This is a grams to moles stoichiometry problem. Lets give it a try! :)
The first step is ALWAYS going to be our given over one. Our given is what we know, which is the 40 grams of HF, or, hydroflouric acid.
So our first step looks like this:
40g HF
1
Next, we need to cancel out the grams of HF. By doing this, we will place the grams of HF on the BOTTOM of the 2nd step, so it cancels out. This particular mass will be the molar mass, which is 20.01. On top will be MOLES of HF, which is 1. Although it says 2 moles in our balanced equation, when we calculate mass, the moles in that step is always one. So far, our equation looks like this:
40g HF 1mol HF
x
1 20.01g HF
Our third and final step is to cancel out the moles of HF to find our moles of Sn. For this step we will use our balanced equation. The balanced equation shows we have 2 moles of HF and 1 of Sn. So, we would place the HF on the bottom, and Sn on the top as that is what we are trying to find. Our full equation looks like this:
40g HF 1mol HF 1mol Sn
x x
1 20.01g HF 2mol HF
To find our answer, we will multiply all of our top numbers together, which is 40. Then, we will multiply all of our bottom numbers together, which is 40.02. Then, we will divide our top total by our bottom total.
40
40.02
This gives us 0.999.
So, 0.999 moles of Sn, Tin, are produced from 40 grams of Hydroflouric Acid.
:)
