Chemical equation: A+B2→AB2 How many moles of product can be made from 0.70 mol of A and 1.0 mol of B2?

Initial number of moles of A = 0.7 mol
Initial number of moles of B₂ = 1.0 mol

Balanced equation for the reaction :
A + B₂ → AB₂
Mole ratio A : B₂ : AB₂ = 1 : 1 : 1

If all 0.7 mol of A completely reacts, B₂ needed = (0.7 mol) × (1/1) = 0.7 mol < 1.0 mol
Hence, B₂ is in excess, and A completely reacts (limiting reactant/reagent).

Number of moles of A (completely) reacted = 0.7 mol
Number of moles of AB₂ made = (0.7 mol) × (1/1) = 0.7 mol

A + B₂ → AB₂.
Note that the mole ratio of A to B₂ is 1:1, so 0.70 mol of A will react with only 0.70 mol of B₂. The other 0.30 mol of B₂ does not get to react; A is the limiting reactant.