Molecular Formula from Simplest Formula

Sample Problem #2

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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

C₄H₁₀

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