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10 Nov 2019

a)Calculate pH of saturated solution of dicalcium phosphate(CaHPO4) in water if its solubility is 3.16x10-4 m.

CaHPO4(aq)-> Ca^2+(aq) + HPO4^2-(aq)

Although phosphoric acid is a triprotic acid, consider only pKa''since pKa' and pKa''' will not contribute to this dissociation:

H2PO4^-(aq)->H+(aq)+ HPO4^2-(aq), pKa''=6.8

Assume that:

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Nestor RutherfordLv2

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For problem s 1 and 2 , assume an ac tivity coefficient of 1 for all substances and no effect of ionic strength. Eliminate terms in quadratic solutions for [H + ] only if the weak acid is dissociated < 5%. Reported pKa values can vary depending on the conditions under which they were measured ; therefore, in solving the following problems use the pKa values given with the problems.

1c . What is the pH of 45 mM H3PO4 ? (For c - f: Phosphoric acid (H3PO4 ) is a triprotic acid; pKa 1 = 2.12, pKa 2 = 7.21, pKa 3 = 12.32 Acetic acid pKa = 4.75)

d . What is the ratio of H2PO4 - to H3PO4 at a pH of 3?

e . If 2 volumes of 18 mM KOH are mixed with 1 volume of 22.5 mM H 3 PO 4 , what will be the pH of the final mixture?

f . If 15 Î¼moles of acetic acid is generated in a 1.0 ml enzymatic reaction buffered by 50 mM Na - phosphate (pH 7.0), what will be the final pH of the reaction mixture? Would the change in pH be smaller or larger if the reaction were buffered by 50 mM Na phosphate ( pH 6.0 )? Explain you answer; no calculations required.

You work in a medical research lab with a biochemist who asks you to make a 1.00 L phosphate buffer stock solution with pH = 7.4. The total phosphorus concentration is to be 0.250 M. In the storeroom, there are the following reagents: (1) a 3.00 M phosphoric acid stock solution, (2) solid sodium dihydrogen phosphate, (3) solid sodium monohydrogen phosphate, (4) solid, sodium phosphate, (5) a 3.00 M HCl(aq) stock solution, (6) a 3.00 M NaOH(aq) stock solution, and (7) deionized water.

(i) (2 pts) To make an optimum buffer, it is important that the pKa of the acid part is close to the target pH of the buffer. Which phosphate species has a pKa value closest to pH = 7.4?

(ii) (4 pts) Choose this phosphate species (in i) as the only phosphorus component. What amounts of this substance and any other reagents (grams of solid(s); mL of any solution(s) and water) would you need to make the buffer with pH = 7.4?

(iii)(4 pts) Choose this phosphate species (in i) and either its conjugate acid or base as the only two phosphorus components. What amounts of this substance, its conjugate acid or base, and water (grams of solid(s); mL of any solution(s) and water) would you need to make the buffer with pH = 7.4?

PLEASE HELP ANSWERING THE 3 PRELAB QUESTIONS FOR A BUFFER EXPERIMENT. Here is the back ground information:

1. Imagine you add some HCl to two solutions. One contains a pH buffer and the other is just plain water with no buffer. a. How would the addition of HCl affect the pH of these solutions? b. Would these solutionsâ pHs change to the same extent if the same amount of HCl were added? Why or why not?

2. If a solution contained formic acid (HCO2H) at a concentration of 0.414 M and its conjugate base, formate (HCO2 ï ), at a concentration of 0.303 M, what would the pH of the solution be? The pKa of formic acid is 3.77

3. You are asked to make a buffer to a pH of 3.00. Which acid (and its conjugate base) would be a good choice for this? Explain your choice.

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