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Molecular Formula and Simplest Formula Example Problem

Determining the Molecular Formula From Simplest Formula

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Formula Problem

The simplest formula for butane is C2H5 and its molecular mass is about 60. What is the molecular formula of butane?

Solution

First, calculate the sum of the atomic masses for C2H5. Look up the atomic masses for the elements from the Periodic Table. The atomic masses are found to be:

H is 1.01
C is 12.01

Plugging in these numbers, the sum of the atomic masses for C2H5 is:

2(12.0) + 5(1.0) = 29.0

This means the formula mass of butane is 29.0. Compare the formula mass (29.0) to the approximate molecular mass (60). The molecular mass is essentially twice the formula mass (60/29 = 2.1), so the simplest formula must be multiplied by 2 to get the molecular formula:

molecular formula of butane = 2 x C2H5 = C4H10

Answer

The molecular formula for butane is C4H10.

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