Calculate the concentration in #"g dm"^-3# of chloride ions in a #"0.25-M"# potassium chloride solution?
Your goal here is to convert the concentration of chloride anions from moles per cubic decimeter to grams per cubic decimeter.
Now, you know that potassium chloride is soluble in water, so it will dissociate completely in aqueous solution to produce potassium cations and chloride anions.
#"KCl"_ ((aq)) -> "K"_ ((aq))^(+) + "Cl"_ ((aq))^(-)#
Now, every mole of potassium chloride that you dissolve in this solution will produce
#["Cl"^(-)] = ["KCl"] = "0.25 M"#
The molarity of the chloride anions tells the number of moles of chloride anions present for every
In this case, you have
#0.25 color(red)(cancel(color(black)("moles Cl"^(-)))) * "35.453 g"/(1 color(red)(cancel(color(black)("mole Cl"^(-))))) = "8.86 g"#
Since this represents the mass of chloride anions present in
#color(darkgreen)(ul(color(black)("concentration Cl"^(-) = "8.9 g dm"^(-3))))#
The answer must be rounded to two sig figs, the number of sig figs you have for the molarity of the solution.