Water Chemistry Cation Determination

In summary, the water sample has a total dissolved solids (TDS) of 1397 mg/L and an excess charge of 3.21 meq/L favoring anions. To identify the missing ion B, the concentrations of all the positive and negative ions must be converted to electric charge and balanced. The correct answer for the missing ion B is Cd+2 with a molecular weight of 112.8 and a mass concentration of 230 mg/L as CaCO3.
  • #1
biker.josh07
5
0
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.
 
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  • #2
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.
 
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1. What is water chemistry cation determination?

Water chemistry cation determination is a process used to identify and measure the concentration of positive ions, or cations, in a water sample. This information is important in understanding the quality and composition of water, and can also help determine the appropriate treatment methods for different types of water.

2. How is cation determination performed?

Cation determination is typically performed through a series of chemical tests, such as titrations or colorimetric analysis, that use reagents to react with specific cations in the water sample. The resulting color change or other measurable reaction can then be used to calculate the concentration of each cation present.

3. What are some common cations found in water?

Some of the most common cations found in water include calcium, magnesium, sodium, and potassium. These cations can come from natural sources, such as rock formations, or from human activities, such as industrial processes or agricultural runoff.

4. Why is cation determination important for water quality?

Cation determination is important for water quality because it can help identify potential contaminants or other substances that may affect the safety or usability of the water. It can also provide information about the overall mineral content of the water, which can impact its taste, appearance, and potential for scale formation.

5. How can cation determination results be used?

The results of cation determination can be used to inform decisions about water treatment and management. For example, if high levels of certain cations are found, specific treatment methods may be needed to remove them. Cation determination can also be used for regulatory purposes, such as monitoring compliance with water quality standards.

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