04-12-2018, 04:00 PM

Hi!!

I have a problem with one of the approximate formulas (for a mixture of two weak acids) which (it) derives from the expression:

[H+]= ka_1/(Ka_1+ [H+]) *C_1 + ka_2/(Ka_2+ [H+])*C_2

At this point my textbook says that there are two main possible approximation:

1) If [H+]>Ka_1, Ka_2 :

[H+]^2 = Ka_1*C_1 + Ka_2*C_2

2) If Ka_1>>Ka_2 and C_1 >(or equal) C_2 :

[H+]= sqrt (Ka_1*C_1)

My PROBLEM is to understand how i can arrive at the expression [H+]= sqrt (Ka_1*C_1) , using the assumptions Ka_1>>Ka_2 and C_1 >(or equal) C_2, starting from the expression:

[H+]= ka_1/(Ka_1+ [H+]) *C_1 + ka_2/(Ka_2+ [H+])*C_2

Which are the mathematical-logical steps that i have to follow in order to get the approximate formula 2)???

Thanks very much!!!!

I have a problem with one of the approximate formulas (for a mixture of two weak acids) which (it) derives from the expression:

[H+]= ka_1/(Ka_1+ [H+]) *C_1 + ka_2/(Ka_2+ [H+])*C_2

At this point my textbook says that there are two main possible approximation:

1) If [H+]>Ka_1, Ka_2 :

[H+]^2 = Ka_1*C_1 + Ka_2*C_2

2) If Ka_1>>Ka_2 and C_1 >(or equal) C_2 :

[H+]= sqrt (Ka_1*C_1)

My PROBLEM is to understand how i can arrive at the expression [H+]= sqrt (Ka_1*C_1) , using the assumptions Ka_1>>Ka_2 and C_1 >(or equal) C_2, starting from the expression:

[H+]= ka_1/(Ka_1+ [H+]) *C_1 + ka_2/(Ka_2+ [H+])*C_2

Which are the mathematical-logical steps that i have to follow in order to get the approximate formula 2)???

Thanks very much!!!!