Video Transcript
What is the M r of xenon difluoride, XeF2, where Xe equals 131 and F equals 19?
The question is asking to find M r, or the relative molecular mass, of xenon difluoride. First off, the definition of relative molecular mass is the ratio of the average mass of one molecule of an element or compound to one twelfth of the mass of an atom of carbon-12. This means we need to work out the mass of a molecule of xenon difluoride on a scale where an atom of carbon-12 is equal to a mass of 12. To do this, we need to add up the atomic masses of its constituent atoms or add up the relative atomic masses of all of the atoms in the compound.
Similar to the definition of relative molecular mass, the relative atomic mass of an element is the average mass of one atom of an element on a scale where an atom of carbon-12 has a mass of 12. Notice at the end of the question the relative atomic masses are provided for both elements in the compound xenon difluoride. Recall the relative atomic mass of an element is unitless. This is why xenon’s relative atomic mass of 131 and fluorine’s relative atomic mass of 19 are not followed by units.
Now that we have the relative atomic masses of each type of element in the compound, we need to make sure we add up the relative atomic masses of all of the atoms in the compound by looking at the chemical formula provided in the question. We can see the chemical symbol for xenon, which is Xe, and the chemical symbol for fluorine, which is F. We can also see a subscript two to the right of fluorine’s chemical symbol, which indicates there are two fluorine atoms in the compound. And in the case of xenon, there is no subscript shown in the formula provided, which means there is one xenon atom in one unit of this compound.
Now, we need to sum the atomic masses of all of the atoms, which means we will add 131 for one atom of xenon to the product of 19 times two for two atoms of fluorine. This gives an answer of 169, which will be unitless because like the relative atomic mass, the relative molecular mass is unitless as well. Therefore, the M r, or relative molecular mass, of xenon difluoride is 169.
