In this lab, you prepared a buffer and then determined its pH, but in a laborato

Question

In this lab, you prepared a buffer and then determined its pH, but in a laboratory setting it is usually the other way around – you need a buffer at a particular pH, and this dictates its preparation. Describe how you would prepare 100. mL of 0.1000 phosphate buffer at pH = 6.5, given appropriate glassware, sodium phosphate, monobasic (NaH2PO4, pKa = 7.20) and sodium phosphate, dibasic (Na2HPO4, pKa = 12.35) at 25°C. Your answer should include calculations. (Note: The moles of each component must add to the buffer moles. i.e. A- + HA = 0.250 mol)

Explanation / Answer

To prepare this buffer we use use NaH2PO4/Na2HPO4 as acid/base component

pH = 6.50

pKa = 7.20

Using hendersen-hasselbalck equation,

pH = pKa + log(Na2HPO4/NaH2PO4)

6.50 = 7.20 + log(Na2HPO4/NaH2PO4)

(Na2HPO4) = 0.2(NaH2PO4)

we know,

NaH2PO4 + Na2HPO4 = 0.1 M x 0.1 L = 0.01 mol

NaH2PO4 + 0.2(NaH2PO4) = 0.01 mol

NaH2PO4 = 0.01/1.2 = 0.0083 mol

mass NaH2PO4 = 0.0083 mol x 119.98 g/mol = 0.996 g

Na2HPO4 = 0.01 - 0.0083 = 0.0017 mol

mass Na2HPO4 = 0.0017 mol x 141.96 g/mol = 0.241 g

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