To find the number of moles of Sr(NO3)2 used in the preparation of a 3.5 M solution, we need to first determine the number of moles of solute in the solution. Since the concentration of the solution is 3.5 M, this means that there are 3.5 moles of solute in 1 liter of the solution. Since the volume of the solution is 2.50 L, this means that there are 3.5 * 2.50 = <<3.5*2.5=8.75>>8.75 moles of solute in the solution.
Since the solute in the solution is Sr(NO3)2, this means that there are 8.75 moles of Sr(NO3)2 in the solution. Therefore, the number of moles of Sr(NO3)2 used in the preparation of the solution is 8.75 moles