presence of excess of oxygen and copper(II) oxide. Carbon and hydrogen in the compound are oxidised to carbon dioxide and water respectively. C x H y + (x + y/ ) O → x CO + (y/ ) H O The mass of water produced is determined by passing the mixture through a weighed U-tube containing anhydrous calcium chloride. Carbon dioxide is absorbed in another U-tube containing concentrated solution of potassium hydroxide.
These tubes are connected in series (Fig. . ). The increase in masses of calcium chloride and potassium hydroxide gives the amounts of water and carbon dioxide from which the percentages of carbon and hydrogen are calculated.
Let the mass of organic compound be m g, mass of water and carbon dioxide produced be m and m g respectively; Percentage of carbon= Percentage of hydrogen = Problem . On complete combustion, . g of an organic compound gave .198g of carbon dioxide and .1014g of water. Determine the percentage composition of carbon and hydrogen in the compound.
. Percentageof carbon . . % Percentage of hydrogen .
Nitrogen There are two methods for estimation of nitrogen: (i) Dumas method and (ii) Kjeldahl’s method. (i) Dumas method: The nitrogen containing organic compound, when heated with copper oxide in an atmosphere of carbon dioxide, yields free nitrogen in addition to carbon dioxide and water. C x H y N z + (2x + y/ ) CuO → x CO + y/ H O + z/ N + (2x + y/ ) Cu Traces of nitrogen oxides formed, if any, are reduced to nitrogen by passing the gaseous mixture over a heated copper gauze. The mixture of gases so produced is collected over an aqueous solution of potassium hydroxide which absorbs carbon dioxide.
Nitrogen is collected in the upper part of the graduated tube (Fig. . ). Let the mass of organic compound = m g Volume of nitrogen collected = V mL Room temperature = T K Volumeof