Molarity is defined as number of moles of the solute in 1 liter of solution.
Molarity = Moles of solute (mol) / Volume of the solution (L)
moles of KCl = mass (g) / molar mass (g/mol)
= 10.0 g / 74.5 g/mol
= 0.134 mol
Hence molarity = 0.134 mol / 0.500 L
= 0.268 mol/L
