Using the following formula 4hf+s1o2> s1f4+2h2o
how many moles of hf will react completly with 9.0 moles
offio2?

36 moles of HF

Explanation:

Let's consider the following reaction

4 HF + SiO₂ → SiF₄ + 2 H₂O

The molar ratio of HF to SiO₂ is 4 mol HF:1 mol SiO₂. We can use a conversion fraction to find the moles of HF that will react completely with 9.0 moles of SiO₂.

9.0 mol SiO₂ × (4 mol HF / 1 mol SiO₂) = 36 mol HF

36 moles of HF will react completely with 9.0 moles of SiO₂.

i believe you are right for all except the 3rd ( look at sqdancefan his answer is much better)