1) To calculate the molarity of NaClO₃ the first thing you need to do is transform 57 g of it in mole. For this, you use the following equation:
mole = mass / molar mass
molar mass of NaClO₃ = 23 + 35.5 + (3×16) = 106.5 g/mol
mole = 57/106.5
mole = 0.535 moles
2) Now you transform 300 mL of water into liters. You just need to divide it to 1000. So you have 0.3 liters of it.
3) Now you use the following equation to calculate the molarity:
M = n/V
M = Molarity
n = moles
V = volume of the solution
M = 0.535/0.3
M = 1.78 moles/Liter
Answer: 1.78 moles/Liter