Question

If 3.5 mL of distilled water is added to 1.5mL of a 0.150 M solution of CoCl2, what is the concentration of the resulting solution?

M1V1=M2V2

M1= 0.150
V1= 1.5mL
M2= 3.5?

Answer 1

I suppose we are to assume that the volumes are additive.
M1V1 = M2V2
0.150*1.5 = M2*(3.5+1.5)
M2 = 0.045M

Another way.
You start with 1.5 mL and end up with 5.0 mL (if the solutions are additive) so 0.150M x (1.5/5.0) = 0.045M