The required number of moles=( 250/1000)×2
(no. of moles=molarity × volume)
so,required mass of HNO3=0.5×63
(given mass=no. of moles × molar mass)
70 grams of HNO3 are present in 100 grams of the solution
so,1 gram will be present in 100/70 grams of solution
hence 31.5 grams will be present in (100/70)×31.5 grams of solution.
so amount of concentrated nitric acid solution used is 45 grams.