For NaCl, seminormal = semimolar = 0.50 M

Suppose that you have 1 L of solution. Then moles NaCl = M NaCl = 0.50 moles NaCl / L of solution

The mass of the solution can be found using the density.

1L x (1000 mL / 1 L) x (1 cm^3 / mL) x (1.16 g / cm^3) = 1160 grams of solution in 1 L

That 1 L contains 0.50 moles NaCl.

0.50 moles NaCl x (58.5 g NaCl / 1 mole NaCl) = 29.3 g NaCl.

So the mass of H2O in the solution is 1160 g - 29.3 g = 1131 g H2O

molality NaCl = moles NaCl / kg H2O = 0.50 / 1.131 = 0.44 molal NaCl