Question

How many mL of a 2.0M NaBr solution are needed to make 200.0 mL of 0.50M NaBr?

Answer 1

This is a classical C1V1 = C2V2 question. We are given the initial concentration (2.0M), the final concentration (0.5M) and the final volume (200mL). We are asked how many mL of 2.0 M NaBr we require to make up the final concentration and volume, so we are calculating the initial volume (V1)

Therefore:
C1V1 = C2V2
2.0M(V1) = (0.5M)(200.0 mL)
V1 = 50 mL
So 50 mL of NaBr is needed to make a dilution to 0.5M in 200 mL.

Answer 2

To solve this we use the equation, 

M1V1 = M2V2

where M1 is the concentration of the stock solution, V1 is the volume of the stock solution, M2 is the concentration of the new solution and V2 is its volume.

2.0 M x V1 = 0.50 M x 200 mL

V1 = 50 mL of 2.0M NaBr is needed