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Question

A sample of magnesium was burnt in air to give a mixture of MgO and Mg3N2. The ash was dissolved in 60 meq. of HCl and the resultant solution was back-titrated with NaOH. 12 meq. of NaOH was required to reach the end point. An excess of NaOH was then added and the solution was distilled. The ammonia was then trapped in 10 meq. of a second acid solution. Back-titration of this solution required 6 meq. of the base. The percentage of magnesium burnt will be (as nearest integer) :

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Solution

Let the total number of moles of Mg used for MgO and Mg3N2 be a and b respectively.
2Mg+O22MgO
Before reaction a 0
After reaction 0 a

3Mg+N2Mg3N2
Before reaction b 0
After reaction 0 b3

Now, a+b3 moles of MgO and Mg3N2 are present in the mixture.

The reactions with HCl are as follows:
MgO+2HClMgCl2+H2O

Mg3N2+8HCl3MgCl2+2NH4Cl

The solution contains a mole of MgCl2 from MgO and b mole of MgCl2 from Mg3N2 and 2b3 mole of NH4Cl.

Also, moles of HCl used for this purpose = 2a+8b3 for MgO for Mg3N2

Now, mole of HCl or meq. of HCl (monobasic acid) =6012=48
2a+8b3=48...(i)

Further,
mole of NH4Cl formed = mole of NH3 liberated = mole of HCl used for absorbing NH3

2b3=4
b=6

From eq. (i), 2a+8×63=48
a=16

Thus, % of Mg used for Mg3N2=6(6+16)×100=27.27%

So, the nearest integer is 27.

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