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# Calculating the molecular formula of an organic compound. Worked example, using fundamental formula and balanced equations.

&#61623; 1 dm3 of hydrocarbon A at 1 atmosphere pressure is shown to have a mass of 2.333g. Given that the molar volume of a gas at S.T.P (ie 25°C, 1 atm) is 24 dm3.

1) Calculate the molecular formula of A

2) A is a member of which homologous series?

&#61623; 15 cm3 of a mixture of gaseous hydrocarbon A and methane is combusted in the presence of excess oxygen, write balanced equations to show the complete combustion of A and of methane.

Are the reactions that you have described exothermic or endothermic?

&#61623; After the combusted mixture was cooled to room temperature the new volume of gas was recorded. When this remaining volume of gas was passed through aqueous sodium hydroxide the volume of gas decreased by 30 cm3.

Using this information, calculate the composition of the mixture.

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SOLUTIONS

1) 1 mole of a gas at S. T. P. occupies 24 dm3. Let x be the Mol. Wt. of the hydrocarbon,

So,
x : 24 = 2.333 : 1

and x = 24 x 2.333

x = 55.992 or 56

From this molecular weight we can ...

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