What mass of iron 3 chloride contain 2.35 x 10 to the 23rd chloride ions

Iron (III) chloride's chemical fromula is FeCl₃  Since Fe (III) is Fe³⁺ and Cl is Cl⁻.  Given that, you know that 3 moles of Cl⁻ combines with 1 mole of FeCl₃.  
First you need to find the number of moles of Cl⁻ is contained in  2.35x10^23 atoms of Cl⁻.  To do this you need to divide the number of Cl⁻ atoms by Avogadro's number (6.02x10^23atoms/mol) to get 0.3902 moles of Cl⁻.  Then you multiply 0.3902 by 1/3 (using the fact that there is 1mol Fe³⁺ per 3mol Cl⁻) to get the number of moles of FeCl₃ which is 0.1301mol FeCl₃.  Then to find the mass FeCl₃, you need to multiply the number of moles of FeCl₃ (which is 0.1301mol) by the molar mass of FeCl₃ (which is 162.15g/mol) to get 21.09g FeCl₃.
therefore the amount of FeCl₃ is 21.09g FeCl₃.

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