The mass of a piece of metallic sodium, the amount of the substance of which is 0.5 mol.

Given:
n (K) = 0.5 mol

Find:
m (K) -?

Solution:
1) Using the periodic system of D.I.Mendeleev, calculate the molar mass of potassium, which will be numerically equal to the relative molecular mass:
M (K) = Mr (K) = Ar (K) = 39 g / mol;
2) Calculate the mass of potassium:
m (K) = n (K) * M (K) = 0.5 * 39 = 19.5 g.

Answer: The mass of potassium is 19.5 g.