Question

What mass of each of the two salts are required in order to prepare 1.00 L...

What mass of each of the two salts are

required in order to prepare 1.00 L of an

0.100 M phosphate buffer solution of pH

7.4 using NaH2PO4 and Na2HPO4?

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Homework Answers

Answer #1

Mass of NaH2PO4 = 4.6420g

Mass of Na2HPO4 = 8.7035g

Explanation

Henderson equation is

pH = pKa + log([ A-]/[HA])

7.4 = 7.20 + log ( [A-]/[HA])

log([A-]/[HA]) = 0.20

[A-]/[HA] = 1.584

[A-] = 1.584[HA]

Buffer concentration is 0.100M

therefore'

1.584[HA] + [HA] = 0.100M

2.584 [HA] = 0.100M

[HA] = 0.03869M

[A-] = 0.100M - 0.03869M = 0.06131M

Volume of buffer = 1L

mole of NaH2PO4 required = 0.03869

mole of NaHPO4 = 0.06131M

Mass of NaH2PO4 required = 0.03869 mol* 119.98g/mol =4.642g

Mass of Na2HPO4 required = 0.06131mol * 141.96g/mol=8.7035g

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