# How many formula units are contained in .67 g CaO?

Feb 3, 2017

$0.67$ grams of CaO contain $7.2 \times {10}^{22}$ formula units.

#### Explanation:

First you need to determine the number of moles in $\text{0.67 g CaO}$. Then you will multiply the number of moles by $6.022 \times {10}^{23} \text{formula units/mol}$.

To determine the molar mass of a compound, add the atomic weight on the periodic table in g/mol times each element's subscript.

Since the formula unit $\text{CaO}$ has no subscripts, they are understood to be $1$.

Molar Masses of Calcium and Oxygen

$\text{Ca:}$$\text{40.078 g/mol}$
$\text{O":}$$\text{15.999 g/mol}$

Molar Mass of Calcium Oxide

("40.078 g/mol"xx1) + ("15.999 g/mol"xx1)="56.077 g/mol"

Determining Moles CaO
To determine the number of moles of formula units in the given mass of $\text{CaO}$, divide its given mass by its molar mass.

(0.67cancel"g" "CaO")/(56.077cancel"g"/"mol" "CaO")="0.01195 mol CaO"

I'm keeping some extra digits in order to reduce rounding errors. The final answer will be rounded to two significant figures.

Determine Formula Units of CaO
Now multiply the number of moles of formula units by $6.022 \times {10}^{23} \text{formula units/mol}$.

$0.01195 \cancel{\text{mol"xx(6.022xx10^23 "formula units")/cancel"mol"=7.2xx10^22 "formula units CaO}}$