Calculating PH of weak acids and bases

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Jasmin Argueta 1K
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Joined: Fri Sep 28, 2018 12:16 am

Calculating PH of weak acids and bases

Postby Jasmin Argueta 1K » Thu Jan 17, 2019 9:25 pm

The problem we did during lecture involved finding the ph and ionization of acetic acid in .10M CH3COOH , with K=1.8x10^-5. I was bit confused why we weren't suppose to use the quadratic equation, I heard the professor saying to omit this step since its a weak acid?

Mindy Kim 4C
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Joined: Fri Sep 28, 2018 12:25 am

Re: Calculating PH of weak acids and bases

Postby Mindy Kim 4C » Thu Jan 17, 2019 9:46 pm

When the Ka value of the acid is less than 10^-3, we can approximate the value of x by assuming the value of x being subtracted from 0.10 to be insignificant. Even if we subtract the value of x from 0.10, the value will still essentially be 0.10 because of how small x will be.

Samantha Chung 4I
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Joined: Wed Oct 03, 2018 12:16 am

Re: Calculating PH of weak acids and bases

Postby Samantha Chung 4I » Thu Jan 17, 2019 10:17 pm

It is because the Kc is very small, that the change in negligible. I think the example that Dr. Lavelle gave was if you have a million dollars and you take for instance, a dollar away, the value is still very close to one million, so you can approximate it to one millon. After you do that, it is no longer a quadratic equation.

Diana Bibireata 1B
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Joined: Fri Sep 28, 2018 12:23 am

Re: Calculating PH of weak acids and bases

Postby Diana Bibireata 1B » Thu Jan 17, 2019 10:23 pm

Because the K value is so small you can use the approximation method. In this case the x value will be so small that you can assume the value (0.1-x) will be approximately equal to 0.1. You can use this method whenever K is less than 10^-3.

chloewinnett1L
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Joined: Fri Sep 28, 2018 12:20 am

Re: Calculating PH of weak acids and bases

Postby chloewinnett1L » Fri Jan 18, 2019 12:32 am

The Ka is incredibly small (less than 10^-3), and so we don't need to use the quadratic equation because we assume the molar concentration of the original protonated molecule is going to be essentially unchanged at equilibrium. Though some of the molecule will be deprotonated, it is such a small number that when we use sig figs, the overall concentration will not budge. For example, if the concentration at equilibrium was 0.5-x and x=.0000001, it would not make sense to include the "-x" term in the calculation. It complicates beyond the degree of precision we are employing for our purposes.

Aria Soeprono 2F
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Joined: Fri Sep 28, 2018 12:27 am

Re: Calculating PH of weak acids and bases

Postby Aria Soeprono 2F » Fri Jan 18, 2019 8:55 am

As long as x is less than 5% of the initial value, than you know it is okay to approximate it to be 0 (in relation to the initial value, NOT the other values).


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