# Question Video: Determining the Relative Formula Mass of Aluminum Hydroxide Chemistry • 10th Grade

What is the relative formula mass of Al(OH)₃? [Al = 27, H = 1, O = 16]

02:36

### Video Transcript

What is the relative formula mass of AlOH3, where the relative atomic mass of aluminum is 27, hydrogen is one, and oxygen is 16?

To begin to solve this problem, the definition of relative formula mass, denoted as M r, is the sum of the relative atomic masses of constituent elements of the chemical formula. In other words, it is the sum of the number of each type of element multiplied by the elements relative atomic mass, denoted as A r.

Since we have been provided the chemical formula AlOH3, let’s identify how many atoms of each element are in the formula. Beginning with aluminum. We can see there is no subscript for Al, only an implied number one, indicating there is only one atom of aluminum. When looking at the hydroxide ion, OH, we notice there is a subscript three on the outside of the parentheses. This means that for every one aluminum ion, there are three hydroxide ions. Therefore, there are three atoms of oxygen and three atoms of hydrogen.

Now that we have identified the number of atoms for each element, let us multiply each by the relative atomic mass that was provided in the question for each element. Aluminum has a relative atomic mass of 27, which we will multiply by one for a product of 27. Oxygen has a relative atomic mass of 16, which we will multiply by three, for a product of 48. Finally, hydrogen has a relative atomic mass of one, which we will multiply by three for a product of three.

Now we will add together all of the relative atomic masses for all of the elements in the chemical formula: 27 plus 48 plus three, which gives us a relative formula mass of 78. Note that since the relative atomic mass for an element is a unitless measure, meaning it does not contain units after the number, the relative formula mass we obtain by summing all of the relative atomic masses together is also unitless.

Therefore, what is the relative formula mass of AlOH3? The answer is 78.