Help with Biochemistry Homework: Formic Acid, pH & Buffer Solutions

[stads_29]

Homework Statement

Hey, have this question which i just can't wrap my head around, any help would be appreciated.

Formic Acid (HCOOH) - pKa = 3.8 is a weak acid that partially dissociates to form its conjugate base (HCOO-) and releases protons into aqueous solutions. Whats the pH of a solution in which (HCOOH) = .7M and (HCOO-) = .15M?

Then, if the pH of sol. of Formic acid has been titrated to a final pH of 2.8, Whats the ratio of conjugate base : Weak acid?

And suppose your given stock solutions of formic acid and sodium salt of the conjugate base : (HCOOH) = 3.OM and (HCOONa) = 1.5M,
How would you make a .1 M buffer(total formic acid + sodium formate concentration), 100mL total at pH 3.5, using above solutions?

Homework Equations

The Attempt at a Solution

Hasselback equation

 

[chemisttree]

Homework Statement

Hey, have this question which i just can't wrap my head around, any help would be appreciated.

Formic Acid (HCOOH) - pKa = 3.8 is a weak acid that partially dissociates to form its conjugate base (HCOO-) and releases protons into aqueous solutions. Whats the pH of a solution in which (HCOOH) = .7M and (HCOO-) = .15M?

Maybe this will start the neurons firing...

If you consider the equilibrium of formic acid to be

HCOOH <-----> H+ + HCOO-

and you know the concentration of HCOO-, how much H+ will you necessarily have?

Then, if the pH of sol. of Formic acid has been titrated to a final pH of 2.8, Whats the ratio of conjugate base : Weak acid?

And suppose your given stock solutions of formic acid and sodium salt of the conjugate base : (HCOOH) = 3.OM and (HCOONa) = 1.5M,
How would you make a .1 M buffer(total formic acid + sodium formate concentration), 100mL total at pH 3.5, using above solutions?

Homework Equations

The Attempt at a Solution

Hasselback equation

Yes, write out the Henderson Hasselbalch equation and see if you notice anything...

pH = pKa + Log([A-]/[HA])

 

[stads_29]

ok got 2.3 as pH, is that right? Still not sure bout ratio. got 4.1 for the first part. Please help on the last part!? haven't clue what to do with buffer

 

[Borek]

2.3 - show how you get there.

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