The following reaction represents the decomposition for water:

Question

2H2O - 2H + O
How many molecules of hydrogen are produced from the decomposition of 12.2 grams of water into its elements?

Answer 1

This is a regular stoichiometry problem. All of these are done the same way. Print this out and remember it.
1. Write the equation and balance it.
Your equation is not correct AND it isn't balanced. It should be
2H2O ==> 2H2 + O2
2. Convert 12.2 g H2O to moles. moles = grams/molar mass.
3. Using the coefficients in the balanced equation, convert moles H2O to moles of hydrogen.
4. IF you wanted grams, then g = moles x molar mass. I would convert to grams even though the problem doesn't ask for it.
5. The problem asks for molecules of H2. You can use the moles from step 3 and calculate it knowing that 1 mole H2 contains 6.022 x 10^23 molecules OR you can use the grams from step 4 and calculate it knowing that 2 g H2 will contain 6.022 x 10^23 molecules.