To 400 g of 18% potassium hydroxide solution was added 100 g of a solution containing 0.5 mol of the same substance

To 400 g of 18% potassium hydroxide solution was added 100 g of a solution containing 0.5 mol of the same substance. The mass fraction of the solute in the resulting solution is?

KOH – potassium hydroxide.
Working formula: W = (mr.v – wa / mr – pa) * 100%. (the mass of the solute is divided by the mass of the solution and multiplied by one hundred percent).
400 * 18/100 = 72g is the mass of potassium hydroxide in its solution.
72/56 = 1.3 mol is the amount of hydroxide substance.
1.3 + 0.5 = 1.8 mol is the amount of hydroxide substance in the resulting solution.
1.8 * 56 = 100.8 g is the mass of potassium hydroxide in the resulting solution.
400 + 100 = 500g – the mass of the resulting solution.
100.8 / 500 * 100 = 20.16% – mass fraction of potassium hydroxide in the resulting solution. round up to an integer – we get the answer number 1 – 20%.