# Chemical Forums

## Chemistry Forums for Students => Analytical Chemistry Forum => Topic started by: AussieKenDoll on March 15, 2019, 01:13:55 AM

Title: how to prepare an acetic acid-sodium acetate buffer of the same pH and I=0.1?
Post by: AussieKenDoll on March 15, 2019, 01:13:55 AM
how to prepare an acetic acid-sodium acetate buffer of pH=5 but whose ionic strength is 0.1 from 0.1M acetic acid-sodium acetate buffer of pH=5?
Title: Re: how to prepare an acetic acid-sodium acetate buffer of the same pH and I=0.1?
Post by: Borek on March 15, 2019, 03:50:23 AM
You have to show you attempts at solving the problem to receive help, this is a forum policy.

http://www.chembuddy.com/?left=buffers&right=toc
Title: Re: how to prepare an acetic acid-sodium acetate buffer of the same pH and I=0.1?
Post by: Babcock_Hall on March 15, 2019, 09:57:20 AM
What do you know about ionic strength and pKa values?
Title: Re: how to prepare an acetic acid-sodium acetate buffer of the same pH and I=0.1?
Post by: AussieKenDoll on March 15, 2019, 02:29:00 PM
On acetic acid-sodium acetate, there are CH3COO-,CH3+, Na+,H+ ions

0.1=1/2 [(C(+1)^2+C(-1)^2+C(+1)^2+C(-1)^2 )]
0.1=2C
C=0.05 mol dm^(-3)
CV=K
0.1mol dm^(-3)×V1=0.05 mol dm^(-3)×V2
V2/V1 =2
Make 0.05M Acetic acid/Sodium acetate buffer by taking a specific volume of the buffer and diluting buffer 2 times to achieve 0.05M.
Is this correct way of preparing an acetic acid-sodium acetate buffer of the same pH and I=0.1?
Title: Re: how to prepare an acetic acid-sodium acetate buffer of the same pH and I=0.1?
Post by: AWK on March 15, 2019, 03:40:48 PM
For electrolyte of type 1:1 (sodium acetate) good approximation for I is c(sodium acetate). The acetic acid in this solution dissociates much less than 1 %.
What does mean CH3 ?
Title: Re: how to prepare an acetic acid-sodium acetate buffer of the same pH and I=0.1?
Post by: Babcock_Hall on March 15, 2019, 03:42:36 PM
There is no such thing as a CH33 ion in an acetate buffer.
Title: Re: how to prepare an acetic acid-sodium acetate buffer of the same pH and I=0.1?
Post by: AussieKenDoll on March 15, 2019, 03:48:58 PM
OMG sorry there is no CH3 lol
only CH3COO-, Na+,H+ ions exists
Title: Re: how to prepare an acetic acid-sodium acetate buffer of the same pH and I=0.1?
Post by: AussieKenDoll on March 15, 2019, 03:53:17 PM
0.1=1/2 [(C(+1)^2+C(-1)^2+C(+1)^2]
0.1=3C
C=0.034 mol dm^(-3)
CV=K
0.1mol dm^(-3)×V1=0.034 mol dm^(-3)×V2
V2/V1 =2.94
Make 0.05M Acetic acid/Sodium acetate buffer by taking a specific volume of the buffer and diluting buffer 2.94 times to achieve 0.034M.
Is this correct way of preparing an acetic acid-sodium acetate buffer of the same pH and I=0.1?
Title: Re: how to prepare an acetic acid-sodium acetate buffer of the same pH and I=0.1?
Post by: AWK on March 15, 2019, 04:19:05 PM
how to prepare an acetic acid-sodium acetate buffer of pH=5 but whose ionic strength is 0.1 from 0.1M acetic acid-sodium acetate buffer of pH=5?
There is something wrong with this statement. Buffer with I=0.1 will contain ~0.1 M sodium acetate.
0.1M acetic acid-sodium acetate buffer of pH=5 shows ionic strength ~0.06 to 0.07. To change its ionic strength to 0.1 not changing pH you should add both sodium acetate and acetic and calculations will be quite difficult.
Title: Re: how to prepare an acetic acid-sodium acetate buffer of the same pH and I=0.1?
Post by: Babcock_Hall on March 15, 2019, 04:50:51 PM
Sometimes an electrolyte is added to increase the ionic strength.  Sodium chloride would be one possibility.
Title: Re: how to prepare an acetic acid-sodium acetate buffer of the same pH and I=0.1?
Post by: AWK on March 15, 2019, 05:12:35 PM
Sometimes an electrolyte is added to increase the ionic strength.  Sodium chloride would be one possibility.
0
But then pH will change slightly (~0.02 ?)
Title: Re: how to prepare an acetic acid-sodium acetate buffer of the same pH and I=0.1?
Post by: Babcock_Hall on March 15, 2019, 05:52:22 PM
AWK,

Are you referring to the change in pKa due to change in ionic strength?  If so, then I agree that pH will change.
Title: Re: how to prepare an acetic acid-sodium acetate buffer of the same pH and I=0.1?
Post by: AussieKenDoll on March 15, 2019, 11:22:09 PM
Can someone please show me how do calculate it? I never done it! At least give me a link to how to solve this problem!
Title: Re: how to prepare an acetic acid-sodium acetate buffer of the same pH and I=0.1?
Post by: AWK on March 16, 2019, 03:40:51 AM
In the second post of this thread Borek gave you link to his lecture on buffers.