Density of dissolved and non-dissolved salts

Question

I am doing a calculation, which needs to be 100% correct. My question is as follows; Does the density of dissolved salt differentiate from the density of non-dissolved salts?

My specific problem is a solution of 75% potassium formate in water. When the salt is dissolved, the salt molecule is broken down to K+ and COOH-. This, at least in my mind have to affect the density of the dissolved ions from the density of crystalline salt (non-dissolved). This in turn, would mean that a calculation of the exact density can not be calculated from the density of pure non-dissolved potassium formate (@ 1.91kg/l).

Is there any table I can look up the density of the ions directly, or does anyone have any tips to point me in the right direction? Are this difference negligible? Or are my hypothesis just plain wrong? All answers are appreciated.

Answer 1

Poutnik has nicely explained in his answer what you have to do to get reliable data. Unfortunately, tabulated density data of potassium formate is hard to found. However, I have found two reliable datasets (Ref. 1 & 2) with very agreeable data sets at two extremes (densities of 5-25% solutions in Ref.1 and that of 50-78% solutions in Ref.2, which was mentioned by Jon Custer in his comment):

Now, since the graph has good agreement of data $(R^2 = 0.9971)$, and using the straightline equation, you can find any density of given solution. Note that the data is for solutions at $\pu{20 ^\circ C}$.

References:

1. Åke Melinder, Thermophysical Properties of Aqueous Solutions Used as Secondary Working Fluids," Ph.D. Thesis 2007, Royal Institute of Technology, Stockholm, Sweden (ISBN 978-91-7178-707-1).
2. Tao Wen, Meng Wang, Yi Chen, Weifeng He, and Yimo Luo, "Thermal properties study and performance investigation of potassium formate solution in a falling film dehumidifier/regenerator," International Journal of Heat and Mass Transfer 2019, 134, 131-142 (DOI: https://doi.org/10.1016/j.ijheatmasstransfer.2019.01.031).

Answer 2

Better than "density of dissolved salt" is using the term "density of the salt solution", which if a function of the particular solution concentration.

As there is no law of total volume conservation, the volume of final solution may differ (is usually little smaller for salts) from the sum of volumes of solvent and (bulk) salt to be dissolved.

The density of a particular salt solution cannot be calculated from basic principles. It has to be determined experimentally, or interpolated from already known tabulated solution density data.

For common salts, there are often published tabulated data of solution densities for given mass concentrations or mass percentages.

If no solution density data are available, you can use as a very rough first approximation the calculated density from the total masses and total volumes of the solvent and the (bulk) salt. BUT, it is highly recommended to measure density (or calculate it from the mass and the volume) at least for one or two solutions close to solution concentrations of your interest.

Then, you can use an interpolation function, available in numerical mathematical methods, to interpolate density data.

Look also on the related question Why water volume went up by almost the same amount after adding salt?

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There is also no salt molecule $\ce{HCOOK}$. It is just a summary formula, expressing stoichiometric ratio of ions $\ce{K+}$ and $\ce{HCOO-}$ 1:1 in a salt crystal or its solution.