Question

A sample of $\text{Al}{{\text{F}}_{\text{3}}}$ contains $3.0\times {{10}^{24}}$ ${{\text{F}}^{\text{-}}}$ anions. The number of formula units in this sample is:
A.$9.0\times {{10}^{24}}$
B.$3.0\times {{10}^{24}}$
C.$0.75\times {{10}^{24}}$
D.$1.0\times {{10}^{24}}$

Answer 1

The formula unit in chemistry refers to the empirical formula of any ionic or covalent network solid compound which is used as an independent entity for stoichiometric calculations. It can be calculated from the smallest ratio of the number of atoms present in a molecule.

Complete step by step answer:
The molecular formula of aluminium fluoride is $\text{A}{{\text{l}}_{\text{2}}}{{\text{F}}_{6}}$. In this formula, if we calculate the smallest ratio of the stoichiometry of the atoms in the compound then we will get $\dfrac{2}{6}=\dfrac{1}{3}$.
Hence, the empirical formula of aluminium becomes$\text{Al}{{\text{F}}_{\text{3}}}$.
Hence, it can be said that,
$3.0$ ${{\text{F}}^{\text{-}}}$ Anions = 1 formula unit of aluminium fluoride or $\text{Al}{{\text{F}}_{\text{3}}}$.
Therefore, $3.0\times {{10}^{24}}$ ${{\text{F}}^{\text{-}}}$anions = $1.0\times {{10}^{24}}$ formula unit of aluminium fluoride or$\text{Al}{{\text{F}}_{\text{3}}}$.

So, the correct answer is option D.

Note:
Consider for example, sodium chloride in which there is one sodium cation and one chloride anion in one formula unit of it. In potassium oxide there are two cations of potassium and one anion of oxygen in its formula unit. In short, the formula unit shows the kinds and the numbers of atoms present in the smallest representative unit of the substance.
To find out the molecular mass of a compound we just need to first find out the formula unit mass of the compound and then multiply it by the common factor of the stoichiometric concentrations of the compound. For example in the above example, we need to calculate the mass of $\text{Al}{{\text{F}}_{\text{3}}}$ and then just multiply it by two to get the molecular mass of aluminium chloride.