1. Find the molar mass of NO2:

* Nitrogen (N) has a molar mass of 14.01 g/mol

* Oxygen (O) has a molar mass of 16.00 g/mol

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

2. Convert grams to moles:

* Moles = (mass in grams) / (molar mass)

* Moles = 19.6 g / 46.01 g/mol = 0.426 mol

3. Use Avogadro's number to find the number of molecules:

* Avogadro's number = 6.022 x 10^23 molecules/mol

* Number of molecules = (moles) * (Avogadro's number)

* Number of molecules = 0.426 mol * 6.022 x 10^23 molecules/mol

* Number of molecules = 2.56 x 10^23 molecules

Therefore, there are approximately 2.56 x 10^23 molecules in 19.6 g of NO2.