FE Exam Review - FE Environmental/Civil - Softening - Water Hardness

[@oliviabustillo7679]

Hi, could you please explain when to use the two different definitions for TH? You said its Ca2+ + Mg2+, but you also said its the sum of CH + NCH. I'm confused as to when I would use each definition. Thanks in advance!

[@directhubfeexam]

Hello! Both defintions are correct. Total hardness is the summation of the MAJOR "multi-valent" cations which tend to be Calcium (Ca2+) and Magnesium (Mg2+). There are other cations that contribute to the total hardness, but these are negligible when compared to the high concentrations we get from Ca2+ and Mg2+. Total hardness is also defined as the Carbonate Hardness (CH) and Non-carbonate Hardness (NCH). The carbonate hardness consists of the hardness has the bicarbonate ion attached to either Ca2+ and Mg2+. This is easier to remove. The non-carbonate hardness DOES not have the bicarbonate ion (HCO3-) attached to the Ca2+ and Mg2+. This is much harder to remove and requires the addition of soda ash. Check out the summary notes taken directly from the DirectHUB course: drive.google.com/file/d/1T6FLrGjyA-0bdA8EXdd8TuMkcXiZw1IT/view?usp=drive_link

[@fatemehsohrabi7596]

So if HCO3 is considered as hardness why in calculation we just measured for Ca2 and Mg2?

[@directhubfeexam]

Hi Fatemeh, good question. The keyword here is "associated" meaning "connected with" the bicarbonate anion. For carbonate hardness removal the calcium and magnesium metals are attached to the bicarbonate anion as shown by the chemical equation provided for lime softening on page. 341 in FE Handbook 10.0.1. drive.google.com/file/d/1W3N5n5C93Zx3v9cjnKxne8GWI5gKxpYm/view?usp=sharing So the calcium and magnesium are still the metals that make the water hard and we remove the hardness (i.e. calcium and magnesium metals) through the softening process by using soda ash and lime which ultimately precipitate out these metals. Hope this helps!

[@anniez2305]

@@directhubfeexam in the video you say that TH = NCH +CH. If HCO3- is the CH why wouldn't we add the Ca+Mg+HCO3

[@directhubfeexam]

@@anniez2305 Hi Annie, sorry for the confusing video definition for carbonate hardness (CH). It should have said: "those metals associated with the bicarbonate anion HCO3 are called carbonate hardness (CH)." So based on that definition, carbonate hardness will consist of the Mg and Ca metals that are bonded to the bicarbonate ion HCO3. The non-carbonate hardness (NCH) is when the Mg and Ca metals are bonded to other anions (not including HCO3). I hope this makes sense!

[@dand7056]

@@directhubfeexam so basically if hco3 is by itself then it doesn't count toward total hardness. If there was a MgOh2 then we would add that too correct?

[@directhubfeexam]

@@dand7056 Hello Dan, yes exactly. HCO3- by itself does not contribute to the total hardness. HCO3- is the alkalinity. Which is a measure water capacity to neutralize acids - or in other words, the water's capacity to resist changes in pH. The higher the alkalinity, the more resistance to changes in pH. MgOH2 would not add to the hardness. Anything associated with the bicabonate and carbonate would. For example: MgHCO and MgCO3 Please refer to the following bar diagram and figure to reinforce your understanding. drive.google.com/file/d/1dAdELneWVb02YsAExr78Nr1lR42tzvxc/view?usp=sharing drive.google.com/file/d/16eXsMD_aK3ohoBWka5vpAjmOq1lK-L1v/view?usp=sharing

[@SkalaWalla]

Hello, in your other video you say that CH is the minimum between ALK and TH. Why dont you calculate the ALK in this problem? Thanks kzbin.info/www/bejne/gXqplX-alK15n80

[@directhubfeexam]

So for this one I think we are only asked for the total hardness (TH). TH = Ca + Mg (always) The carbonate hardness (CH) is the hardness due to the Calcium (Ca) and Magnesium (Mg) ions associating with bicarbonate (HCO3-) and carbonate (CO32-). CH is minimum value between alkalinity (HCO3-) and total hardness (Ca + Mg). In this case: CH = Total Hardness = 110 mg/L ALK = HCO3- = 160 mg/L

[@SkalaWalla]

@@directhubfeexam Thanks! you are truly changing the world!