Buffer Assignment

pH                                                   concentration(M)                                                   mL

7.8                                                  1.0                                                                              100.00

Eq) Buffer=Stotal/L

Where Stotal=moles (A) + moles (B)

Ptotal=mol (H2PO4-)+mol (HPO42-)

PH=pKa + log (moles B/molesA)

Determine the Mass of Each Component

Recall that buffers are formed from conjugate acid/base pairs.Using the information given about your assigned buffer, determinehow much of each component (acid and base) you will need in orderto prepare it in the lab. (This will require a system of equationsbecause there are two “unknowns.”)

Note: the conjugate acid in this case is H2PO4−, and theconjugate base is HPO42−.

1. Using equation (3) in your lab manual, solve for the ratio of(moles B/moles A).

2. From the ratio, solve for the moles B in terms of moles A.(Hint: you are not solving for the actual number of moles B at thispoint, you are only solving for the relationship of B to A.)

3. Using your concentration and volume, solve equation (1) inthe lab manual for Stot.  

4. Stot is the total moles of solute components (the sum of youracid and base combined). Using equation (2) in the lab manual, andsubstituting your answer to question 2 in for “moles B,” solve forthe quantity of moles A.

5. Solve for the quantity of moles B by inserting your numericalvalue for moles A into the relationship found in question 2.

6. Determine how many grams of each component must be measuredout in order to make the buffer solution. Note that the conjugateacid comes as a solid of NaH2PO4H2O, and the conjugate base comesas a solid of Na2HPO47H2O.

Determine pH Values of a Buffered System

7. Suppose that you pour half of your buffer solution into aseparate beaker and add 1.00 mL of 0.103 M HCl into it. What pHwould you expect to measure with a pH probe? Show all work. (Hint:Is HCl an acid or a base? Is it strong or weak? How would it affectyour buffer components?)

8. Suppose you add 1.00 mL of 0.098 M NaOH into the other halfof your buffer solution. What pH would you expect to measure with apH probe? Show all work. (Hint: Is NaOH an acid or a base? Is itstrong or weak? How would it affect your buffer components?)

Determine pH Values of an Unbuffered System

9. What pH would you expect to measure if you placed 52.5 mL ofDI water into a beaker and added 1.00 mL of 0.103 M HCl? Show allwork.

10. What pH would you expect to measure if you placed 51.3 mL ofDI water into a beaker and added 1.00 mL of 0.098 M NaOH? Show allwork.