Net Charge

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heflicra000
Posts: 8
Joined: Fri Nov 15, 2019 5:26 pm

Net Charge

Post by heflicra000 » Fri Dec 20, 2019 5:53 pm

Hello, I am confused as to why some hydroxides are considered negatively charged in this problem while others are not. The question asks to identify the net charge of both molecules at pH = 7. The hydroxides on the left molecule are not negatively charged, while the hydroxides bonded to the phosphorous are as seen in the molecule on the right. I'm assuming this has something to do with them bonded to a P atom. Hoping for some clarification. Thank you!
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NS_Tutor_Mathias
Posts: 466
Joined: Sat Mar 30, 2019 8:39 pm

Re: Net Charge

Post by NS_Tutor_Mathias » Sun Dec 22, 2019 4:20 am

The key concept to be aware of here are the carboxylic acid and phosphoric acid functional groups, versus the two simple alcohol groups. Alcohols have pKa around 15 or higher - well outside of the range we are asked about here. Carboxylic acids tend to be in the pKa 2-3 range, and the first pKa (that for losing the first proton off phosphate) is similarly also around 2. The second proton from phosphoric acid can be a bit tricky - I would expect this to be deprotonated about 50% of the time, so on average only contributing -0.5, but if I had to decide on an integer net charge for the right-hand molecule, I would go ahead and say -3.

Don't worry too much about the charge of phosphate at physiological pH, besides knowing that it should be about -1.5 but is often simply thought of as -2.
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