An acid compound found in plants and vegetables such as rhubarb, has a mass percent composition of 26.7% C, 2.24% H, and 71.1% O. It can interfere with respiration and cause kidney or bladder stones. If a large quantity of rhubarb leaves is ingested, this acid can be toxic. The lethal dose (LD50) in rats for this acid is 375 mg/kg. Rhubarb leaves contain about 0.5% by mass of the acid. If the acid has an experimental molar mass of 90.0 g/mol, what is its molecular formula?