### Video Transcript

The displayed formula for
1,3,5-tribromocyclohexane is shown below. What is its relative formula
mass? Give your answer to the nearest
whole number. The relative atomic mass of carbon
is 12, hydrogen is one, and bromine is 80.

Let’s move the displayed formula
for 1,3,5-tribromocyclohexane over to make room for the following. Relative formula mass, denoted as
𝑀 𝑟, is defined as the sum of the relative atomic masses of constituent elements
of the chemical formula. To determine the relative formula
mass of this compound, we first need to determine the chemical formula from the
displayed formula provided in the question. Then we need to add the relative
atomic masses, denoted by 𝐴 𝑟, of all of the elements in the chemical formula.

Upon analyzing the displayed
formula, we can count six carbon atoms represented by the chemical symbol C, nine
hydrogen atoms represented by the chemical symbol H, and three bromine atoms
represented by the chemical symbol Br. By taking the total number of atoms
of each element and writing them as a subscript to the right of their corresponding
chemical symbol, we find that the chemical formula for 1,3,5-tribromocyclohexane is
C6H9Br3.

Now that we have the chemical
formula, we can use the relative atomic masses provided in the question for carbon,
hydrogen, and bromine to calculate the relative formula mass for
1,3,5-tribromocyclohexane. The relative formula mass equation
for this compound is equal to six of the relative atomic mass of carbon plus nine of
the relative atomic mass of hydrogen plus three of the relative atomic mass of
bromine. Next, we substitute the relative
atomic masses into the equation. We perform the calculation and
determine the relative formula mass of 1,3,5-tribromocyclohexane is 321.

Note that relative formula mass is
a unitless measure. Therefore, the relative formula
mass of 1,3,5-tribromocyclohexane is 321.