# Question Video: Calculating the Relative Formula Mass of 1,3,5-Tribromocyclohexane from Its Structural Formula Chemistry • 10th Grade

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. [C = 12, H = 1, Br = 80]

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### 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.