How many h hydrogen atom are in the product

There are a total number of 4 hydrogen atoms in the product, this can be obtained by multiplying by the numerical coefficient to the H2 subscript for hydrogen. This will give you 2 H2 = 4 H atoms. Another way you can think of it, is the following:

1 H20 molecule = 2 H
2 H20 molecules = 4 H.

Ok so co(g) is not pronoused that. the only clue is that youpronouse it the oppisote way. 

C, small, hard spheres with insignificant volumes