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Question
Solve the following problem:
Carbohydrates are compounds containing only carbon, hydrogen, and oxygen. When heated in the absence of air, these compounds decompose to form carbon and water. If 310 g of a carbohydrate leaves a residue of 124 g of carbon on heating in absence of air, what is the empirical formula of the carbohydrate?
Solution
Given: Mass of carbon residue = 124 g, mass of carbohydrate = 310 g
To find: Empirical formula of the carbohydrate
Calculation: Since the 310 g of compound decomposes to carbon and water and the mass of carbon produced is 124 g, the remaining mass would be of water.
∴ Mass of water = 310 – 124 = 186 g
Moles of C = `"Mass of C"/"Atomic mass of C"=124/ 12` = 10.33 mol
Moles of water = `"Mass of water"/"Molar mass of water"=186/18` = 10.33 mol
The ratio of number of moles of C : water = C : H2O = 1 : 1
Hence, empirical formula = CH2O
Empirical formula of the carbohydrate = CH2O
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