# Question 2694b

Jun 17, 2017

The formula mass is 399.86 u.

#### Explanation:

The calculation might be easier if you first multiply all atoms inside the parentheses by the subscript 3.

Then "Fe"_2("SO"_4)_3# becomes ${\text{Fe"_2"S"_3"O}}_{12}$.

Now, we can calculate the formula mass.

$\boldsymbol{\text{No."color(white)(m)"Atom"color(white)(mm)A_text(r)color(white)(mm) "Mass}}$
$\textcolor{w h i t e}{l} 2 \textcolor{w h i t e}{m m m l} \text{Fe} \textcolor{w h i t e}{m m l} 55.84 \textcolor{w h i t e}{m l l} 111.68$
$\textcolor{w h i t e}{l} 3 \textcolor{w h i t e}{m m m l} \text{S} \textcolor{w h i t e}{m m m} 32.06 \textcolor{w h i t e}{m m} 96.18$
$12 \textcolor{w h i t e}{m m m} \text{O} \textcolor{w h i t e}{m m l l} 16.00 \textcolor{w h i t e}{m l l} 192.00$
$\textcolor{w h i t e}{m m m m m m m l l} \text{Total = 399.86}$

The formula mass is 399.86 u.