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