Question 154
Emilia wishes to create a basic solution with $7\%$ hydroxide $(\mathrm{OH})$ ions. She has three solutions of different bases available: $10\%$ rubidium hydroxide $(\mathrm{Rb}(\mathrm{OH}))$, $8\%$ cesium hydroxide $(\mathrm{Cs}(\mathrm{OH}))$, and $5\%$ francium hydroxide $(\mathrm{Fr}(\mathrm{OH}))$. (The $\mathrm{Rb}(\mathrm{OH})$ solution has both $10\%$ $\mathrm{Rb}$ ions and $10\%$ $\mathrm{OH}$ ions, and similar for the other solutions.) Since francium is highly radioactive, its concentration in the final solution should not exceed $2\%$. What is the highest possible concentration of rubidium in her solution?