How many molecules of nitrogen dioxide are in 1.28 g of nitrogen dioxide no2?

Hello! 

Let's first find the molecular mass of nitrogen dioxide, knowing that by Avogadro's Law for each mole of a substance we have 6.02 * 10²³ molecules.

N = 1*14 = 14 amu
O = 2*16 = 32 amu
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molecular mass of nitrogen dioxide = 14 + 32 = 46 g/mol

How many molecules of nitrogen dioxide are in 1.28 g of nitrogen dioxide NO2?

46 g ---------------- 6,02*10²³ molecules
1,28 g -------------  y molecules