n(H2SO4)=cV=0.5*0.25=0.125mol
m(H2SO4)=nM=0.125*98=12.25g
需要98%的浓硫酸的质量是12.25/98%=12.5g
V(H2SO4)=m/ρ=12.5/1.84=6.8mL
需要浓硫酸6.8mL.