Dilution

C₁ V₁ = C₂ V₂

Solve for

Stock solution

C₁ concentration

V₁ volume to take

Final solution

C₂ concentration

V₂ final volume

Take 25 mL of the 2 mol/L solution, then make up to 500 mL with solvent.

Solvent to add 475 mL Dilution factor 20×