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Answer:
Approximately formula units (
).
Explanation:
Refer to a modern periodic table for the relative atomic mass of magnesium () and chlorine (
):
:
.
:
.
In other words, the mass of of
atoms would be (approximately)
.
Likewise, the mass of of
atoms would be approximately
.
One formula unit of the ionic compound includes exactly as many atoms as there are in the given formula. The formula mass of a compound is the mass of
of the formula units of this compound.
The formula includes one
atom and two
atoms.
Hence, every formula unit of would include the same number of atoms: one
atom and two
atoms. There would be
of
atoms and
of
atoms in
of
formula units.
Thus, the mass of of
formula units would be equal to the mass of
of
atoms plus the mass of
of
atoms. (The mass of
of each atom could be found from the relative atomic mass of each element.)
.
In other words, the formula mass of is
.
Therefore, the number of formula units in of
would be:
.
Multiple by Avogadro's Number
to estimate the number of formula units in
:
.