How many moles of nitrogen gas, n2, are there in 16.8 l of this gas at stp

  0.75 mol

Explanation:

- For any type of gas, the molar volume (the volume of one mole of     substance) at Standard Temperature and normal Pressure  is 22.4 L.

- Using cross multiplication,   1 mol    →   22.4 L

                                                        ?       →   16.8 L

So number of moles of N2 in 16.8 L  =  ( 16.8 L × 1 mol ) / 22.4 L = 0.75 mol ..

if you mean hydrogen and oxygen then water is the product