Question

How many mols are in 8.00 g `BeF_2`?

Answer 1

There are approximately 0.170 moles of `BeF_2` in 8.00 grams of `BeF_2`.

Explanation:

We can determine the number of moles of a compound in a given number of grams of that compound (and vice versa) using dimensional analysis, or conversions.

If you are uncomfortable with conversions, perhaps the best way to look at them starting out is how you can arrange the ratios so that the units cancel appropriately and leave you with the units you're looking for on your final answer.

In this case, we are looking for an answer with units of moles. We're starting with 8.00 grams of `BeF_2`, so we know we need to somehow get rid of grams and pick up moles.

In nearly every case you will need the molecular weight of the compound you are dealing with, and this one is no different, so the first step is to calculate the molecular weight of `BeF_2`. This can be done by adding the molecular weights (or atomic masses) of each of its components, i.e. one beryllium atom and two fluorine atoms.

From the periodic table, we can see that the molecular weight of one Be atom is 9.012 g/mol, and one fluorine atom has a molecular weight of 18.998 g/mol. Thus, the molecular weight of `BeF_2` is

`(9.012 + 18.998 + 18.998 = 47.008)g/(mol)`

We can now use dimensional analysis to determine how many moles of `BeF_2` are present in 8.00 grams of `BeF_2`. For units to cancel, they must be on opposite sides of the vinculum (fraction bar), so we'll want to arrange our ratios accordingly.

8.00 grams `BeF_2` x `(1 mol BeF_2)/(47.008 grams BeF_2)`

If we multiply, "grams `BeF_2`" cancel, as it is present identically in both the numerator and denominator, leaving us with units of moles. We can then perform the indicated division (`8.00/47.008`), giving us an answer of approximately 0.170 moles.

These types of problems can take some getting used to, but with enough practice, it will become second nature! Being able to quickly convert between grams and moles of a compound is an invaluable skill in chemistry.

tl;dr:
1. Determine molecular weight of `BeF_2`
2. Use conversions with g/mol `BeF_2` and 8.00g `BeF_2`.