Answer: m= 0.69 m or mol/kg
Explanation: Molality is expressed as moles per unit kilograms of solvent or m= n / kg
First convert 25.0 g NaCl to moles
25.0 g NaCl x 1 mole NaCl / 58 g NaCl
= 0.43 moles NaCl
Next convert 625 g H2O to kilograms
625 g H2O x 1 kg / 1000 g H2O
= 0.625 kg H2O
Substitute the values in the formula
m = n / kg
= 0.43 mole NaCl / 0.625 kg
= 0.69 m or mole / kg