How many molecules of water are produced if 2.00 g of ammonia and 4.00 g 02 are reacted together?

This is a limiting reagent problem. How do I know? Because BOTH reactants are given.

1. Write and balance the equation.
2. Convert 2.00 g NH3 to moles. moles = grams/molar mass
3. Convert 4.00 g O2 to moles same procedure.
4. Using the coefficients in the balanced equation, convert moles NH3 form step 2 to moles of the product.
5. Same process convert moles O2 in step 3 to moles of the product.
6. Usually, these two answers don't agree which means one of them is wrong. The correct answer, in limiting reagent problems, is ALWAYS the smaller one.
7. Using the smaller, it can be converted to molecules H2O remembering that 1 mole of H2O contains 6.022E23 molecules.
Post your work if you get stuck.