How many molecules of water are formed when 8 g of oxygen reacts with NH3? 4NH3+5O2------>6H2O+4NO

This is a stoichiometry problem. The balanced reaction is:

`~4NH_3 + ~5O_2 -gt ~6H_2O + ~4NO`

Step 1: Convert grams of oxygen (`~O_2` ) to moles. 

First, determine the molar mass of `~O_2` . The molar mass of a substance is determined by multiplying the atomic mass of each element in the substance by its subscript and then adding the resulting answers together.

Molar mass of `~O_2` = 2(15.9994) = 31.9988 g/mol

Second, convert the given grams of oxygen to moles by dividing by the molar mass.

      (8 g)(1 mol/ 31.9988 g) = 0.250 mol `~O_2`

Step 2: Multiply the answer from Step 1 by a ratio of the coefficients of `~O_2` and `~H_2O` .

First, identify the coefficient of each substance as indicated in the chemical equation.

   The coefficient of `~O_2` is 5.

   The coefficient of `~H_2O` is 6.

Second, write the coefficients as a ratio. The ratio can be written two ways:

   5 moles `~O_2` /6 moles `~H_2O`   OR  6 moles `~H_2O`/5 moles `~O_2`

Choose the ratio with the moles of the given substance (`~O_2` ) on the bottom.

Third, multiply the answer from Step 1 with the coefficient ratio that you just chose.

   (0.250)(6 mole/5 mole) = 0.30 mole `~H_2O`

Step 3: Convert moles of `~H_2O` to molecules.

Multiply the answer to Step 2 with the conversion factor, 1 mole = 6.02 x `~10^23` molecules. Orient the conversion factor such that unit "mole" is in the denominator. This causes the unit "mole" to cancel out, leaving your answer in "molecules."

   (0.30 mole)(6.02 x `~10^23` molecules/1 mole) = 1.80 x `~10^23` molecules

Rounded to proper significant figures, this answer would be:

   2 x `~10^23` molecules