The reaction is as follows:
K2Cr2O7+3SO2+H2SO4→K2SO4+Cr2(SO4)3+H2O
Thus, 1 mole of potassium dichromate reacts with 3 moles of sulphur dioxide.
Number of millimoles of potassium dichromate =50×0.01=0.5 millimoles.
Number of millimoles of sulfur dioxide 3×0.5=1.5 millimoles.
The molarity of the sulphur dioxide solution =1.5100=0.015 M =15×10−3 M.