[horseb0x]

The aim of the experiment was to calculate the solubility product of CaSO4 from its ionic strength. The procedure was to prepare a series of NaNO3 solutions of varying concentrations and add 2g of CaSO4 to each of these solutions then titrating these solutions with EDTA. The main thing confusing me is what the NaNO3 has to do with all this. From the results we got from titrating each solution its clear that NaNO3 increases the solubility of CaSO4 but they don't explain anything about the role NaNO3 plays in this experiment in my lab book so all I can do at the moment is guess. I'm guessing I have to include the Na+ and NO3- ions in the calculation of the ionic strength of the solutions.

According to my lab book if plot log[Ca2+] against sqrt(I) / (1 + 1.25sqrt(I)) I'll get a line with an intercept of sqrt(logKsp) and I can see that I couldn't have gotten different values for I without the NaNO3 but I still don't understand. I'm starting to get a glimpse of whats going on here but there are lots of variables involved here so its not an easy concept to visualise.

[Borek]

I'm guessing I have to include the Na+ and NO3- ions in the calculation of the ionic strength of the solutions.

From what I guess that's exactly the idea of the experiment - check how the solubility changes with ionic strength and extrapolate to ionic strength of zero. Note that I is not zero even if there is no sodium nitrate present, hence extrapolation.