Q.Find the formula mass of CaSO4. If atomic mass of Ca = 40.1 u, S = 32.1 u and O = 16.0 u

Some compounds, most notably ionic solids like sodium chloride, don't actually exist as separate, discrete molecules — instead their positive and negative ions are arranged in a continuous, repeating three-dimensional crystal lattice (in NaCl, for instance, each sodium ion is surrounded by six chloride ions at equal distance, and vice versa). For such substances, the term 'molecular mass' doesn't strictly apply, so chemists instead use 'formula mass' — the sum of the atomic masses of all the atoms shown in the compound's simplest formula unit (e.g. NaCl's formula mass = atomic mass of Na + atomic mass of Cl = 23.0u + 35.5u = 58.5u) — even though that formula unit is not a real, isolated molecule.