a$.$ An experiment was conducted in the laboratory on the effect of pH on the rate of corrosion of a rectangular mild steel specimen; the specimen in the acidic medium has an initial weight of $7\mathrm{.}356$ g $.$ Calculate the rate of corrosion of the specimen after $3$ weeks when the weight became $7\mathrm{.}176$ g $.$ The specimen has the following dimensions; length is $50$ mm $,$ breadth is $10$ mm and a thickness of $1$ mm