In this tutorial we will determine,
The concentration of arsenic in unknown samples of contaminated water
The mass percent of arsenic in the samples
This tutorial is recorded using Ubuntu Linux version 18.04
ChemCollective Vlabs version 2.1.0
Java version 11.0.8
To follow this tutorial you should be familiar with, ChemCollective Vlabs interface.
For the prerequisite tutorials, please visit this website.
Here I have opened the Vlabs interface.
Click on File menu and select Load Homework option.
Default Lab Setup dialog box opens.
Double-Click on Quantitative Analysis folder.
You will see two problems in this folder.
Click on Gravimetric Determination of Arsenic.
Click on OK button at the bottom of the dialog-box.
Workbench 1 opens with a Stockroom Explorer on the left.
It contains required chemicals and Problem Description.
Double-click on the Problem Description.
The problem description opens in a new tab.
The aim of this experiment is to determine the amount of arsenic present in soil samples.
Read the description given here.
We have to design an experiment to determine the concentration of arsenic.
This is based on the reaction of silver ion with arsenate ion.
We have to also determine the mass percent of arsenic in the soil samples.
Close the problem description window.
Stockroom Explorer is provided with all the required chemicals.
Two samples containing unknown quantities of arsenic are provided here.
Solutions of 1 Molar silver nitrate and 0.01 Molar pure sodium arsenate are also provided.
Using gravimetric analysis we will determine the amount of arsenic present in these samples.
About Gravimetric Analysis:
It is a method which involves measurement of masses in a precipitation reaction.
This method works for a solution where either of the ions present can be precipitated.
An ion in solution is precipitated out, filtered and dried. Its mass is then related to the original ion.
Gravimetric analysis relies on stoichiometry.
Steps involved in gravimetric analysis.
Write the relevant equation for the analysis.
Find the stoichiometry of precipitated compound to the original salt in the soluble form.
Find the mass of salt in grams from moles of salt.
Grams of salt is equal to moles of the salt multiplied by Molecular weight of the salt.
Determine the mass percent of the salt.
This slide shows the reactions involved in this analysis.
Arsenate present in the soil sample reacts with sliver ion to form silver arsenate.
Silver arsenate is insoluble in water.
Gravimetric analysis relies on stoichiometry.
From this displacement reaction we know that 1 mole of sodium arsenate gives 1 mole of silver arsenate.
Let us go back to the workbench.
From the Stockroom Explorer, double-click on Sample 1 flask.
It is now added to the workbench.
Click on the Sample 1 flask.
In the solution info panel the amount of liquid present in the flask is shown as 100 ml.
The solution is clear and has no solids in it.
All the arsenic present in it is in the soluble form.
We will add an excess of silver nitrate to this sample.
This will convert all the arsenate to insoluble sliver arsenate.
Double-click on 1 Molar silver nitrate flask in the stockroom explorer .
One Molar silver nitrate is added to the workbench.
From the glassware menu, select 10 mL Pipet.
Place the Pipet in 1 Molar silver nitrate solution.
Withdraw 10 mL of silver nitrate solution.
Take the filled pipet and place it on the Sample 1 flask.
We will pour aliquots of 1ml of silver nitrate to the Sample 1 flask.
Pour 1 ml of 1 Molar silver nitrate to the flask.
On the solution info panel, click on the solid radio button if not clicked.
Observe the amount of silver arsenate precipitate.
Add 1 more ml of silver nitrate to the flask.
You will see an increase in the amount of precipitate on the solution info panel.
Add a few more ml of silver nitrate to the flask.
After a few additions you don't see any increase in the amount of precipitate.
I have added 5 ml of silver nitrate to the sample 1 flask.
Now I don't see any increase in the amount of precipitate.
This indicates that all the arsenate ions are now, in the form of insoluble silver arsenate precipitate.
In the solution info panel the amount of silver arsenate is given in grams as well as moles.
Note the amount in your observation book.
Here are the calculations:
Number of moles of silver arsenate is multiplied by molecular weight of arsenic.
This will give you the amount of arsenic in grams present in Sample 1.
Calculate the percentage of arsenic in the soil Sample 1 as shown here.
Tabulate the results of sample 1 as shown here.
Follow the same procedure and find the mass percent of arsenic in Sample 2.
I will open a new workbench.
Follow the same steps and find the number of moles of silver arsenate precipitate.
The solution info panel shows the amount of silver arsenate.
Note the amount in your observation book.
Here are the values for Sample 2.
The amount of arsenic in the unknown soil samples were found to be 59.9 milligrams and 37.4 milligrams per Kg of soil.
The World Health Organization safe limit for arsenic in drinking water is 10 micrograms per litre.
We conclude that both samples 1 and 2 have high levels of arsenic contamination.
Let us summarize In this tutorial we have determined, the concentration of arsenic in unknown samples of contaminated water.
The mass percent of arsenic in the samples.
As an assignment Using gravimetric analysis, design an experiment to find the amount of salt present in a bag of chips.
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This tutorial is contributed by Snehalatha Kaliappan and Madhuri Ganapathi from IIT-Bombay. Thank you for watching.