You are instructed to create 800. mL of a 0.74 M phosphate buffer with a pH of 7

Question

You are instructed to create 800. mL of a 0.74 M phosphate buffer with a pH of 7.1. You have phosphoric acid and the sodium salts NaH2PO4, Na2HPO4, and Na3PO4 available

Na2HPO4 = base component buffer

NaH2PO4 = acid component 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?

How many grams of base are needed for the buffer?

Explanation / Answer

pH for buffer by Hendersen-Hasselbalck equation,

pH = pKa + log([HPO4^2-]/[H2PO4-])

7.1 = 7.2 + log([HPO4^2-]/[H2PO4-])

[HPO4^2-] = 0.794[H2PO4-]

[H2PO4-] + [HPO4^2-] = 0.74 M x 0.8 L = 0.592 mol

[H2PO4-] + 0.794[H2PO4-] = 0.592 mol

[H2PO4-] = 0.33 mol

[HPO4^2-] = 0.592 - 0.33 = 0.262 mol

Moles of acid component needed = 0.33 mol

Moles of Base component needed = 0.262 mol

Molarity of acid component needed = 0.33 mol/0.8 L = 0.4125 M

Molarity of Base component needed = 0.262 mol/0.8 L = 0.3275 M

Grams of acid component needed = 0.33 mol x 119.98 g/mol = 39.593 g

Grams of Base component needed = 0.262 mol x 141.96 g/mol = 37.193 g

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