Solve Back Titration Problem to Find Ammonium Sulfate Mass in Fertiliser

In summary, the experiment involved dissolving 1.301g of lawn fertiliser in de-ionised water and diluting it to 250mL. A 20mL aliquot was taken and reacted with excess 0.1M NaOH. The remaining NaOH was titrated with 0.09933M HCl, with an average titre of 16.335mL. After calculations, it was determined that the mass of ammonium sulfate in the fertiliser was 0.3098g. However, this value did not align with the expected range and there may have been errors in the experimental procedure.
  • #1
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Please Help! Back titration!

Find the mass of ammonium sulfate in lawn fertiliser.

The experiment in a nutshell:

A sample of 1.301g lawn fertiliser is dissolved in de-ionised water and diluted to 250mL. A 20mL aliquot is taken and an excess of 0.1M [tex]NaOH[/tex] is added. The remaining [tex]NaOH[/tex] is titrated with 0.09933M [tex]HCl[/tex].

The average titre was 16.335mL.


I get 0.3098g as my mass for ammonium sulfate. But for it to be concordant with my gravimetric analysis, that value should be between 0.600g-0.700g!
Can someone please tell me what's gone wrong or what to do to ge the right answer? Thanks!

My working:

Original n(NaOH) = [tex]0.1 \times 0.02[/tex] = 0.002 mol

[tex]HCl + NaOH -> NaCl + H_2O[/tex]

[tex]n(HCl) = 0.09933 \times 0.016355[/tex] = 0.00162454 mol

[tex]n(NaOH)_{unreacted with ammonium sulfate}[/tex] = 0.00162454 mol

[tex]n(NaOH)_{reacted with ammonium sulfate}[/tex] = 0.002 - 0.00162454 = 0.000375458mol

The ionic reaction between ammonium sulfate and NaOH is as follows:

[tex]NH_4^{+} + OH^{-} -> NH_3 + H_2O[/tex]

[tex]n(NH_4^{+}) = n(OH^{-}) = 0.000375458 mol[/tex]

[tex][NH_4^{+}]_{20.00mL aliquot} = \frac{0.000375458}{0.02} = 0.01877 M[/tex]

[tex][NH_4^{+}]_{20.00mL aliquot} = [NH_4^{+}]_{250.00mL volumetric flask} = 0.01877 M[/tex]

[tex]n(NH_4^{+})_{250mL volumetric flask} = 0.01877 \times 0.25 = 0.00469 mol[/tex]

[tex]n(NH_4^{+}) = 2 \times n((NH_4)_2SO_4)[/tex]

so [tex]n((NH_4)_2SO_4) = \frac{0.00469}{2} = 0.002345[/tex] mol

[tex]m((NH_4)_2SO_4) = 0.002345 \times 132.1 = 0.3098g[/tex]
 
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  • #2


How much 0.1 M NaOH did you add?
 
  • #3


I have the same question as post #2. You apparently also confused the volume of aloquat with the unstated volume of NaOH.
 

1. What is a back titration and how is it used to find the mass of ammonium sulfate in fertiliser?

A back titration is a type of chemical analysis that involves adding an excess amount of a known reagent to a sample and then titrating the remaining excess with a second reagent. This is useful in cases where the reaction between the sample and the first reagent is slow or incomplete. In the case of finding the mass of ammonium sulfate in fertiliser, the first reagent would be excess hydrochloric acid, which reacts with the ammonium sulfate to form ammonium chloride. The remaining excess of hydrochloric acid is then titrated with a known concentration of sodium hydroxide. The amount of sodium hydroxide used in the titration can then be used to calculate the amount of ammonium sulfate present in the sample.

2. How do you perform a back titration to find the mass of ammonium sulfate in fertiliser?

To perform a back titration to find the mass of ammonium sulfate in fertiliser, first, a known amount of the fertiliser sample is dissolved in water. Then, excess hydrochloric acid is added to the solution and the mixture is heated to ensure complete reaction. The remaining excess of hydrochloric acid is then titrated with a known concentration of sodium hydroxide until the endpoint is reached. The amount of sodium hydroxide used in the titration can then be used to calculate the amount of ammonium sulfate present in the sample.

3. What is the endpoint and how is it determined in a back titration?

The endpoint is the point at which the reaction between the excess reagent and the titrant is complete. In the case of a back titration to find the mass of ammonium sulfate in fertiliser, the endpoint is determined by adding a few drops of an indicator, such as methyl orange, to the solution. The indicator will change color when all of the excess hydrochloric acid has been neutralized by the sodium hydroxide, indicating that the endpoint has been reached.

4. What are the sources of error in a back titration to find the mass of ammonium sulfate in fertiliser?

One of the main sources of error in a back titration is the presence of impurities in the sample. These impurities can react with the reagents, leading to inaccurate results. Another source of error is human error in measuring and adding the reagents, as well as in determining the endpoint. It is important to carefully follow the procedure and use precise measuring tools to minimize these errors.

5. How can the back titration method be improved for more accurate results?

To improve the accuracy of the back titration method, it is important to use high-quality reagents and carefully measure and add them to the solution. The sample should also be finely ground to ensure complete reaction. Another way to improve accuracy is to perform multiple trials and take an average of the results. Using a more precise indicator or using a more sensitive instrument for titration, such as a pH meter, can also help to improve accuracy.

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