Chemistry - Volumetric determination

In summary, the speaker is asking for help understanding how to determine the percentage of chloride present in an unknown sample through a Fajans method titration using silver nitrate. They have provided the details of the experiment, including the amount of sample and silver nitrate used. They are unsure of how to calculate the final mass needed to determine the percentage of chloride. The expert explains the process of using the concentration of silver nitrate and the net ionic equation to calculate the number of moles of chloride present, which can then be used to find the percentage of chloride by mass in the original sample.
  • #1
h6872
10
0
Hi... I x-posted this to the other chemistry board..but that might've been the wrong place...

I recently performed an experiment to determine the % Cl (by mass) of an unknown chloride sample containing both potassium and sodium. To determine this, we did a Fajans method titration using 0.1 M AgNO3.
A 1.0233g sample of the unknown chloride sample was diluted and titrated with the silver nitrate. Approximately 15.70 mL of the silver nitrate was used.
Okay, so, I understand that to determine the % Cl by mass, I should determine a final mass and divide it by the original mass of the entire chloride sample. This may seem like a stupid question, but what final mass are we determining? From the total volume of silver nitrate used, I can figure out the moles of silver nitrate used. Where do I go from here?
Please help me!
Thanks,
Heather
 
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  • #2
You say you used "Approximately 15.70 mL of the silver nitrate"...but what was the concentration of the AgNO3 ? This will determine the number of moles used.

The net ionic equation of the titration you preformed was,
Cl- (aq) + Ag+ (aq) --> AgCl (s)

1 mole of Chloride ions react with 1 mole of Silver +1 ions to form 1 mole of Silver Chloride precipitate.

By knowing the number of moles of AgCl formed or the number of Ag+ ions used to fully react with the Chloride ions in solution, one can determine the number of moles of Cl- originally present.
Apparently, you know the number of moles of AgNO3 used (by way of the volume used * concentration). Since 1 mole of AgNO3 disassociates to form 1 mole of Ag+ ions, if you know the # of moles of AgNO3, you know the # of moles of Ag+ (1:1 ratio), and thus you know the # of moles of Cl- ions originally present in solution.

If you know the number of moles of Cl-, you can calculate the mass of the Cl- since the Cl- ion has the same molar mass as Chlorine (the extra electron is negligible).
Once you calculate the mass of the Cl-, you can find the % Cl- by mass in the original, mystery, substance since you know the original substance's mass.
% Cl- by mass = (mass of Cl- / mass of total substance) * 100
 
  • #3


Hello Heather,

It sounds like you are on the right track with understanding the concept of determining the %Cl by mass. To answer your question about what final mass we are determining, it is the final mass of the precipitated silver chloride. In the Fajans method, the silver nitrate is added to the chloride sample until all of the chloride ions have reacted and formed a precipitate with the silver ions. This precipitate can then be filtered, dried, and weighed to determine the final mass of silver chloride.

To calculate the %Cl by mass, you will divide the final mass of silver chloride by the original mass of the entire chloride sample. This will give you the mass percent of chloride in the sample. From there, you can use the molar mass of chloride to convert to moles and then determine the mole ratio between chloride and either potassium or sodium to determine the percentage of each in the sample.

I hope this helps clarify the process for you. Don't hesitate to reach out if you have any further questions. Chemistry can be complex, but it's all about breaking it down step by step. Keep up the good work!
 

1. What is volumetric determination in chemistry?

Volumetric determination is a method of quantitative analysis in chemistry that involves measuring the volume of a solution needed to react completely with a known amount of another substance. This technique is used to determine the concentration of a substance in a solution.

2. What are the common types of volumetric determination techniques?

The common types of volumetric determination techniques include acid-base titration, redox titration, complexometric titration, and precipitation titration. Each technique involves the use of a burette to measure precise volumes of solutions.

3. How does acid-base titration work in volumetric determination?

In acid-base titration, a known volume of an acid or base solution is added to a solution of unknown concentration until the reaction reaches the equivalence point, where the acid and base have completely neutralized each other. The volume of the known solution is used to calculate the concentration of the unknown solution.

4. What equipment is needed for volumetric determination?

The equipment needed for volumetric determination includes a burette, pipette, and volumetric flask for accurate measurement of solutions, as well as a standardized solution of known concentration and an indicator for titration techniques.

5. What are some common sources of error in volumetric determination?

Some common sources of error in volumetric determination include inaccurate measurement of solutions due to human error, variations in temperature or atmospheric pressure, and impurities in the solutions used. It is important to take precautions and carefully follow procedures to minimize these sources of error.

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