If 2.00 mL of the 0.15 M Co(NO3)2 solution is diluted to 10.00 mL with water, what is the resulting Co(NO3)2 concentration

Answer 1

Going from 2.00 ml to 10.00 mls is a 5-fold dilution, so the final Co(NO3)2 concentration will be 1/5 the original. That would be 1/5 x 0.15 M = 0.03 M

Looking at it another way, and using the often taught V1M1 = V2M2, we have...

(2.00 ml)(0.15 M) = (10.0 ml)(x M)

x = (2)(0.15)/10

x = 0.03 M

Looking at it in yet a 3rd way:

2.00 ml x 1 L/1000 ml x 0.15 mol/L = 0.00030 moles of Co(NO3)2 in the 2.00 ml

Putting this into 10.0 mls (0.01 L) we have 0.0030 moles/0.01 L = 0.03 M