Question

Given:

pH: 7.60

Concentration (M): 0.050

mL: 100mL

Determine the Mass of Each Component

Recall that buffers are formed from conjugate acid/base pairs. Using the information given about your assigned buffer, determine how much of each component (acid and base) you will need in order to prepare it in the lab. (This will require a system of equations because there are two “unknowns.”) Note: the conjugate acid, in this case, is H2PO4−, and the conjugate base is HPO42−.

Equations 1 > [buffer= Stot/L

Equations 2 > Stot= Moles A + Moles B

Equations 3 > pH = pKa + log (moles B/moles A)

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 this point, you are only solving for the relationship of B to A.)

3. Using your concentration and volume, solve equation (1) in the lab manual for Stot. 2

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

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

6. Determine how many grams of each component must be measured out in order to make the buffer solution. Note that the conjugate acid comes as a solid of NaH2PO4H2O, and the conjugate base comes as a solid of Na2HPO47H2O. Determine pH Values of a Buffered System

7. Suppose that you pour half of your buffer solution into a separate beaker and add 1.00 mL of 0.103 M HCl into it. What pH would 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 affect your buffer components?) 3

8. Suppose you add 1.00 mL of 0.098 M NaOH into the other half of your buffer solution. What pH would you expect to measure with a pH probe? Show all work. (Hint: Is NaOH an acid or a base? Is it strong 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 of DI water into a beaker and added 1.00 mL of 0.103 M HCl? Show all work.

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

Answer 1

$H_{2}PO_{4}^{^{-}}\leftrightharpoons HPO_{4}^{^{2-}}$

Concentration= 0.050 M

pH= 7.60

s= Moles A (H2PO4-) + Moles B (HPO42-)

S/ 0.1 L= moles/litre= 0.050

S= 0.050 X0.1= 0.0050

A+B= 0.0050 moles in 0.1L buffer solution

in 100mL buffer A+B= 0.0050

pH = pKa + log ( B/ A)

7.60= 7.21 ( from literature ) + log (B/A)

log (B/A)= 7.60-7.21= 0.39

B/A= 100.39

(1) B/A= 2.45, here B/A indicating ratio of the moles of B and moles of A

(2) moles of B= 2.45 X moles of A

(3) Stot=A+B= 0.0050

(4) A+ 2.45A= 0.005

3.45A= 0.005

A= 0.005/3.45=0.00145 moles

(5) B= (0.00500-0.00145)=0.00355 moles

(6) Molecular weight of Na2HPO4, 7H2O (B)= 268.07 g/ mole

The amount of Na2HPO4, 7H2O (B)required= 268.07 X 0.00355= 0.9516 g

Molecular weight of NaH2PO4, 7H2O (B)= 137.99 g/ mole

The amount of NaH2PO4, H2O (A)required= 137.99 X 0.00145= 0.2001 g

The pH of the solution = 7.60