Determine the chemical formula and molar mass of lead(II) trioxonitrate(V).

The chemical name lead(II) trioxonitrate(V) refers to lead(II) nitrate, Pb(NO₃)₂.
We need the atomic masses: Pb = 207 g/mol, N = 14 g/mol, O = 16 g/mol.
Molar mass of Pb(NO₃)₂ = Pb + (2 × N) + (6 × O)
Molar mass = 207 g/mol + (2 × 14 g/mol) + (6 × 16 g/mol)
Molar mass = 207 g/mol + 28 g/mol + 96 g/mol
Molar mass of Pb(NO₃)₂ = 331 g/mol

Moles = mass / molar mass
Moles of Pb(NO₃)₂ = 12.2 g / 331 g/mol
Moles of Pb(NO₃)₂ ≈ 0.03686 mol

We assume the density of water is 1 g/mL (or 1 g/cm³).
Volume of water = mass / density
Volume of water = 21 g / (1 g/mL) = 21 mL
To convert mL to dm³: 1 dm³ = 1000 mL
Volume of water = 21 mL × (1 dm³ / 1000 mL)
Volume of water = 0.021 dm³

Solubility = moles of solute / volume of solvent (in dm³)
Solubility = 0.03686 mol / 0.021 dm³
Solubility ≈ 1.755 mol/dm³
Rounding to two significant figures (based on the given mass of 21 g of water):