The ratio of chlorine to hydrogen chloride in the reaction

h2(g) + cl2 (g) -> 2 hcl(g) is

It is a simple stoichiometry problem. We have been given with balanced equation and 5.00 moles of HCl and have to calculate the moles of H2. 

From the balanced equation it is being clear that 2 moles of HCl are produced by 1 mole of H2. so, 1 mole of HCl will be produced by 1/2 moles of H2. So, the mole ratio of H2 and HCl is 1:2 
to calculate the moles of H2 for the given moles of HCl, multiply the moles of HCl by the mole ratio of H2 and HCl. 

5.00 mole HCl x (1 mole H2/ 2 mole HCl) 
= 2.50 mole H2 
so, to produce 5.00 moles of HCl, 2.50 moles of H2 are required.

During photosynthesis, plants transform electromagnet energy from the sun into chemical energy.