I think your question is: "A solution with a concentration of 0.5 m can be made by dissolving the solid in 100 mL of water. If the known [tex] M_{r} [/tex] of KCl is 74.5, the mass should be _____.
Assume that the density of water is 1.00 g/mL.
Then the mass of the water is 100 g.
0.5 m KCl = 0.5 mol KCl/kg water
∴ Moles of KCl = 100 g H₂O × [tex] \frac{0.5 mol KCl}{1000 g H_{2}O } [/tex] = 0.05 mol KCl
Mass of KCl = 0.05 mol KCl × [tex] \frac{74.5 g KCl}{1 mol KCl} [/tex]= 3.725 g KCl
Note: The answer can have only 1 significant figure, because that is all you gave for the molality of the solution.
To the correct number of significant figures, the correct answer is 4 g KCl.
