Chemistry QUANTATIVE ESTIMATION OF CARBON AND HYDROGEN

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QUANTATIVE ESTIMATION OF CARBON AND HYDROGEN

Quantative Analysis :

After determining the elements present in the organic compound, the subsequent step is to find out the percentages of elements in the compound. The methods used for determining the percentage of elements involve combustion, oxidation and precipitation etc. of an organic compound such that they are converted into inorganic compounds, which can be easily estimated by gravimetric or volumetric techniques.

After determining the elements present in the organic compound, the subsequent step is to find out the percentages of elements in the compound. The methods used for determining the percentage of elements involve combustion, oxidation and precipitation etc. of an organic compound such that they are converted into inorganic compounds, which can be easily estimated by gravimetric or volumetric techniques.

Estimation of Carbon and Hydrogen :

`text(Liebig's Combustion Method)` : A known mass of the organic compound is heated in a current of dry oxygen (free from `CO_2`) in the presence of cupric oxide till all the carbon is oxidised to carbon dioxide and all the oxygen is oxidised to water.

`C_x H_y + (x + y/4) O_2 -> x CO_2 + y/2 H_2O`

Water is absorbed in a previously weighed `U`-tube containing anhydrous calcium chloride or anhydrous magnesium perchlorate. Carbon dioxide is absorbed in a previously weighed `U`-tube containing a strong solution of potassium hydroxide or ascarite (`NaOH` + `CaO`). The weights of carbon dioxide and water thus formed are determined and the amounts of carbon and hydrogen in the organic compound can be calculated as

Moles of `CO_2` formed `= (text(Mass of)quad CO_2 quad text(formed))/44 =` Moles of `C` in `CO_2`

= Moles of `C` in organic compound

Mass of carbon in organic compound `= (text(Mass of)quad CO_2 quad text(formed))/44 xx 12`

Percentage of carbon in organic compound `= 12/44 xx (text(Mass of)quad CO_2 quad text(formed))/(text(Mass of organic compound)) xx 100`

Moles of `H_2O` formed `= (text(Mass of)quad H_2O quadtext(formed))/18`

Moles of `H` in `H_2O` `= (text(Mass of) quadH_2O quadtext(formed))/18 xx 2 =` Moles of `H` in the organic compound.

Mass of hydrogen in the organic compound `= (text(Mass of)quad H_2O quadtext(formed) xx 2 xx 1)/18`

Percentage of hydrogen in organic compound `= 2/18 xx (text(Mass of)quad H_2O quadtext(formed))/text(Mass of organic compound) xx 100`

`text(Liebig's Combustion Method)` : A known mass of the organic compound is heated in a current of dry oxygen (free from `CO_2`) in the presence of cupric oxide till all the carbon is oxidised to carbon dioxide and all the oxygen is oxidised to water.

`C_x H_y + (x + y/4) O_2 -> x CO_2 + y/2 H_2O`

Water is absorbed in a previously weighed `U`-tube containing anhydrous calcium chloride or anhydrous magnesium perchlorate. Carbon dioxide is absorbed in a previously weighed `U`-tube containing a strong solution of potassium hydroxide or ascarite (`NaOH` + `CaO`). The weights of carbon dioxide and water thus formed are determined and the amounts of carbon and hydrogen in the organic compound can be calculated as

Moles of `CO_2` formed `= (text(Mass of)quad CO_2 quad text(formed))/44 =` Moles of `C` in `CO_2`

= Moles of `C` in organic compound

Mass of carbon in organic compound `= (text(Mass of)quad CO_2 quad text(formed))/44 xx 12`

Percentage of carbon in organic compound `= 12/44 xx (text(Mass of)quad CO_2 quad text(formed))/(text(Mass of organic compound)) xx 100`

Moles of `H_2O` formed `= (text(Mass of)quad H_2O quadtext(formed))/18`

Moles of `H` in `H_2O` `= (text(Mass of) quadH_2O quadtext(formed))/18 xx 2 =` Moles of `H` in the organic compound.

Mass of hydrogen in the organic compound `= (text(Mass of)quad H_2O quadtext(formed) xx 2 xx 1)/18`

Percentage of hydrogen in organic compound `= 2/18 xx (text(Mass of)quad H_2O quadtext(formed))/text(Mass of organic compound) xx 100`

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