Question

Amino-acid organic base gave the following results on analysis. $ C=78.6%,H=84%,N=13%. $ $ 0.4gm $ of the platinic chloride left on ignition $ 0.125gm $ of $ pt $ . The molecular formula of the base is:
(A) $ {{C}_{7}}{{H}_{9}}N $
(B) $ {{C}_{6}}{{H}_{8}}{{N}_{2}} $
(C) $ {{C}_{7}}{{H}_{9}}{{N}_{2}} $
(D) $ {{C}_{6}}{{H}_{8}}N $

Answer 1

Hint: We know that a stronger base readily donates its pairs of electrons. Chemical formula of benzylamine is $ \left( {{C}_{6}}{{H}_{5}}C{{H}_{2}}N{{H}_{2}} \right)or\left( {{C}_{7}}{{H}_{9}}N \right) $ and respectively. In both lone pairs are present on nitrogen atoms whereas in benzylamine, nitrogen is attached to the saturated carbon.

Complete step by step solution:
Let us denote monoacidic base by base by B which is $ B=(xHyNz) $ and its molecular weight be W
Thus we can say that $ {{B}_{2}}[Pt(C{{l}_{6}})]\to Pt $ [Half reaction to show $ 1:1 $ mole ratio]
Now we have to calculate number of moles of platinic chloride and platinum which are $ \dfrac{0.4}{408+W} $ and $ \dfrac{0.125}{195} $ respectively,
Now we know that one mole of platinic chloride gives 1 mole of platinum;
 $ \dfrac{0.4}{408+W\times 2}=\dfrac{0.125}{195} $
 $ W=\dfrac{78+51}{0.25} $
 $ \Rightarrow W=108 $
Now that we got the value of W we have substitute value in one mole of each compound to get actual number of moles present in molecular formula
 $ C=78.6%,H=84%,N=13%. $
Here the number of moles of carbons in one mole of compound is $ \dfrac{108\times 0.786}{12}=7 $
Similarly the number of moles nitrogen in 1 mole of compound is $ \dfrac{108\times 0.13}{14}=1 $
And the number of moles hydrogen in 1 mole of compound is $ \dfrac{108\times 8.4}{100\times 1}=9 $
Thus, this corresponds to $ {{C}_{7}}{{H}_{9}}N $
Therefore, correct answer is option A, i.e. the molecular formula of the base is $ {{C}_{7}}{{H}_{9}}N $

Note:
Not that the benzylamine as well as aniline are bases but relatively differs in their basicity, you must know that the lower is the $ p{{K}_{b}} $ value (i.e., numeric measurement of the basicity) , stronger is the base. The $ p{{K}_{b}} $ value of benzylamine is $ 4.70 $ .