i need help on letter a -e. Please help me answer this question. Thank you This

Question

i need help on letter a -e. Please help me answer this question. Thank you

This question has multiple parts. Work all the parts to get the most points. You have the following reagents on hand Solids (pKa of Acid Form is Given) Solutions Benzoic acid (4.19) Sodium acetate (4.74) Potassium fluoride (3.14) Ammonium chloride (9.26) 5.0 M HCI 1.0 M acetic acid (4.74) 2.6 M NaOH 1.0 M HOCI (7.46) What combinations of reagents would you use to prepare buffers at the following pH values? Choose two benzoic acid sodium acetate potassium fluoride ammonium chloride 5.0 M HCl 1.0 Macetic acid 2.6 M NaOH 1.0 M HOCI

Explanation / Answer

pH of buffer can be calculated as follows

pH = pKa + log [[conjugate base]/[acid]

So the acid with pKa closer required pH should be in the combination to get the best combination of acid and its conjugate base.

a) pH = 3.0

So species with pKa closer to 3.0 must be used.

potassium fluoride(KF) has pKa of 3.14, but it is conjugate base of HF. So the given pKa value belongs to HF.

To get HF we have to add HCl to KF

KF(s) + HCl(aq) ---------> HF(aq) + KCl(aq)

So the best combination is

Potassium fluoride and 5.0 M HCl solution

b) pH = 4.0

So species with pKa closer to 4.0 must be used.

Benzoic acid has pKa of 4.19, but to get buffer conjugate base of acid should be present.

To get conjugate base of acid we have to add NaOH to C6H5COOH.

C6H5COOH(s) + NaOH(aq) ---------> C6H5COONa(aq) +H2O(l)

So the best combination is

Benzoic acid and 2.6 M NaOH solution

c) pH = 5.0

So species with pKa closer to 5.0 must be used.

Sodium acetate (CH3COOH) has pKa of 4.74, but it is conjugate base of CH3COOH. So the given pKa value belongs to CH3COOH.

To get the CH3COOH form CH3COONa we have to add HCl to KF

CH3COONa(s) + HCl(aq) ---------> CH3COOH(aq) + NaCl(aq)

So the best combination is

Sodium acetate and 5.0 M HCl solution

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