Approximations

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Ashley McClearnen 1B
Posts: 59
Joined: Fri Sep 28, 2018 12:26 am

Approximations

Postby Ashley McClearnen 1B » Fri Jan 18, 2019 5:26 pm

Why are we able to approximate when trying to find the pH of a weak acid/base?

Shivangi_2J
Posts: 65
Joined: Fri Sep 28, 2018 12:15 am

Re: Approximations

Postby Shivangi_2J » Fri Jan 18, 2019 5:39 pm

In a weak acid or base whose K is smaller than 10^-3, the amount of substance that dissociates is so small compared to the initial molarity that there isn't really a substantial difference between the initial and the initial minus the amount dissociated
for example, we are saying because the amount dissociated, x, is so small, there isn't a significant difference between (.20 - x ) and (.2)

hazelyang2E
Posts: 57
Joined: Fri Sep 28, 2018 12:23 am

Re: Approximations

Postby hazelyang2E » Sat Jan 19, 2019 8:13 am

In addition to what was already said in the previous answer, it is important to understand that when we approximate we are saying that x is an extremely small number, but it is not zero.

Alexandra Albers 1D
Posts: 61
Joined: Fri Sep 28, 2018 12:18 am

Re: Approximations

Postby Alexandra Albers 1D » Sat Jan 19, 2019 11:09 am

Also, if you are doing a problem where you have, for example: K=x^2/(0.2-x) and K is less than 10^-3, (0.2-x) can be approximated to just (0.2) but the x^2 should stay x^2 I believe.

Karishma_1G
Posts: 67
Joined: Fri Sep 28, 2018 12:18 am

Re: Approximations

Postby Karishma_1G » Sat Jan 19, 2019 12:29 pm

You are able to approximate the value of x when the equilibrium constant is less than 10-3. We can approximate because x is extremely small relative to the initial value of the acid/base, so it will not substantially effect the equilibrium concentration of the acid/base.


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