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Topic: Help me find my mistake in calculating the percentage of nitrogen  (Read 669 times)

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Offline nova343

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During estimation of nitrogen present in an organic compound by Kjeldahl’s method, the ammonia evolved from 0.5 g of the compound in Kjeldahl’s estimation of nitrogen, neutralized 10 mL of 1 M H2SO4. Find out the percentage of nitrogen in the compound.
I will explain how i solved this, please help me find my mistake
first i calculated the number of moles of H2S04 involved using the given volume and concentration.
it came out to be 10-2 moles.
In reaction between ammonia and H2SO4, for every 1 mole of sulphuric acid, there are 2 moles of ammonia reacting with it.
so, the number of moles of ammonia must be 2 x 10-2 moles.
ammonia has nitrogen to hydrogen ratio as 1:3.
which means the number of moles of nitrogen atoms is (2 x 10-2)/4.
which is 5 x 10-3
1 mole of nitrogen atom would have a mass of 14 grams which means the mass of nitrogen here is 5 x 10-3 x 14 grams.
which will be 7 x 10-2
the mass of the organic compound is 0.5 gram.
so, to calculate the percentage of nitrogen i did ((7 x 10-2)/0.5) x 100
which comes out to be 14 percent
the answer key says that it is 56 percent
where did i go wrong?

Offline Borek

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Re: Help me find my mistake in calculating the percentage of nitrogen
« Reply #1 on: December 25, 2023, 03:32:41 AM »
ammonia has nitrogen to hydrogen ratio as 1:3.
which means the number of moles of nitrogen atoms is (2 x 10-2)/4.

Think it over.

Is there really 1/4th of mole of N atoms in a mole of the ammonia?
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Offline nova343

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Re: Help me find my mistake in calculating the percentage of nitrogen
« Reply #2 on: December 25, 2023, 03:51:56 AM »
Ah, yes you right
1 molecule of ammonia will have 1 nitrogen and 3 hydrogen
which means 1 mole of ammonia will have 1 mole of ammonia and 3 moles of hydrogen.
Thanks a lot for the insight!

Offline Hunter2

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Re: Help me find my mistake in calculating the percentage of nitrogen
« Reply #3 on: December 25, 2023, 09:46:12 AM »
Who had thought this
Quote
which means 1 mole of ammonia will have 1 mole of ammonia

Offline nova343

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Re: Help me find my mistake in calculating the percentage of nitrogen
« Reply #4 on: December 25, 2023, 02:53:36 PM »
i intended to write 1 mole of nitrogen and 3 moles of hydrogen, somehow managed to write ammonia twice 

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