2024 Environmental and Civil FE Exam - Water Softening - FE Exam Practice

[@directhubfeexam]

Correction: For any softening problem we will use the EQUIVALENT RATIOS. Using this ensures the amount of lime (or soda ash) added will exactly match the amount required to neutralize or precipitate the hardness ions, based on their charge equivalency. The Stoichiometric Ratio isn't accurate enough since it's based only on the balanced chemical equation which tells us the ratio of moles of reactants required to completely react with each other. A better method is to use the equivalent ratio which accounts for both the charge and the amount of substance. It is used because water softening processes typically involve ionic reactions where the focus is on balancing the positive and negative charges rather than just the moles of reactants. The corrected proportion equation should be: 2 equivalent of Lime/1 mol Mg(HCO3)2 = X / 50 mg/L The equivalent ratio for the reaction is equal to the molar ratio. This won't always be the case but it is so for these chemical softening equations.

[@InsideTheScull]

Studying for the PE and no joke, you are the first video I have found where I finally feel like I get how to do these problems, so thank you!

[@directhubfeexam]

This brings me great joy! I’m so glad to hear this was helpful! Thanks for this 😊

[@oliviabustillo7679]

your videos are so helpful, thank you!! Is the equivalent weight formula you used in the handbook?

[@directhubfeexam]

Thank you! The equivalent weight formula is not in the FE Handbook. The only relevant table provided is on page. 344 in the FE Handbook. Definitely know this table!

[@dilhanijayathilake6417]

Hi, I was watching your videos and doing your shared problems to practice for FE. Thank you! Is considering the molar ratio equal to mass concentration right? For example, Mg(HCO3)2 to Lime, 50 mg/L: 100mg/L is not equal to 1:2 molar ratio of them. You can convert 50mg/L into mols/L and 100mg/L to mol/L and see that. Isn't that right?

[@directhubfeexam]

Hello! I'm glad you're finding these videos helpful! Thank you for practicing with me. After testing a few other problems, it looks like the concept is still correct here. There is an equivalent ratio of 1:2 between Mg(HCO3)2 and Ca(OH)2. Therefore, 50 mg/L of Mg(HCO3)2 will give us 100 mg/L of Ca(OH)2. If you stick to this method for the Softening portion of the FE exam, you'll get these problems correct.

[@directhubfeexam]

Sorry, I should have stated this from the start. We are using the equivalent ratio and not the molar ratio. It just happens to be the case the molar ratio is equal to the equivalent ratio for these water softening equations.

[@kenlua6064]

Im getting 68.42 mg/L . I am just relearning this again though so I might be mistaken but why didnt you use the atomic weight / molecular weight of Mg(HCO3)2? 50 mg/L Mg(HCO3)2 x (1 / 146.3 g/mol ) x (2 CaCO3 / 1 Mg(HCO3)2 x 100.1 g/mol = 68.42 mg/L

[@dilhanijayathilake6417]

I got the same answer 68.4

[@directhubfeexam]

@@dilhanijayathilake6417 when determining the amount of lime and soda ash needed, we will always use the equivalent ratio method in the video. Stick to this for any softening question where we are asked to find the lime and soda ash dose when given water quality data which can be drawn on a bar chart.

[@sepidehmansoori7686]

I am so sorry. But I think you made a mistake calculating the amount of soda needed before turning it into CaCo3. Maybe I am confused but please check the answer again and let me know.. Thanks

[@alirahmaninezhad895]

@@directhubfeexam I'm pretty sure we have to follow the stoichiometric relationship between components if we want to find the equivalent of each them in a reaction. It would be great if you tell me "WHY" this chemistry concept is not valid in these types of problems? What makes it different? It does not make sense to me. The 2:1 ratio is comparing the number of the moles of Mg(HCO3)2 and lime, and it is not the weight ratio! So we have to first convert the mass to the mole number, then use that ratio to find the required mole of lime and then required mass of the lime. Although instead of molarity concept, we can use simply Normality, to find the mass of lime based on the mass of Mg(HCO3)2. However, in both approaches, the required lime based on CaCO3, is 68.3 mg/L.

[@directhubfeexam]

I think I found the answer. For any softening problem we will use the EQUIVALENT RATIOS. Using this ensures the amount of lime (or soda ash) added will exactly match the amount required to neutralize or precipitate the hardness ions, based on their charge equivalency. The Stoichiometric Ratio isn't accurate enough since it's based only on the balanced chemical equation which tells us the ratio of moles of reactants required to completely react with each other. A better method is to use the equivalent ratio which accounts for both the charge and the amount of substance. It is used because water softening processes typically involve ionic reactions where the focus is on balancing the positive and negative charges rather than just the moles of reactants. In this case, if we examine equation 4: - The balanced chemical equation, the STOICHIOMETRIC ratio tells us that 1 mole of magnesium bicarbonate reacts with 2 moles of calcium hydroxide giving us a molar ratio of 1:2. - The equivalent ratio for the reaction is 1:2 (this requires a video by itself but see image below), which aligns with the stoichiometric ratio but is justified by the NEED TO BALANCE THE CHARGES and not just the moles. snipboard.io/BupPyc.jpg