What is the total number of #AlF_3# molecules in a sample of #AlF_3# containing #3.01 * 10^(23)# fluoride ions ?
There are 1.00 × 10²³ formula units of AlF₃.
This is a problem about conversion factors.
AlF₃ is an ionic compound. Ionic compounds do not form molecules. Instead, we speak of formula units.
We see that one formula unit (FU) of AlF₃ contains three fluoride ions.
The conversion factor is either
We can use either one, depending on our calculation. In this case, we would write
3.01 × 10²³ F⁻ ions × conversion factor → FU of AlF₃
We must choose the ratio that makes the units cancel to give "formula units" in the answer. We choose the first one.
3.01 × 10²³ F⁻ ions ×
Notice how the “F⁻ ions” in the numerator and denominator cancel to give the desired units of “FU AlF₃”.
If we had used the other ratio, we would have gotten the units of “(F⁻ ions)²/FU AlF₃”. This is obviously wrong.
Always use units in your calculations. Then you can choose the correct conversion factors to get the desired units in your answers.