phosphoric acid is a triprotic acid with the following pKa Phosphoric acid is a

Question

phosphoric acid is a triprotic acid with the following pKa Phosphoric acid is a triprotic acid with the following pKa values: pK,' = 2.148 pK,,-7.198 pK,,-12.375 You wish to prepare 1.000 L of a 0.0500 M phosphate buffer at pH 7.720. To do this, you choose to mix the two salt forms involved in the second ionization, NaH2PO4 and Na2HPO4, in a 1.000 L volumetric flask and add water to the mark. What mass of each salt will you add to the mixture? Number Number Nah,Po,- | mass Na, HPO, 10 rn ass What other combination of phosphoric acid and/or its salts could be mixed to prepare this buffer? (Check all that apply) HPO4 and NaH2PO4 H3PO4 and Na2HPO4 H3PO4 and Na PO4 NaH2PO4 and NayPOa Na2HPO4 and NayPO4

Explanation / Answer

pH = 7.72

the best option will be pKa2 , since it is the nearest

this includes

NaH"PO4 and Na2HPO4

then

pH = pKa + log(Na2HPO4/NaH2PO4)

7.72 = 7.198 + log(Na2HPO4/NaH2PO4)

(Na2HPO4/NaH2PO4) = 10^(7.72 -7.198) = 3.3265

total molarity --> 0.05*1 =0.05

(Na2HPO4 + NaH2PO4) = 0.05

3.3265*NaH2PO4 + NaH2PO4 = 0.05

NaH2PO4 = (0.05)/(3.3265+1) = 0.01155

mass of NaH2PO4 = mol*MW = 0.01155*119.9770 = 1.3857 g of NaH2PO4  

Na2HPO4 = 3.3265*0.01155 = 0.03842

mass Na2HPO4 = mol*MW = 0.03842*141.9588 = 5.4540 g of Na2HPO4  

we require:

H3PO4 + Na2HPO4

H3PO4 + Na3PO4

Related Questions

Navigate

• Browse (All)
• Browse P
• Subjects

---

---

Integrity-first tutoring: explanations and feedback only — we do not complete graded work. Learn more.