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

According to Avogadro's Law, same volume of any gas at standard temperature and pressure will occupy same volume. And one mole of any Ideal gas occupies 22.4 dm³ (1 dm³ = 1 L).

Data Given:

                  n = moles = ?
                  V = Volume = 16.8 L

Solution:
                As 22.4 L volume is occupied by one mole of gas then the 16.8 L of this gas will contain

                           = ( 1 mole × 16.8 L) ÷ 22.4 L
                    
                           = 0.75 moles

Result:
           16.8 L of Nitrogen gas will contain 0.75 moles at standard temperature and pressure.

While unproven, it uses facts to support part of a conclusion. it is still factual and has predictive power, however.