molarity is equal to the number of moles of solute per liter solution.
number of moles solute: (molar mass of perchloric acid is 100.45 g/mol)
35.0 mL (1.67 g/mL) 0.705 (mol/100.45 g) = 0.41 moles
liter solution = 2000 ml (1 L/1000 ml) = 2 L
molarity is equal to 0.41 moles/ 2 L =0.205 M