please help!

[Premed828]

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Hi! every time I think I have mastered a problem in acid/base I end up getting it wrong the next time it comes up. I am struggling with the estimations and logs and antilogs. Can someone please help me out by explaining this problem as well as any tips to solving similar problems. Thanks!

if the pkA1 value for H2CO3 is 6.4 what is the PH of a 10^-3 M solution of this acid?

 

[puravida85]

Hi! every time I think I have mastered a problem in acid/base I end up getting it wrong the next time it comes up. I am struggling with the estimations and logs and antilogs. Can someone please help me out by explaining this problem as well as any tips to solving similar problems. Thanks!

if the pkA1 value for H2CO3 is 6.4 what is the PH of a 10^-3 M solution of this acid?

Two ways

1) TBR Shortcut

pH = 1/2 pka + 1/2 -log [HA]

pH= (1/2)6.4 + 1/2 - log [.001] = 3.2 + 1.5 = ~4.7.

Ok your prob. wondering how I got .001 into 1.5. Well, the log of .001 is -3. Cause I had to move the decimal 3 spaces over to the right to make it bigger. Half that is 1.5. Simple log tips. What if it had said .003 instead of .001, well we know it's bigger than .001 so it's not exactly -3. So it lets write it out in scientific notation, 3.0 x10^-3. Because log(3) = ~.5 , we take 3 -.5 = -2.5 = log of .003. With this logic, you can reverse and go from antilog to log. Trust me. Just use a calculator and just play with it, you'll get it.

2) You can set up the equation and solve for [H+]

antilog [-6.4] * [.001] = [A-][H+]

3.98x10^-10 = X^2

x = ~ 2x10 ^-5

X = [H+] = 2x10^-5

-log [H+] = ~4.7

Side note: Strong acids just take the -log [HA]. And for bases just solve for pOH using 14 = pOH + pH

The last calculations involve Henderson's equation and this is usually good for titrations and buffers. If you need help with this, PM me. GL!