We use the following equation to find the concentration of the diluted solution:
M1V1 = M2V2
where M1V1 refers to the concentration and volume of the initial solution, respectively, and M2V2 refers to the concentration and volume of the final solution, respectively.
In this case, we are trying to find the final concentartion, M2:
M2 = M1V1/V2 = (12.0 M * 0.025 L)/ 0.500 L) = 0.6 M