Calculate the mass percent of 3.57 g KCl dissolved in 41.1 g H2O.

Question

I get the answer 8.7 but the assignment says I am wrong, Could someone please explain what I am doing wrong?

Answer 1

The reason why you got the answer wrong is because you did 3.57g KCl x 100 = 8.7%

41.1g H₂O

This is incorrect because mass percent is the grams of the substance divided by the grams of the whole solution. Water is not the solution, it is part of the solution along with KCl. So you have to add the water and KCl together to get the grams of the whole solution.

Mass of the solution = 3.57g KCl + 41.1g H₂O

Mass of the solution = 44.67g

So it should be set up as: mass percent = 3.57g KCl x 100 = 7.99 %

44.67g solution

Answer 2

mass percent means grams of solute per 100 grams of solution

mass of solute = 3.57 g

mass of solution = 3.57 g + 4.1 g = 44.67 g

mass % KCl = 3.57 g / 44.67 g (x100%) = 7.99% KCl by mass

Your mistake was not including the mass of KCl in the total mass of the solution.