The formula of chromium chloride is CrCl3 and the formula for
silver chloride is AgCl. The relevant formula unit masses are
158.36 for chromium (III) chloride and 143.32 for silver chloride.
The gram atomic masses of chlorine, chromium, and silver are
35.453, 51.996, and 107.866 respectively. Therefore, the mass
fraction of chloride in chromium (III) chloride is
[3(35.453)/158.36] or 0.671628 and the mass fraction of chloride in
silver chloride is 35.453/143.32 or 0.24737. Therefore, to form
75.0 g of silver chloride, (0.24737)(75.0) or 18.55 g of chloride
is needed, and this amount of chloride is contained in 18.55/0.6716
or 27.6 g of chromium (III) chloride, to the justified number of
significant digits.