Question

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 CaCO3 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: g CaCO (15-7) Maco, = mpol(Ca2+ + Mg2JKT Olmo 12, co, 24)"100fcaco' mmol (Ca2+ Ca2+ + Mo2+ 1 mol CaCO, 100 (15-7) 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+ + Mg+ in units of mmol/L, multiplied by a factor of 100 (the molar mass of CaCO,).

Answer 1

Given concentration=15 mmol/L

Substituting value in equation 15.7

100 g CaCO3 I mol (Ca+2 + Mg+2y 1 mol CaCO3 Cat2+Mg)x 1 mol CaCO3

=15x100 mg/L=1500 mg/L

So the answer is 1500 (to be entered in box)