a solution consisting of isobutyl bromide and isobutyl chloride is found to have a refractive index of 1.3931 at 20 degrees celsius. The refractive indices at 20 degrees celsius of isobutyl bromide and isobutyl chloride are 1.4368 and 1.3785. determine the molar composition in percent of the mixture by assuming a linear relation between the refractive index and the molar composition of the mixture?