The solubility of mercury (I) iodide is $\mathrm{5.5\ fmol/L}$ in water at $25\ \mathrm{^\circ C}$. What is the standard Gibbs energy of dissolution of the salt? The reaction is $$\ce{Hg2I2(s) -> Hg2^2+(aq) + 2I- (aq)}$$
Here is what I think I need in order to solve the question:
$$\Delta G=-RT\ln(K)$$ where $K$ will be the solubility constant. When the question says “solubility of mercury (I) iodide is $\mathrm{5.5\ fmol/L}$” I don’t exactly understand what this means.
The units make it seem like a concentration to me.