Gout is caused by an error in metabolism that leads to a buildup of uric acid in body fluids, which is deposited as slightly soluble sodium urate (C₅H₃N₄O₃Na) in the joints. If the extracellular [Na⁺] is 0.15 M and the solubility of sodium urate is 0.085 g/100. mL, what is the minimum urate ion concentration (abbreviated [Ur⁻]) that will cause a deposit of sodium urate?