Calculation of % w/w of NaCl in the solution:

Step 1: Calculate the molecular weight of NaCl
Molecular weight of NaCl = atomic weight of Na + atomic weight of Cl
= 23 + 35.5
= 58.5 g/mol

Step 2: Calculate the mass of NaCl in the solution
Mass of NaCl = molality x molecular weight x mass of solvent
Given, molality of NaCl solution = 1.2 m
Mass of solvent is not given, assume it to be 1000 g (1 L of water)
Mass of NaCl = 1.2 x 58.5 x 1000
= 70,200 g

Step 3: Calculate the % w/w of NaCl
% w/w of NaCl = (mass of NaCl / total mass of solution) x 100
Total mass of solution = mass of solvent + mass of solute
= 1000 + 70,200
= 71,200 g
% w/w of NaCl = (70,200 / 71,200) x 100
= 98.59%

Explanation of the calculation:

The given data provides the molality of the NaCl solution, which is the number of moles of solute per kilogram of solvent. Using this information, we can calculate the mass of NaCl in the solution using the formula for molality. Then, we can calculate the % w/w of NaCl by dividing the mass of NaCl by the total mass of the solution and multiplying by 100. This gives us the percentage of NaCl by weight in the solution.