Question

calculate the mass in kilograms of 6.7x10^21 molecules of NO2

Answer 1

You need to use Avogadro's number 6.0221415 × 10^23 which explains how many molecules go per mole, and then you just need to calculate the molecular weight of NO2 (just combine the molecular weight of one N atom and two O atoms)
so the calculus is:
x molecules of NO2 * 6.0221415 × 10^23 molecules/mole = x moles of NO2
x moles of NO2 * molecular weight of NO2 (g/mole) = x g of NO2