Animated Solution for Chemistry - Organic Chemistry: Which of the following is a false statement?

Imagine you are a chemical detective. You've just synthesized a brand new organic compound in the lab, or perhaps you've extracted a mysterious substance from a rare plant. You know it contains carbon and hydrogen, but what else is in there? Is there nitrogen? Sulphur? Phosphorus? Halogens?

Qualitative analysis tells you what elements are present, but to truly understand the molecule and determine its empirical and molecular formulas, you need to know exactly how much of each element is there. This is the realm of quantitative analysis. Over the centuries, brilliant chemists have developed ingenious, highly specific methods to trap and measure these elements. Let's explore the classic techniques mentioned in our problem to uncover the false statement.

Dumas and Kjeldahl

Nitrogen is a crucial component of many organic molecules, from amino acids to complex pharmaceuticals. To estimate its percentage, chemists rely primarily on two legendary techniques.

The Dumas Method is the universal approach. The organic compound is heated with copper(II) oxide in an atmosphere of carbon dioxide. The carbon and hydrogen are oxidized to $C{O}_2$ and $H_{2}O$, while the nitrogen is released as free $N_2$ gas. Any oxides of nitrogen formed are reduced back to $N_2$ by passing the gases over a heated copper gauze. The nitrogen gas is then collected over an aqueous solution of potassium hydroxide (which absorbs the $C{O}_2$), and its volume is carefully measured to calculate the mass of nitrogen.

The Kjeldahl's Method, mentioned in option (d), is another workhorse of the chemistry lab. Here, the organic compound is heated with concentrated sulphuric acid ($H_{2}S{O}_4$). The nitrogen in the compound is quantitatively converted into ammonium sulphate, $(N{H}_4{)}_{2}S{O}_4$. This mixture is then treated with an excess of sodium hydroxide ($NaOH$) to liberate ammonia gas ($N{H}_3$). The ammonia is absorbed in a known volume of standard acid, and the unreacted acid is titrated to find out how much ammonia was produced. Because Kjeldahl's method is indeed a standard technique for nitrogen estimation, statement (d) is absolutely true.

A Tool for Sulphur and Halogens

Now we arrive at the heart of the confusion: the Carius method.

The Carius Method is a robust technique designed specifically for the estimation of halogens, sulphur, and phosphorus. It involves heating a known mass of the organic compound with fuming nitric acid ($HN{O}_3$) in a sealed, thick-walled glass tube known as a Carius tube. The high pressure and strong oxidizing environment ensure that the organic matter is completely destroyed.

If we are estimating halogens, silver nitrate ($AgN{O}_3$) is added to the tube. The halogens are converted into their respective silver halides (like $AgCl$, $AgBr$, or $AgI$), which are then filtered, washed, dried, and weighed.

If we are estimating sulphur, the fuming nitric acid oxidizes the sulphur present in the compound into sulphuric acid ($H_{2}S{O}_4$). This acid is then precipitated as barium sulphate ($BaS{O}_4$) by adding an excess of barium chloride ($BaC{l}_2$) solution. The precipitate is weighed to determine the sulphur content.

Because the Carius tube is explicitly used for sulphur estimation, statement (a) is perfectly true. However, the Carius method is never used for nitrogen estimation. Heating a nitrogenous compound with fuming nitric acid would produce a chaotic mixture of nitrogen oxides that cannot be easily captured and weighed in this setup. Therefore, statement (b) is the false statement we are looking for.

Let's not forget option (c), which details the estimation of phosphorus. When an organic compound containing phosphorus is heated with fuming nitric acid in a Carius tube, the phosphorus is oxidized to phosphoric acid ($H_{3}P{O}_4$).

To quantify this, we add a special reagent called magnesia mixture (a solution containing $MgC{l}_2$, $N{H}_{4}Cl$, and aqueous $N{H}_3$). This causes the phosphorus to precipitate out as magnesium ammonium phosphate:

This initial precipitate can be slightly variable in its water content. To get a perfectly stable, weighable compound, the precipitate is filtered, washed, and then strongly ignited. The heat drives off ammonia and water, leaving behind a highly stable compound called magnesium pyrophosphate:

The mass of the $M{g}_2{P}_2{O}_7$ is then used to calculate the exact percentage of phosphorus. Thus, statement (c) is a beautifully accurate description of the chemical reality.

By walking through the laboratory procedures, we've confirmed that Kjeldahl's method is for nitrogen, the Carius tube is used for sulphur and phosphorus (yielding $M{g}_2{P}_2{O}_7$), but the Carius method is absolutely useless for estimating nitrogen. The false statement is undeniably (b).