The molecular mass of a compound is the sum of the atomic masses of all the atoms in its chemical formula. To calculate the molecular mass of potassium chloride (KCl), we use the atomic masses of potassium (K) and chlorine (Cl).

The atomic mass of potassium (K) is 39 u, and the atomic mass of chlorine (Cl) is 35.5 u. The chemical formula of potassium chloride is KCl, which indicates one atom of potassium and one atom of chlorine.

Calculation:
Molecular mass of KCl = (1 × Atomic mass of K) + (1 × Atomic mass of Cl)
Molecular mass of KCl = (1 × 39) + (1 × 35.5)
Molecular mass of KCl = 39 + 35.5
Molecular mass of KCl = 74.5 u

Thus, the molecular mass of potassium chloride is 74.5 u.