设此NaOH溶液中溶质的质量为x.
2NaOH+H2SO4=Na2SO4+2H2O
80 98
x 49g*25%
80/x=98/12.25
x=10g
NaOH溶液中溶质的质量分数是10/80*100%=12.5%
答：此NaOH溶液中溶质的质量分数是12.5%