Hydrogen fluoride is used in the manufacture of Freons (which destroy ozone in the stratosphere) and in the production of aluminum metal. It is prepared by the reaction CaF2 + H2SO4 ---> CaSO2 + 2HF In one process, 6.15 kg of CaF2 is treated with an excess of H2SO4 and yields 2.75 kg of HF. Calculate the percent yield of HF.