It is estimated that about 0.02 m3 of pyrite (density 5 g/cm3) has "disappeared" from a rock face that is adjacent to a lake (original pH=6.7) with dimensions 100 m x 40 m x 10 m. You think that the "disappearance" is due to leaching of the pyrite into the lake. To prove your point, what would the new pH of the lake have to be?