硫酸钡与硫酸反应后氢离子物质的量不变,取滤液一半,加入NaOH溶液恰好呈中性,由H++OH-=H2O,可知n(H+)=n(OH-)=0.025L×0.2mol/L=0.005mol,故原硫酸溶液中H+为0.005mol×2,则n(H2SO4)=0.005mol×2× 1 2 =0.005mol,故硫酸的体积为 0.005mol 0.1mol/L =0.05L=50mL,故答案为:50.