At a certain temperature, the solubility of strontium arsenate, sr3(aso4)2, is 0.0480 g/l. what is the ksp of this salt at this temperature?

Given the solubility of strontium arsenate is 0.0480 g/l . we have to convert it into mol/L by dividing it over molar mass (540.7 g/mol)
Molar solubility = 0.0480 / 540.7 = 8.9 x 10⁻⁵ mol/L
Dissociation equation:
Sr₃(AsO₄)₂(s) → 3 Sr²⁺(aq) + 2 AsO₄³⁻(aq)
                             3 s                   2 s
Ksp = [Sr²⁺]³ [AsO₄³⁻]²
       = (3s)³ (2s)²
       = 108 s⁵
Ksp = 108 (8.9 x 10⁻⁵) = 5.95 x 10⁻¹⁹ 

You use the atomic number to find out how many protons and neutrons there are in the atom. you use the atomic mass to find out how many protons and electrons there are in the atom.