How many molecules of sulfur trioxide are in 78.0 grams?

Molecular formula of sulfur trioxide is SO₃
Molar mass of SO₃ is -  32 + (16 x 3 ) = 80 g/mol
1 mol of any substance consists of 6.022 x 10²³ units. In 1 mol of SO₃ there are 6.022 x 10²³ molecules of SO₃
in 80 g of SO₃ - there are 6.022 x 10²³ molecules of SO₃
Therefore in 78 g of SO₃ - there are 6.022 x 10²³ /80 x 78 = 5.871 x 10²³ SO₃ molecules