It's probably best to set this up as a ratio. You know that for every 5g of concrete, of that 18% (i.e. 0.9g) is cement and 0.03% (i.e. 0.0015g) is chloride.

So the ratio of cement to chloride is 0.9 : 0.0015 = 9 : 0.015 = 1 : 0.0016666667.

So for every gram of cement, there is 0.00166666667 grams of chloride.

You were close, but I think you had the answer as a percentage rather than as a weight.