To determine the molality of seawater, we need to first calculate the molar mass of NaCl.

The molar mass of NaCl is the sum of the atomic masses of Na and Cl:
Na: 22.99 g/mol
Cl: 35.45 g/mol

Molar mass of NaCl = 22.99 g/mol + 35.45 g/mol = 58.44 g/mol

Now, we need to find the mass of NaCl in 100 g of seawater (since seawater is 3.50 mass % NaCl):

Mass of NaCl in 100 g of seawater = 100 g x 0.0350 = 3.50 g

Next, we convert the mass of NaCl to moles:
3.50 g NaCl x (1 mol / 58.44 g) = 0.0599 mol NaCl

The molality of a solution is defined as the moles of solute per kilogram of solvent. Since the solvent in this case is water, and water has a density of approximately 1 kg/L, we can approximate 1 kg of seawater to be approximately 1 L.

Therefore, the molality of seawater is:
0.0599 mol / 1 kg = 0.0599 mol/kg

So, the molality of seawater is approximately 0.0599 mol/kg.