There are two components in the NaOH solution, NaOH and H2O. The mole fraction of a component is given by the formula
Number of moles of H2O = Mass/Molar mass = 36/18 = 2 moles
Number of moles of NaOH = 4/40 = 0.1 moles.
Total number of moles = 2 + 0.1 = 2.1 moles.
Mole fraction of H2O = 2/2.1 = 0.952
Mole fraction of NaOH = 0.1/2.1 = 0.048
Specific gravity is the density of a substance divided by density of water. Here, the density is 1g mL-1. Mass of the solution is the sum of mass of NaOH and mass of H2O = 4 + 36 = 40g.
Volume of solution = Mass/density
Volume of solution = 40mL.
Molarity = 0.1 moles/0.04L = 2.5M.
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