Answer the following questions based on an aqueous 7.28 M copper(II) bromide stock solution

Question

Determine the concentration (in M) of bromide ions in a solution prepared by adding 399 mL of water to 10.3 mL of the stock solution of copper(II) bromide solution.

I got 1.83e-1, 0.183 and 0.188 and they were all wrong

Answer 1

(10.3 ml)(7.28 M) = (409.3 ml)(x M)

x = 0.183 M CuBr₂

Since there are 2 moles of Br^- for each mole of CuBr₂ the answer would be

[Br^-] = 2 x 0.183 M = 0.366 M Br^-