Astandard solution contained 0.8 mg/ml. a student took 2 ml of the standard solution and added 10 ml of water. what is the new/final concentration?

To get the concentration of the second solution let us use the following formulae

C1V1=C2V2 where C1 is concentration of first solution and V1 is the volume of solution first solution. on the other hand C2 is the concentration of second solution and V2 is the volume of second solution.

therefore

0.8×2=(2+10)×C2
   1.6 =12×C2
1.6/12=C2
C2     = 0.1333mg/mL

If the standard solution contains 0.8 mg/mL of a substance and there are 2 mL of the solution, then that quantity contains 0.8*2 = 1.6 mg of the substance. Adding 10mL of water, the new volume will be 2mL + 10mL = 12 mL and there will still be the same 1.6 mg of the substance. Thus the new concentration will be 1.6mg/12mL = 0.133 mg/mL.