Question

29.5mg of an organic compound containing nitrogen was digested according to Kjeldahl’s method and the evolved ammonia was absorbed in 20ml of 0.1 M HCl solution. The excess of the acid required 15ml of 0.1M NaOH solution for complete neutralization. The percentage of nitrogen in the compound is:
A. 59.0
B. 47.4
C. 23.7
D. 29.5

Answer 1

To solve this question, we do not need the chemical formula of the organic compound containing nitrogen. We can just find the moles of the ammonia released in the reaction as it contains the nitrogen from that organic compound itself. Then we can find the percentage of nitrogen easily.

Complete step by step solution:
-Molarity, also called molar concentration, is a measure of the solute in a solution. It is defined as the ratio of the moles of the solute per liter of the solution. Its unit is mol/L and is denoted by the letter M.
M = n/v where M=molarity
n = moles of solute
v = litres of solution.
-We need to remember that the moles of a solute do not change if we dilute a solution. Only the solvent is added during dilution and moles depend on the solute and not the solvent.
So, from the above equation, we can conclude that moles=vol x molarity
n = M x v
-We can see that the moles of the original solutions are not given in the question. We can find it and then use it to find the percentage of nitrogen in the compound.
-There are 2 reactions of the acid, one with ammonia and the other with NaOH. Both these reactions use the acid completely. The reaction with NaOH can be shown as
$HCl+NaOH\to NaCl+{{H}_{2}}O$
Thus we see that 1 mole of both the compounds are being used up in the reaction. So, the moles of NaOH will be equal to the moles of HCl. As we have seen above, we can easily find the moles from molarity. So, the moles of HCl consumed with the base will be equal to
Moles of HCl = molarity x vol = 15x0.1=1.5
-When HCl reacts with ammonia, the reaction is written as
$HCl+N{{H}_{3}}\to N{{H}_{4}}Cl$
So we can see that here also, 1 moles are utilized by both the reactants.
-The total moles of HCl will be 20x0.1=2. 1.5 moles are used with the base. It means that the moles of HCl left for the reaction with ammonia are 2-1.5=0.5 moles.
-Thus we can say that 0.5 moles of ammonia are present in the organic compound. Now we need to find the percentage of nitrogen from this. From the definition of moles, we can see,
$moles=\dfrac{wt.\text{ in grams}}{\text{Molecular wt}\text{.}}$

-So we can find the wt. of nitrogen as moles x Molecular wt. =0.5x14=7
The weight of the organic compound is given to us and it is 29.5. Thus the percentage of nitrogen in the compound will be found from simple percentage formula from maths as
$\dfrac{7}{29.5}\times 100=23.7$

Therefore the correct option is C.

Note: One thing to keep in mind is that the molarity depends on the volume of the solution and not the solvent.
-Solution = solute + solvent.
So, in a solution, the volume is added not only by the solvent but also by the solute. There are certain concepts like molality where we need the physical quantities of solvent and not of solution.