how do you find the limiting reactant?

I usually tackle these questions in a slightly different way to those who answered before me.

I first work out the moles of the species you are looking at, so in this case that is HF and SiO2

so you start with 6 mol of HF, and 4.5 mol SiO2

so from the stoichiometry you can work out how much of one species you need to completely react with the other.

so for example

n(HF)needed = 4 * n(SiO2)

= 4 * 4.5

= 18 mol

so you need 18 mol of HF to completely react with the SiO2, but you only have 6 mol, so this means that once all the HF has been used up there will still be some SiO2 left, and HF is limiting how far the reaction proceeded, ie limiting reagant

you could also work out the moles of SiO2 needed, and still come to the same conclusion. because you would need less SiO2 than you have.

hope this makes sense, i found doing it this way was alot more logical and made more sense to me :)

For a complete reaction, the no of moles of HF must be 4x the moles of SiO2. If you have 4.5 moles of SiO2, then you need 18 moles of HF to fully react. Anything less means that there is not enough HF, and that "limits" the amount of SiO2 that can react, as well as the amount of reaction products.

basically you look how many reactions you can proform with each amount. THis can becone kinda like a unit conversion

6molHF ( 1 rxn / 4 HF) = 1.5 reactions

4.5molSiO2 (1rxn/1SiO2) = 4.5 reactions

So the amount of HF you have limits the number of reactions you can have.