What is the formula mass of ethyl alcohol, ${\mathrm{C}}_2{\mathrm{H}}_5\mathrm{OH}$?

Step 1: Formula mass:

• “The formula mass of a substance is defined as the sum of the atomic masses of constituent atoms in an ionic compound.”

Step 2: Atomic mass of each atom in Ethyl alcohol $\left(C_2{H}_5\mathrm{OH}\right)$:

Ethyl alcohol ( ${\mathrm{C}}_2{\mathrm{H}}_5\mathrm{OH}$ ) has two Carbon$\left(\mathrm{C}\right)$ atoms, six Hydrogen$\left(\mathrm{H}\right)$ atoms, and one Oxygen$\left(\mathrm{O}\right)$ atom.

$$\begin{gathered} {\text{Atomic mass of one Carbon atom= 12 g mol}}^{-1} \\ \text{Atomic mass of two Carbon atoms = 2 × 12 g/mol = 24}\mathrm{g}{\mathrm{mol}}^{-1} \\ \text{Atomic mass of one Hydrogen atom = 1}\mathrm{g}{\mathrm{mol}}^{-1} \\ \text{Atomic mass of six Hydrogen atoms = 1× 6 g/mol = 6}\mathrm{g}{\mathrm{mol}}^{-1} \\ \text{Atomic mass of one Oxygen atom = 16}\mathrm{g}{\mathrm{mol}}^{-1} \end{gathered}$$

Step 3: Formula mass of Ethyl alcohol (${\mathbf{C}}_{\mathbf{2}}{\mathbf{H}}_{\mathbf{5}}\mathbf{OH}$):

${\text{Formula mass = (24 + 6 + 16) g mol}}^{-1}\text{= 46}\mathrm{g}{\mathrm{mol}}^{-1}$

Thus, the formula mass of ${\mathrm{C}}_2{\mathrm{H}}_5\mathrm{OH}$ is $46\mathrm{g}{\mathrm{mol}}^{-1}$.