Question

The amount of nitrogen in an organic substance can be determined by an analytical method called the Kjeldahl method, in which all the nitrogen in the organic substance is converted to ammonia. The ammonia, which is a weak base, can be neutralized with hydrochloric acid, as described by the equation NH 3 ( aq ) + HCl ( aq ) ⟶ NH 4 Cl ( aq ) If 21.0 mL of 0.150 M HCl ( aq ) is needed to neutralize all the NH 3 ( g ) from a 2.25 g sample of organic material, calculate the mass percentage of nitrogen in the sample.

Answer 1

Answer:

The mass percentage of nitrogen in the sample is 1.96%.

Explanation:

$Moles (n)=Molarity(M)\times Volume (L)$

21.0 mL of 0.150 M HCl

Moles of hydrogen chloride = n

Volume of hydrogen chloride solution = 21.0 mL = 0.021 L

Molarity of the hydrogen chloride = 0.150 M

$n=0.150 M\times 0.021 L=0.00315 mol$

$NH_3 (aq) + HCl(aq)\rightarrow NH_4Cl ( aq )$

According to recation, 1 mole of HCl neutralizes 1 mole ammonia.Then 0.00315 moles of HCl will neutrtalize:

$\frac{1}{1}\times 0.00315 mol=0.00315 mol$ of ammonia

1 mole of ammonia has 1 mole of nitrogen atom . Then 0.00315 moles of ammonia will have = 1 × 0.00315 mol = 0.00135 mol

Mass of 0.00315 moles of nitrogen= 0.00315 mol × 14 g/mol = 0.0441 g

Mass of the sample = 2.25 g

The mass percentage of nitrogen in the sample:

$\%=\frac{0.0441 g}{2.25 g}\times 100=1.96\%$