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Homework Help: Water Chemistry Cation Determination

  1. Aug 5, 2011 #1
    I'm having some trouble with this one

    A water sample contains the following ions (all in mg/L except HCO3-)

    K+=70, Sr+2=20, Ca+2=180, Mg+2=80, HCO3-=230 mg/L as CaCO3-2, SO4-2=164, NO3-=37, Cl-=400, CO3-2=35, and B=?? has a valence number of 2 but the sign is unknown.

    TDS(Total Dissolved Solids)=1397 mg/L

    Assume the water is balanced, identify the missing ion B as being Fe+2, Ba+2, Cd+2 or HPO4-2.

    So I worked out the Equivalent Weights and converted the concentrations to the respective normalities.Then I added up all the positive and negative normalities and found a difference of 3.21meq/L favouring the anions.

    My problem is when I work out the mass concentration for the B ion I convert the HCO3- to its mass concentration 280.6 mg/L rather than using its alkalinity of 230 mg/L as CaCO3.However this gives me the wrong answer with a molecular weight of 81.2 which doesn't correspond to anything but I get the right answer if I use 230 and it works out to be Cd+2 because you get a molecular weight of 112.8.so yeah this doesn't make sense.
  2. jcsd
  3. Aug 11, 2011 #2


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    Try converting your cations and anions into electric charge as well. Think of balancing the moles of positive charge and moles of negative charge. For example, if you have 100 mL solution of potassium chloride of concentration 0.050 meq/L, then you will have 0.005 meq of positive charge and 0.005 meq of negative charge. Do that for all the ions and anions to determine the excess charge in moles, equivalents, meq or whatever.

    Only one cation will meet the double requirement of providing both the mass (from the TDS number) and the missing charge to exactly counter the excess charge.
    Last edited: Aug 11, 2011
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