Answer:
There are 0.006 moles of Cl- in the solution.
Step-by-step explanation:
To solve this problem you should think in te concept of molar mass, the molar mass of Cl- is 35.45 grs/mol (it has the same mass as Cl, because electrons doesn´t contribute with significant mass). So,
35.45 grs --------- 1 mol Cl-
0.213 grs --------- x = 0.006 moles Cl-
There are 0.006 moles of Cl- in the solution.