Question

How do you calculate the formula mass of aluminium carbonate?

Answer 1

Formula masses are the mass of an ionic compound. Molecular formula or Ionic formulas are formed by the combination of anion and cation. In an ionic compound one cation is surrounded by various numbers of anion, similarly one anion is surrounded by various numbers of cations.

Complete step by step answer:
Useful formula:
Formula mass = Mass of each and every atom present in a given ionic compound multiplied by the number of atoms present in an ionic compound and it is expressed in atomic mass unit (amu).
Let ${{\operatorname{A}}_{x}}{{B}_{y}}$ is an ionic compound, where x and y are the number of cation and anion respectively. The formula mass of an ionic compound ${{\operatorname{A}}_{x}}{{B}_{y}}$ can be calculated as follows
Formula mass = (mass of cation A $\times$ number of cation) $+$ (mass of anion B $\times$ number of anion)

Let us find the formula of aluminium carbonate for which we have to calculate the formula mass
Symbol of Aluminium $=Al$
Aluminium belongs to group 13 family in which 3 valence electrons are present i.e., 3 electrons are present in the valence shell of aluminium which it readily loses and forms $+3$ oxidation state and it is found that in most of its compound aluminium shows $+3$ oxidation state and here also it represent the same oxidation state i.e., $+3.$
In a carbonate ion, carbon and oxygen atom are present whose formula is equal to $CO_{3}^{2-}$
By criss cross valency method we can find the formula of aluminium carbonate, In this method by cross multiplying the valency of ions we can neutralize the charge of an ion as well as we can find the formula of an ionic compound.
$\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,Al\,\,\,\,\,\,\,\,\,\,\,\,\,CO_{3}^{2-}$
$Charge\,\,\,\,\,\,\,\,\,+3\,\,\,\,\,\,\,\,\,\,\,\,\,-2$
By cross multiplying method we can write the formula mass of aluminium carbonate as $A{{l}_{2}}{{(C{{O}_{3}})}_{3}}.$
Now the mass of Aluminium $=27\ amu,\ carbon=12amu,\ oxygen=16amu$
Formula mass $=(2\ \times \ mass\ of\ Aluminium)+(3\ \times \ mass\ of\ Carbon)+(9\times mass\ of\ Oxygen)$
Formula mass $=(2\times 27)+(3\times 12)+(9\times 16)$
Formula mass $=(54+36+144)\ amu$
Formula mass $=234\ amu$

Final answer:
The formula mass of Aluminium carbonate $=234\ amu$

Note:
Formula masses are calculated the same as molecular mass, the only difference is that the formula mass is calculated for ionic compounds and they are expressed in amu while the molecular mass is calculated for the covalent compound where they are expressed in gm/mol.