Chemistry and Chemical Reactivity (9th Edition)

Number of moles of $OH^-$ $1.71\ g\div 171.34\ g\ Ba(OH)_2/\ mol\ Ba(OH)_2\times2\ mol\ OH^-/mol\ Ba(OH)_2=0.0200\ mol$ Concentration: $C=0.0200\div(250/1000)=0.0798\ mol/L$