Asample containing 0.213g cl- is dissolved in 50.0ml of water how many moles of cl- are in the solution?

There are 0.006 moles of Cl- in the solution.

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.