You are instructed to create 900. mL of a 0.29 M phosphate buffer with a pH of 7.8. You have phosphoric acid and the sodium salts NaH2PO4, Na2HPO4, and Na3PO4 available. (Enter all numerical answers to three significant figures.)

H3PO4(s) + H2O(l) <----> H3O+(aq) + H2PO4−(aq) Ka1 = 6.9 ✕ 10−3
H2PO4−(aq) + H2O(l) <----> H3O+(aq) + HPO42−(aq) Ka2 = 6.2 ✕ 10−8
HPO42−(aq) + H2O(l) <----> H3O+(aq) + PO43−(aq) Ka3 = 4.8 ✕ 10−13

Which of the available chemicals will you use for the acid component of your buffer?

A. H3PO4
B. NaH2PO4
C. Na2HPO4
D. Na3PO4

Which of the available chemicals will you use for the base component of your buffer?

A. H3PO4B. NaH2PO4C. Na2HPO4D. Na3PO4

When we have to do a buffer solution we always have to choose the reaction that has the pKa closer to the desired pH value. When we find the pKa values we will obtain:

[tex]pKa_1=-Log[6.9x10^-^3]=2.16[/tex]

[tex]pKa_2=-Log[6.2x10^-^8]=7.20[/tex]

[tex]pKa_3=-Log[4.8x10^-^13]=12.31[/tex]

The closer value is pKa2 with a value of 7.2. Therefore we have to use the second reaction. In which [tex]H_2PO_4^-^1[/tex] is the acid and [tex]HPO_4^-^2[/tex] is the base. Therefore the answer for the first question is B and the answer for the second question is C.