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0.10 M Phospate buffer at pH 7.0 total 100.0 ml. 30mL of 0.20M NaH2PO4, 20 mL...

0.10 M Phospate buffer at pH 7.0 total 100.0 ml. 30mL of 0.20M NaH2PO4, 20 mL of 0.20M Na2HPO4 and 50.0 mL of water were used to prepare the buffer.

The average pKa is 6.76

Using the average pKa value calculated in 3, calculate the pH of the pH 7.0 buffer solution that is expected after the addition of 1.0 mL of 1.0M HCl.

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Answer #1

The added moles of HCl and those of the initial buffer components are calculated:

n HCl = M * V = 1 M * 0.001 L = 0.001 mol

n H2PO4- = 0.2 M * 0.03 L = 0.006 mol

n HPO4-2 = 0.2 M * 0.02 L = 0.004 mol

HCl reacts with HPO4-2 (decreasing it) and forms H2PO4- (increasing it), the new pH is calculated:

pH = pKa + log (n HPO4-2 / ​​n H2PO4-) = 6.76 + log (0.004 - 0.001 / 0.006 + 0.001) = 6.39

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