What do you mean by "molar composition"? Do you refer to the "molar fraction" which is amount of substance / sum of the amounts of all substances or "molar concentration", which is amount of substance / volume of the solution? In the latter case, please take into account that volumes are usually not additive, therefore you should fill your solution with solvent up to a certain amount of volume and not simply add the volumes (even when the differences might be small). Finally, I would use the molar weight of one formula unit of SiO2, determine from the mass the number of units and thereby calculate molar fraction or molar concentration.