 0.05 cm   → measurement: 23.00 cm3

Note that

• The scale in the burette is upside down.
• All measurements have 2 decimals and the last digit will be 0 or 5.
• All measurements will have 3 or 4 significant digits.
• The last digit will always be 0 or 5 depending on the position of the lower meniscus.

#### Pre Lab:

Define the following words:
Titration- _____________________________________
Neutralization_________________________________

Write the neutralization reaction for HCl reacting with NaOH. ____________________
What is the pH of the solution at the end point of the titration?___________________

#### Procedure:

1. The flask is filled with 10 mL of unknown concentration of HCl. Record the volume on your data table.
2. Add phenolphthalein to the flask. The indicator will not change color since it is colorless in presence of an acid. 3. The next step is to perform the rough titration. You will now start adding base (0.25M NaOH) from the buret into the flask to neutralize the acid.  The flask is gently swirled as the base is added. A pink color should appear as the base is added, but it will disappear as the flask is swirled. The end-point of the titration will be reached when 1 drop of base makes the solution turn and stay pink.Most likely you  will overpass the ending point, so most probably you will go to far. The following are images of the initial and final readings for the rough titration.

• Record this values on a suitable data table.
• Do not forget to put the units on the table headers and

#### Rough Titration Measurements  You will perform three other titrations and will decide which data you will use for your calculations.

Be careful when you are near the ending point. Most likely part of the solution will turn pink, so you need to swirl the solution all the time. #### REAL Titration Measurements

First titration  #### Second titration  #### Third titration  #### Calculations:

1. Calculate the number of moles of acid contained in the 10 mL of solution that were placed in the flask.
2. calculate the mole ratio and hence the number of moles of base contained in the volume delivered to the flask using the burette.
3. Use your data table to calculate the average volume of base used for the titration.
4. Determine the Molarity (concentration) of HCl using the data you collected and the titration formula.  (The concentration of the NaOH used was 0.25M)

#### Analysis Questions

1. State the purpose of this experiment.
2. Describe the function of the phenolphthalein in this experiment. How would this experiment be different if you had forgotten to add the phenolphthalein
3. State evidence from the lab that the endpoint was reached.
4. It takes 75ml of a 0.25M HCl solution to neutralize 55ml of a base of unknown concentration. Calculate the concentration of an unknown basic solution.
5. If the actual concentration of the HCl was 0.33M what is your percent error?
The formula for the % error is: 6. Environmental studies usually involve an analysis of precipitation and its response to pollution. To quantify the degree of contamination in natural rain water or snow, titration is used. The process is quick and results are reliable. Since most titration processes do not require expensive or specialized equipment, the test can be performed often and in different areas with relatively little effort.
1. A 1000.0 mL sample of lake water is titrated using 0.100 mL of a 0.100 M base solution. What is the molarity of the acid in the lake water?
2. Based on the molarity of the acid calculated above, what is the pH of the lake water?