An environmental chemist wants to estimate the calcium ion concentration in a wastewater sample by precipitating, drying, and weighing the calcium oxalate formed. The concentration of calcium ions can then be read off the calibration curve shown in Figure 1.
The chemist filters and collects the wet precipitate from the treated sample.
The wet mass of this precipitate is 15 g15\text{ g}15 g, which is 60%60\%60% water.
Calculate the calcium ion concentration in the wastewater sample.
Use Figure 1 to support your answer.