Question

Provide a detailed description of how you would prepare one liter of a pH 6.50 buffered solution with a total phosphate concentration of 0.50 M ( [Phosphate]T = formal concentration = 0.50 M) using potassium phosphate monobasic anhydrous and sodium phosphate dibasic heptahydrate.

Answer #1

Here we need to prepare 1L of pH 6.5 buffered solution of
phosphate concentration 0.5 M using potassium phosphate
KH_{2}PO_{4} and sodium phosphate
Na_{2}HPO_{4}

using henderson Hasselbalch equation pH = pKa + log (A/HA) ----(1)

So pKa of phosphoric acid are 2.3, 7.21 and 12.67.

If pH required is 6.5 then we will use pKa as 7.21.

Now [A] will be
Na_{2}HPO_{4} concentration & [HA] will be
KH_{2}PO_{4} concentration _{.}

Therefore equation (!) becomes 6.5= 7.21 + log [A] / [HA]

log [A] / [HA]= - 0.71

[A] / [HA]= 0.1949

Given molarity of the solution is 0.5

To find out required quantity:

HA+ A=0.5

A/HA = 0.1949

By using the above , We get HA=0.4194

A=0.0806

We need to put 0.4194moles of KH_{2}PO_{4} and
0.0806 moles of Na_{2}HPO_{4
}salts in to 1 litre of the solution.

For molecular weight of KH_{2}PO_{4} =136g/mol
&

molecular weight of Na_{2}HPO_{4}
=268.07g/mol.

Thus 0.4194 x 136 = 57.03 g of KH_{2}PO_{4} and
0.0806 x 268.07 = 21.60g of Na_{2}HPO_{4 .}

To prepare 1 L of pH 6.5 buffer solution 57.03g of
KH_{2}PO_{4} and 21.60g of
Na_{2}HPO_{4} should be added.

Prepare 250.00 mL of an phosphate buffer (pKa=7.199) with a
total phosphate ion concentration of 0.020000 M and a pH of
7.100.
Calculate the amount of the Monobasic Potassium Phosphate
(KH2PO4 FW=136.086) required to prepare this solution. Report your
answer in grams.
____________ grams
Calculate the amount of Dibasic Sodium Phosphate
(Na2HPO4 FW=141.959) required to prepare this
solution. Report your answer in grams.
____________ grams

Consider how best to prepare one liter of a buffer solution with
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Ka
pKa
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6.4 x 10-5
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HPO42-
6.2 x 10-8
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CO32-
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6.4 x 10-5
4.19
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Ka
pKa
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6.4 x 10-5
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H2PO4-
HPO42-
6.2 x 10-8
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Weak Acid
Conjugate Base
Ka
pKa
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6.4 x 10-5
4.19
H2PO4-
HPO42-
6.2 x 10-8
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pKa
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C2O42-
6.4 x 10-5
4.19
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HPO42-
6.2 x 10-8
7.21
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CO32-
4.8 x 10-11
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