To the OP

Ok here's the question i need to do.. I'm really lost!

I have 100mM (mmol.l-1) solutions of the protonated and non-protonated forms of various chemicals that are used to produce biological buffers. The chemicals are acetic acid, MOPS (4-morpholinepropane sulphonic acid) and TRIS (2-amino-2-{hydroxymethyl}-1,3-propanediol). Calculate the volume of 100mM (mmol.l-1) of the non-protonated form of TRIS (i.e. TRIS base) that I need to add to 100ml of the 100mM (mmol.l-1) protonated (i.e. TRIS.HCl) form to prepare a buffer with a pH of 7.89. Express your answer to the nearest ml. Do not include the units.

CHEMICAL ???FORMULAWEIGHT???pKa

Acetic Acid???60???4.80

MOPS???209???7.20

TRIS???121???8.30

I believe you need to use the H-H equation but I think I'm just getting overwhelmed by the information provided, if someone could sorta outline the steps it would be really great!

Cheers

I performed a lab similar to this during the semester, gen chem II lab.

here's the general idea, try to follow...

-use the HH equation to find the ratio [Tris base]/[Tris acid form] this is the ratio of concentrations.

-thus you are provided with the volume of the acid as well as its concentration.

100mMV1=(V1+100mL)Mb, V1 is the volume of the Tris base (non-protonated form), Mb is the final concentration of the Tris base

100mL(100mM)=(V1+100mL)Ma, Ma is the final concentration of the acid

Thus

[Tris base]/[Tris acid form] = Mb/Ma=

[100mMV1/(V1+100mL)]/[100mL(100mM)/(V1+100mL)]=[Tris base]/[Tris acid form]

The left side of the equation becomes V1/100mL, thus

V1/100mL=[Tris base]/[Tris acid ], solve for V1 and there's your answer. You might want to check up on the math also.