how many ions of each type are in 125 ML of a solution containing 25.9 g of copper(II) nitrate per liter?

From Wikipedia : Molar mass of Cu(NO₃)₂ = 187.5558 g/mol
Concentration of Cu(NO₃)₂ = (25.9/187.5558) mol/L
No. of moles of Cu(NO₃)₂ = (25.9/187.5558) × 0.125 mol

1 mole of Cu(NO₃)₂ contains 6.022 × 10²³ formula units.

1 formula unit of Cu(NO₃)₂ contains 1 Cu²⁺ ion.
Number of Cu²⁺ ions = (25.9/187.5558) × 0.125 × (6.022 × 10²³) × 1 = 1.04 × 10²²

1 formula unit of Cu(NO₃)₂ contains 2 NO₃⁻ ions.
Number of Cu²⁺ ions = (25.9/187.5558) × 0.125 × (6.022 × 10²³) × 2 = 2.08 × 10²²