You are instructed to create 500. ml_ of a 0.47 M phosphate buffer with a pH of

Question

You are instructed to create 500. ml_ of a 0.47 M phosphate buffer with a pH of 7.6. You have phosphoric acid and the sodium salts NaH_2PO_4, Na_2HPO_4, and Na_3PO_4 available. (Enter all numerical answers to three significant figures.) H_3PO_4(s) + H_2O(/) H_3O+(aq) + H_3PO_4^-(aq) K_a1 = 6.9times10^-3 H_2PO_4^-(aq) + H_2O(/) H_3O+(aq) + HPO_4^2-(aQ) K_a2 = 6.2times10^-8 HPO_4^2_(aq) + H_2O(/) H_3O^+(aq) + PO_4^3-(aq) K_a3 = 4.8times10^-13 Which of the available chemicals will you use for the acid component of your buffer? Which of the available chemicals will you use for the base component of your buffer? What is the molarity needed for the acid component of the buffer? What is the molarity needed for the base component of the buffer? How many moles of acid are needed for the buffer? How many moles of base are needed for the buffer? How many grams of acid are needed for the buffer?

Explanation / Answer

we know that

for buffers

pH = pKa + log [base / acid ]

so

pH = -log Ka + log [ Na2HP04 / NaH2P04]

so

7.6 = -log 6.2 x 10-8 + log [ Na2HP04 / NaH2P04]

[ Na2HP04 / NaH2P04] = 2.468

so

[Na2HP04] = 2.468 [NaH2P04]

given

[ Na2HP04] + [NaH2P04] = 0.47

so

2.468 [NaH2P04] + [NaH2P04] = 0.47

so

[NaH2P04] = 0.1355

so

[Na2HP04] = 0.47 - 0.1355 = 0.3345

now

moles = molarity x volume (L)

so

moles of NaH2P04 = 0.1355 x 0.5 = 0.06775

moles of Na2HP04 = 0.3345 x 0.5 = 0.16725

now

mass = moles x molar mass

so

mass of NaH2P04 = 0.06775 x 120 = 8.13 g

so

molarity of acid component ----> 0.1355

molarity of basic component ---> 0.3345

moles of acid component ---> 0.06775

moles of base component --> 0.16725

mass of acid ---> 8.13 g

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