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Topic: Kjehdahls method  (Read 2136 times)

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Offline fhz

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Kjehdahls method
« on: April 18, 2020, 01:09:22 PM »
How can I calculate this?

"John was determining fertilizer's nitrogen concentration with Kjeldahl's method. He weighted 1,40 grams of fertilizer, and dissolved it to water. (nitrogen is already in NH4+ ions) He added NaOH and then ammonia was formed. Ammonia gas was guided to container, and there was 150,0 ml 0,1M HCL. After distillation, acid solution was titrated with 0,05M NaOH-solution. 60ml of NaOH was spent. Write reaction equations, and calculate fertilizer's nitrogen mass percentage (N%)"

Thank you!!
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Offline chenbeier

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Re: Kjehdahls method
« Reply #1 on: April 18, 2020, 01:42:47 PM »
What are your own ideas how to proceed?
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Offline fhz

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Re: Kjehdahls method
« Reply #2 on: April 18, 2020, 04:13:20 PM »
Quote from: chenbeier on April 18, 2020, 01:42:47 PM
What are your own ideas how to proceed?

To be honest, I have no idea. First I tried to calculate HCl concentration (because we know NaOH concentration), but then I didn't know how to continue.
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Offline chenbeier

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Re: Kjehdahls method
« Reply #3 on: April 18, 2020, 04:20:35 PM »
Correct, if you know the HCl and also the given HCl, then you can calculate the HCl what react with ammonia.you know ammonia and also then nitrogen. You know mass of the fertilizer and so know its content.
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Offline fhz

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Re: Kjehdahls method
« Reply #4 on: April 19, 2020, 05:23:44 AM »
Quote from: chenbeier on April 18, 2020, 04:20:35 PM
Correct, if you know the HCl and also the given HCl, then you can calculate the HCl what react with ammonia.you know ammonia and also then nitrogen. You know mass of the fertilizer and so know its content.

Thank you so much! Now I got the idea. Still I don't know how to write the chemical equations (reactions). The first i get (NH4+ + NaOH -> NH3 + H2O + Na+), but then I don't have a clue how to continue.
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Offline chenbeier

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Re: Kjehdahls method
« Reply #5 on: April 19, 2020, 07:05:20 AM »
Your equation is correct. But you need the reaction of ammonia with HCl and also the neutralization of NaOH with HCl additionally.

So  calculate first how much HCl is left ( titration of 60 ml with 0.05 M HCl)
How much HCl is used in the beginning.
The difference gives you the amount what react with ammonia.
How much ammonia correspond to it.
How much nitrogen correspond and at least how much was in the fertilizer.
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Offline fhz

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Re: Kjehdahls method
« Reply #6 on: April 19, 2020, 09:31:48 AM »
Quote from: chenbeier on April 19, 2020, 07:05:20 AM
Your equation is correct. But you need the reaction of ammonia with HCl and also the neutralization of NaOH with HCl additionally.

So  calculate first how much HCl is left ( titration of 60 ml with 0.05 M HCl)
How much HCl is used in the beginning.
The difference gives you the amount what react with ammonia.
How much ammonia correspond to it.
How much nitrogen correspond and at least how much was in the fertilizer.

Thank you so much! Now I understand this!
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