Your concentration should be undergoing an exponential decay, right: ? Take the natural log of both sides:

Thus the relationship between ln(M) and t is linear, with -r the slope of the line. You can take the logarithms of your M values, and then fit a line to that versus t with a standard least-squares linear regression and extract a value for r in the equation, and using the fact that , obtain your rate constant.

--Kevin C.