At stp how many molecules of nitrogen gas are in 2.24 l

STP stands for Standard Temperature and Pressure. It defines approximately 0° C or 32 Fahrenheit for temperature and 1 atm for pressure.

Volume can be used ONLY for gases to determine their mole number at STP.

One mole gas occupies 22.4 liters (L). The type of gas doesn't matter. It may be 1 mole of oxygen gas (O₂), hydrogen gas (H₂), or nitrogen gas (N₂). Though they have different molar mass, their volumes of one mole equals 22.4 L.

Here is the formula of volume-mole relationship for GASES at STP.

mole= Volume given/ 22.4 L ; n= Vgiven/22.4 L

Apply that formula at the question. n=2.24/ 22.4 L= 0.1 mole N₂ gas 

To find out number of molecules, convert mole into number. 

Actually, 1 mole defines 6*10²³ particles but that number (Avogadro's number) is mostly used for atoms and molecules. 

1 mole apples mean  1*6*10²³ apples.
2 mole pens mean   2*6*10² pens.
1 mole iron (Fe) have 1*6*10²³ iron atoms.

By formula;
mole= number of particles / Avogadro's number ; n= N/ NA; n=N/6*10²³

So, N= n*6*10²³ ; N= 0.1*6*10²³ = 6* 10²² nitrogen molecules.

The answer is 6*10²² N₂ molecules.