small Ka

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lauraxie2e
Posts: 108
Joined: Fri Aug 09, 2019 12:17 am

small Ka

Postby lauraxie2e » Sun Jan 19, 2020 5:42 pm

I know when there is a small Ka in strong acids, you can disregard the change in the denominator when solving for the change to solve for pH, however does this also apply to weak acids?

KarineKim2L
Posts: 100
Joined: Fri Aug 30, 2019 12:16 am

Re: small Ka

Postby KarineKim2L » Sun Jan 19, 2020 5:48 pm

Yes, some of the examples we did in class were on weak bases, and whenever x is less than 5% of the initial concentration, the approximation is valid.

Vincent Leong 2B
Posts: 207
Joined: Fri Aug 09, 2019 12:15 am

Re: small Ka

Postby Vincent Leong 2B » Sun Jan 19, 2020 5:49 pm

Weak acids are the only types of acids that we'll consider Ka values. In strong acids, there will be no Ka value because it will always be undefined as the acid will dissociate completely, leaving no concentration of reactant left.

Matthew Chan 1B
Posts: 111
Joined: Sat Sep 07, 2019 12:16 am

Re: small Ka

Postby Matthew Chan 1B » Sun Jan 19, 2020 5:51 pm

As mentioned above, we only account for the Ka values for the weak acids, since we know that the strong acids basically dissociate 100%. Also, when x is 5% of the initial concentration, we can get rid of the x since it would be such a small negligible amount anyway.

Rosa Munoz 2E
Posts: 105
Joined: Wed Sep 18, 2019 12:21 am

Re: small Ka

Postby Rosa Munoz 2E » Sun Jan 19, 2020 7:18 pm

if the ka is smaller than 10^3 than you may use the approximation but if percent dissociation is larger than 5% then you have to go back and use the quadratic equation

Eesha Chattopadhyay 2K
Posts: 104
Joined: Fri Aug 09, 2019 12:16 am

Re: small Ka

Postby Eesha Chattopadhyay 2K » Sun Jan 19, 2020 7:21 pm

Yes, if the ka is smaller that 10^-3, but if the percentage ionization is greater than 5% then you should go back and do the entire quadratic equation.


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