Why can’t the density of a sodium hydroxide solution be calculated using the superposition method?
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Recently, while studying the density of sodium hydroxide solutions, I found it difficult to locate data on densities for concentrations above 50%. So I tried to calculate the density using the superposition method. At 20 degrees Celsius, the solubility of sodium hydroxide in its saturated solution is 109 grams, with a density of 1.530. Using the relative density of sodium hydroxide, which is 2.12, the volume corresponding to 109 grams is 48.1 cubic centimeters; adding the density of water, which is 100, gives a total volume of 148.1 cubic centimeters. The total mass is 209 grams, resulting in a calculated density of 1.364, which differs significantly from the value obtained from existing data. Why is this? I seek advice from experts! Thank you!
This post was last edited by xzm881122 on 2019-6-25 at 15:25. This cannot be simply calculated, as due to the properties between molecules, solubility and concentration do not follow a simple linear relationship. Due to the gaps between molecules, the volume of the solution is not equal to the volume of the solvent plus the volume of the solute
Sodium hydroxide and water can form NaOH.nH2O hydrates at different temperatures (with n=1/2/3.5/4/5/7, for a total of 6 types). . From the perspective of inorganic chemistry, in any medium with a high amount of hydrates (when there are so many hydrates, it’s no longer true that 1+1=2), using this additive method to calculate density or specific gravity will inevitably result in large errors, either positive or negative. So it’s better to rely on actual measurement data.