What is the molarity of a solution made by dissolving 18.4 g of potassium perchlorate in enough water to fill a 300.00 ml volumetric flask to the calibration mark?

The units of molarity are moles per liter; therefore, in order to compute the molarity, we must first find the moles of potassium perchlorate and the volume in liters:
Moles = mass / molecular mass
Moles = 18.4 / 138.6
Moles = 0.13 mol

Volume = 300 mL
1 L = 1000 mL
Volume = 0.3 mL

Molarity = moles / liters
Molarity = 0.13 / 0.3
Molarity = 0.43 M

Exothermic,  physical change after it is already baked