What is the mass of 3.5 mol of sulfur (VI) oxide?

Given:
n (SO3) = 3.5 mol

To find:
m (SO3) -?

Decision:
1) Using the periodic system of D.I.Mendeleev, calculate the molar mass of SO3, which will be numerically equal to the relative molecular mass:
M (SO3) = Mr (SO3) = Ar (S) + Ar (O) * 3 = 32 + 16 * 3 = 80 g / mol;
2) Calculate the mass of SO3:
m (SO3) = n (SO3) * M (SO3) = 3.5 * 80 = 280 g.

Answer: The mass of SO3 is 280 g.