Question

Ydrogen and fluorine combine according to the equation h2(g) + f2(g) → 2 hf(g) if 5.00 g of hydrogen gas are combined with 38.0 g of fluorine gas, the maximum mass of hydrogen fluoride that could be produced is

Answer 1

 Molar mass (H2)=2*1.0=2.0 g/mol
Molar mass (F2)=2*19.0=38.0 g/mol
Molar mass (HF)=1.0+19.0=20.0 g/mol

5.00 g H2 * 1mol H2 /2 g H2=2.50 mol H2 
38.0 g F2*1mol F2/38.0 g F2=1.00 mol F2

                                   H2(g) + F2(g) → 2 HF(g)
From reaction        1 mol      1 mol
From problem      2.50 mol   1 .00mol

We can see that  excess of H2, and that F2 is a limiting reactant.
So, the amount of HF is limited by the amount of F2.

                                 H2(g) + F2(g) → 2 HF(g)
From reaction                      1 mol       2  mol
From problem                      1.00 mol  2.00mol

2.00 mol HF can be formed.

2.00 mol HF*20.0g HF/1mol HF=40.0 g HF can be formed