Changing pH Using CaO

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Chemical Forums Chemistry Forums for Students => Undergraduate Undergraduate General Chemistry Forum => Topic Topic started by: Steve1956 on January 29, 2011, 07:19:15 PM

Title: Changing pH using CaO Post by: Steve1956 on Steve1956 on January 29, 2011, 07:19:15 PM It's been a long time since I've done this kind of chemistry, so I want to make sure I'm doing doing t his problem problem correct ly. Question: You plan to add CaO t o inc inc rease the pH of a lake. lake. The c urrent urrent ph is 4 and you would like to raise the ph to 6.5, the volume of your lake is 4.0 x 109 L. How much CaO do you need to add? 1 mole of CaO neutralizes 2 moles of H+ My Work: I know the reaction is CaO+H2O ::equil::Ca(OH)2 Start of water: pH=4, pOH=10. [OH-]= 10-pOH = 1x10-10 M OH End Desired water: pH=6.5 pOH=7.5  

=3.16227766x10-8 M OH

Diff of 3.1522776610-8 M OH So, to calculate the amount of CaO I solved for X here (multiplying by 2 because 1 mol of CaO produces two moles of OH-) (X moles of CaO/4x109 L H2O)*2=3.16227766x10-8 M OH x=63.04555 moles of CaO needed then 63.04555 moles of CaO * 56 grams/mol= 3530.5508 g CaO needed to increase the pH. pH.

Title: Re: Changing pH using CaO Post by: Borek on Borek on January 30, 2011, 12:54:53 12:54:53 AM No, you need to remove H+, that means CaO reacts with H+ producing Ca 2+ and water. Your reaction can be seen as a first step in the process - note that OH- will immediately react with H+.

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Title: Re: Changing pH using CaO Post by: Steve1956 on January 30, 2011, 04:59:18 AM I don't get what you're saying here. So what do I need to do?

Title: Re: Changing pH using CaO Post by: Borek on January 30, 2011, 05:03:12 AM Write reaction equation between CaO and H+, calculate how much H+ has to be consumed, calculate how much CaO you need for that.

Title: Re: Changing pH using CaO Post by: Steve1956 on January 30, 2011, 05:12:12 AM Ok, so the reaction then is CaO+2H+ :rarrow: Ca2+ + H2O Now I'm blanking on how to figure out how much H+ has to be consumed.

Title: Re: Changing pH using CaO Post by: Steve1956 on January 30, 2011, 05:30:04 AM ok, so maybe now I know what i'm doing, but as I said its been a long time since i've had gen c hem for t hese problems. To calculate moles of H+ in the volume of water given: Start: 10-4*4 * 109= 400000 moles H+ Want: 10-6.5*4 * 109= 1264.9111 moles H+ so we take the difference to find out how many moles we need to neutralize: 400000-1264.9111=398735.08894 moles of H+ to neutralize since 1 mol CaO neutralizes 2 H+, we divide that number by 2 to give 199367.544468 moles of CaO needed 199367.544468 moles of CaO* 56= 11164582.4902 grams of CaO needed?

Title: Re: Changing pH using CaO Post by: Borek on January 30, 2011, 09:18:36 AM Correct approach, I guess reporting it as 11.2 (or even 11) tonnes will do.

Title: Re: Changing pH using CaO Post by: Steve1956 on January 30, 2011, 10:51:28 AM

http://www.chemicalforums.com/index.php?PHPSESSID=7c15d5851b5e5104361072cf07a4a034&action=printpage;topic=46466.0

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