## Approximations

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

### Approximations

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

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

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

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

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.

Return to “Calculating pH or pOH for Strong & Weak Acids & Bases”

### Who is online

Users browsing this forum: No registered users and 1 guest