In the reaction __h2 + o2 → 2h2o, what coefficient should be placed in front of h2 to balance the reaction?

a. 1

b. 2

c. 3

d. no coefficient is needed

The answer is:  [B]:  " 2 " .

Explanation:

Given the chemical equation:


      ?  H₂ + O₂ → 2 H₂O   ;

      →  We are asked, "What coefficient, if any — should be put in front of the:  " H₂ " ;

          (which is on the "left-hand side" side of the chemical equation given—      the "reactants") ;

     →  to get a balanced chemical equation?

 →  Let us examine the "right-hand side" of this chemical equation—the product(s).  In this case, the "product" given is:  " 2 H₂O " .  

So, on the "right hand side", we have:

     1)  4 "H's" → {Two "H₂" 's = 2 * 2 = "4 H's"} ;  and:
     2)  2 "O's" → { Two "O's").

   So, the left-hand side should have:

     1)  4 "H's" ;  and:
     2)  2 "O's" ;

Let us examine the left-hand side (the "reactants").

                  " ?  H₂  +  O₂ " ; 

On the left-hand side, we already have: 

     1)   " 2  O's " ;   →  one "O₂" = "2 O's" ;  and:
     2)    "2  H's " ;   →  one "H₂" .

Now, we would need:

A "total of "4 H's".  Is there any number we could put as a coefficient on the other reactant, which happens to be:  "H₂" ;  to make a total of "4 H's" ?          

         {Note: There are only these TWO (2) reactants in this chemical equation.}.  

        → The "H₂" ;  as it stands alone, is insufficient—since that would be only                                                                                         "2 H's".
 
     →  Thus, we can rule out:  "Answer choices: [A] and [D]."  

Note:  
→  Choice [A]:  "1" ;  The coefficient, "1";  is generally not (never?) used; and basically would function as the same as:
          →  Choice: [D]:  "no coefficient is needed".
→  Choices [A] & [D]: would leave us with only "2 H's" on the "reactants side" (i.e. "left-hand side of the equation"; and we need FOUR ("4 H's").

Since we are given:  "H₂" ; what coefficient could we put in front of this to get:       "4 H's" ?  (4÷2 =2).  So we could put a "2" in front of the "H₂" ; to get: 
    "4 H's".  The coefficient, "2" , corresponds directly with: 

                                             → Answer choice:  [B]:  "2" .

   { NOTE:  The remaining answer choice, [C], which is, "3" ; is INCORRECT; since 3 "H₂'s" would be "6 H's" ; (since 3 * "2 H's" equal "6 H's") ; which is too many "H's" → We need exactly "4 H's".}.

   So, the correctly balanced equation is:

             2 H₂ + O₂ → 2 H₂O  ; 

     → The coefficient that goes before the "H₂" ; to make this chemical equation balanced, is:  "2" . 

The correct answer is:  [B]:  "2" .  

      → The coefficient that goes before the "H₂" is:  "2" .

         Hope this answer—and {lengthy} explanation—is of help!