No way! You are completely neglecting the volume of the salt dissolved. A thermodynamically exact approach is via partial molar volumes of water, CaCl2, and MgCl2. However, these are very hard to find as they are concentration-dependent. The second obvious way is to find the corresponding tables with solution densities.

Rather than completely neglecting the salts' volume, I would estimate the final solution volume via their bulk density (2.15 g/mL for CaCl2 and 2.32 g/mL for MgCl2). Hence, the approximate volumes are 20/2.15 = 9.3 mL, and 270/2.32 = 116.4 mL. The estimated solution density would be:

(1000 + 20 + 270)/(1000 + 9.3 + 116.4) = 1.15 g/mL

Certainly a better approximation than 1.29 g/mL.

Thank you,

It is true. I had ignored the volume of the salt dissolved. In addition to the item you mentioned, are other parameters also effective?

I'm not sure I understand. Partial molar volumes are exact, however, they are very hard to obtain for many substances as they have to be measured in quite laborous way.

Perhaps there are some empirical formulas for estimating solution density for electrolytes (and their mixtures). Try searching for such models.

You can verify the approach I outlined in my first message - try calculating the density for some well known solutions (KBr, CaCl2 etc.) with various concentrations and compare the result with tabular data. This will give you the idea what level of accuracy can you expected.

You can find densities of aq. sol. of either calcium chloride (CaCl2) and magnesium chloride (MgCl2) at Tables 2-42 and 2.61, respectively, in: R.H. Perry, D.W. Green, J.O. Maloney (Eds.), "Perry's Chemical Engineers’ Handbook", 7th ed., McGraw-Hill, 1997.

I have presented a correlation for the density of NaCl aq. sol. elsewhere at this forum: https://www.researchgate.net/post/How_can_I_dissolve_6_molar_sodium_chloride

Tibor Dubaj how did you get this bulk density? Actually, I need to calculate the density of ZnCl2, LiCl, KCl mixture at 400C Can you give some idea?

Hello, I've just Googled these values. I'm not an expert in this area, but I know that completely neglegting the "volume" of the solute is incorrect. As I wrote, the correct approach would be to find the experimental values of partial volumes of both solute and solvent. Otherwise, you would need to find some sort of empirical model for density of solutions depending on the temperature, solvent density and concentration.