Gout is caused by an error in nucleic acid metabolism that leads to a buildup of uric acid in body fluids, which is deposited as slightly soluble sodium urate (C5H3N4O3Na) in the soft tissues of joints. If the extracellular [Na+] is 0.10 M and the solubility in water of sodium urate is 0.072 g/100 mL, what is the minimum urate ion concentration (abbreviated [Ur - ]) that will cause a deposit of sodium urate?