Question

How many formula units make up \[37.4g\] of magnesium chloride (\[MgC{l_2}\])?

Answer 1

Hint: A mole of a substance contains \[6.023 \times {10^{23}}\] formula units of that substance. This unit is referred to as the Avogadro’s constant. The unit is expressed as \[g/mol\].

Complete step by step answer:
The given question is based on the mole concept of a substance. The term mole is related to the amount of substance present in the compound. By definition a mole is the amount of substance which contains \[6.023 \times {10^{23}}\]number of elementary particles. The elementary particles are ions, atoms or molecules.
A mole is also defined as the amount of substance which is equal to the molar mass of that substance. Thus mole is represented as the ratio of amount of substance and the molecular weight or molar mass of that substance.
\[mole = \dfrac{{amount{\text{ }}of{\text{ }}subs\tan ce}}{{molar{\text{ }}mass{\text{ }}of{\text{ }}subs\tan ce}}\]
For determination of the formula units of a substance, so we need to evaluate the moles of the substance in consideration. The given compound is magnesium chloride. The molar mass of magnesium chloride = atomic mass of \[Mg\] + \[2{\text{ }} \times \] atomic mass of \[Cl\]
$ = 24.305 + 2 \times 35.453 = 95.211g/mol.$
The compound magnesium chloride is given as \[37.4g\]. So the moles of magnesium chloride is
$ = \dfrac{{37.4g}}{{95.211g/mol}} = 0.393moles.$
So the sample contains \[0.393moles\] of magnesium chloride.
Now according to mole concept one mole of magnesium chloride contains \[6.023 \times {10^{23}}\] formula units. Thus the formula units present in \[0.393moles\] of magnesium chloride is
$ = 0.393 \times 6.023 \times {10^{23}} = 2.37 \times {10^{23}}{\text{ }}formula{\text{ }}units$.

Hence $2.37 \times {10^{23}}{\text{ }}{\text{ }}units$make up \[37.4g\] of magnesium chloride (\[MgC{l_2}\]).

Note: The mole of any substance is used to express the exact amount of substance. It is used in determining the amounts of other reagents used for a reaction. It is also used to determine the theoretical yield and the obtained yield in a reaction.