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A 3.00 M solution of sodium chloride (NaCl) has 3 moles of NaCl in every liter of the solution. To find the mass/volume of the 3.00M solution, we must determine the mass of a single mole of NaCl (molar mass), then multiply that by 3. The volume is represented by the number of liters of the solution, which is 1.
The molar mass of sodium chloride is 58.44 g/mole. To find the molar mass of 3 moles, multiply 58.44 by 3. The mass of 3 moles of sodium chloride is thus 175.32 g.
Again, the volume of the compound is 1 liter. This means the mass/volume of the 3.00 M solution of NaCl = 175.32 g/liter.
When NaCl dissolves in water, due to strong bonds between Na+ and water dipoles as also Cl- and water dipoles, the increase in volume is very little, though a huge amount of mass is added. As a consequence, the mass per unit volume should increase and a density greater than 1 is expected (assuming the density of pure water is 1 g/ml at 20 degree Celsius). A 3.00 M solution of NaCl requires 3x58.443= 175.329 g solid NaCl to be dissolved in sufficient water to make a total volume of 3000 ml. Let the volume of water required be 2950 ml. Hence total mass of the solution = 175.329 g (solute) + 2950 g (solvent) = 3125.329 g. Total volume is 3 litres or 3000 ml. So mass/volume works out to be 3125.329/3000 = 1.042 g/ml. As expected, this solution is definitely heavier than pure water.
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