Chromium is a metal, the chemical element with the symbol Cr. In solutions, chromium is presented in the oxidation states +3 or +6; for example, chromium chloride CrCl3. A weight percentage is the common way to express the concentration of a dissolved compound such as chromium, i.e. the ratio of the compound mass to the mass of the solution. Another way to express the concentration that is required for some calculations is to use milligrams (mg) of the compound dissolved in 1mm of the solution.
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Divide the percent concentration by 100, then multiply by the weight of the solution to calculate the mass of chromium. For example, if the weight percent is 5 and the weight of the solution 160 grams, then the mass of chromium is (5/100) x 160 = 8 grams.

Divide by the weight of the solution by the density to calculate the volume of the solution. For example, if the density is 1.1 g/ml, then the volume of the solution is 160/1.1 = 145.5 ml.

Divide the mass of chromium by the volume of the solution to calculate the concentration in grams per milliliters. In this example, the concentration is 8/145.5 = 0.055 g/ml.

Multiply the concentration by 1,000 to convert it to milligrams (mg) per milliliters. In this example, the chromium concentration is 0.055 g/ml x 1.000 = 55 mg/ml.
References
 Towson University: Concentration Units
 "Chemistry"; Raymond Chang; 2007