Limiting reactant

Question

H2+F2——->2HF, in the reaction represented above, what mass of HF is produced by the reaction of 3.0×10 to the power of 23 molecules of H2 with excess F2?

Answer 1

H₂ + F₂ ==> 2HF ... balanced equation

Since the question is asking us to find the mass of product formed, we will want to first convert the molecules of H₂ into moles of H₂ (we could do this at the end of the calculations, but it's just as easy to do it now).

moles of H₂ present (using Avogadro's number):

3.0x10²³ molecules H₂ x 1 mole H₂/6.02x10²³ molecules = 0.498 moles H₂

From the balanced equation, we see that 1 mole H₂ produces 2 moles HF. Therefore, we can now find the theoretical mass of HF produced from 0.498 moles H₂:

0.498 moles H₂ x 2 moles HF/1 mol H2 = 0.996 moles HF formed.

The molar mass of HF = 20.01 g/mole, thus...

0.996 moles HF x 20.01 g/mole = 19.93 g HF = 20. g HF formed (to 2 significant figures)