Here's how to calculate the number of moles of NO2 molecules:

1. Find the molar mass of NO2:

* Nitrogen (N) has an atomic mass of 14.01 g/mol

* Oxygen (O) has an atomic mass of 16.00 g/mol

Molar mass of NO2 = 14.01 g/mol + (2 * 16.00 g/mol) = 46.01 g/mol

2. Use the molar mass to convert grams to moles:

* Moles = Mass / Molar Mass

* Moles = 25.5 g / 46.01 g/mol

* Moles ≈ 0.554 mol

Therefore, there are approximately 0.554 moles of NO2 molecules in 25.5 g of NO2.