How many moles of hci are formed from 3.5 moles of h2 and 3.5 moles of cl2?
a 5
b 7
c 1
d 2
e 0.5

First, we need a reaction formula. 

H₂ + Cl₂ --> 2HCl

as the equation tells you, for every mole of each one of the reactants (H2 and Cl2) is twice for HCl (because of the coefficient)

3.5 x 2= 7 (answer is B)