Calculate the hardness of water in units of mg/L of CaCO3 (see equation 15-7) if your titration at pH = 10 resulted in a concentration of 15 mmol/L. Round your answer to the nearest whole number and enter only the numerical answer into the box. In most natural waters, some magnesium ion is present, so expressing water hardness in units of mg/L of CaCO, is an arbitrary convention that has been adopted for the sake of convenience (or tradition). The conversion from mmol/L of Ca2+ + Mg2+ is made using the following equation: mg Caco, = mmol (Ca2+ + Mg2+)x- 1 mol CaCO3 1 mol (Ca2+ + Mg2+) 100 g CaCO3 mol CaCO3 (15-7) L In effect, we see that the water hardness in units of mg/L of CaCO, is just equal to the total con- centration of Ca2+ + Mg2+ in units of mmol/L, multiplied by a factor of 100 (the molar mass of CaCO3).