Can\'t seem to get the right answer on this one... A water initially contains 40

Question

Can't seem to get the right answer on this one... A water initially contains 40 mg/L of Mg(2+) . The pH of thewater is increased until the concentration of hydroxide ion (OH-)is 0.001000 M. What is the concentration of magnesium ion in thiswater at this pH? Give your answer in milligrams per liter. Assumethat the temperature of the solution is 25 degrees Celsius. Can't seem to get the right answer on this one... A water initially contains 40 mg/L of Mg(2+) . The pH of thewater is increased until the concentration of hydroxide ion (OH-)is 0.001000 M. What is the concentration of magnesium ion in thiswater at this pH? Give your answer in milligrams per liter. Assumethat the temperature of the solution is 25 degrees Celsius.

Explanation / Answer

Initial concentration of Mg 2+ in water = 40mg / L Concentration of hydroxide ionOH- = 0.0010 M we know that if Mg is dissolved in water , it formsMg(OH)2 . that means concentration of Mg ions is equal to half theconcentration of OH ions present in the water. Therefore Concentration of Mg ions in water =(0.0010 /2)M                                                                       = 0.0005M          So 0.0005moles of Mg ions dissolved in 1.0L of water.     mass of Mg ions present per leter ofwater = moles of Mg * molar mass of Mg                                                                      = 0.0005 moles * 24.305 g / mol                                                                      = 0.012g or 12milligrams.                 Therefore0.012 g of Mg ions present in 1.0L of water .              Thatis 12milligrams of Mg per 1.0L of water.                                                                       

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