There should be 1.03 grams of sodium chloride to have 400. mg of Na present. This problem is an application of the concept of mass percent or percentage by mass.
In the problem, the given mass percent 39.0% can be interpreted as 39.0 grams of Na are in 100 g of sodium chloride. Using this ratio, the mass of the compound required to obtain 400. mg of Na can be solved using the dimensional analysis below and the conversion factor between milligrams (mg) to grams (g):
[tex]mass \ of \ NaCl \ = \ 400. \ mg \ Na \ (\frac{1 \ g}{1000 \ mg}) (\frac{100. \ g \ NaCl}{39.0 g})[/tex]
Getting the ratio of the products of the numerator and the products of the the denominator:
[tex]mass \ of \ NaCl \ = 1.02564 \ g[/tex]
Since the number of significant figures in the given quantities is 3, the final answer must also have 3 significant digits. Therefore, the mass of sodium chloride that has 400. mg of Na is 1.03 g.
Keywords: concentration, mass percent
