Question

I know how to do the question, but at the very end, how do you know if it's a buffer or not? What is the formula or rule to tell if something is a buffer or not?

UBC Chem 123 Acid-Base Examples 20: Calculate the pH if 10.0 mL of 0.020 M benzoic acid is added to 40.0 mL of 0.004 M sodium benzoate. The pka of benzoic acid is 4.20. H₂O ² HA + benzoic acil - H₂ot + Á benzoate anion pair conj. acid base Initial moles of benzoic acid, "HA" CHA VA = (0.020M)(0.010) = 2.Oxo-4 moles of benzoate anion, "A" -- VAP = (0.004M)(0.040) = 1.6 XDmoles Initial moles ate anima, "90" moles Assuming there won't be much dissociation of the weak acid or protonation of the conjugate base, we can use the Henderson-Hasselbalch equation here. (There won't be much change because benzoic acid is a weak acid and we are mixing it with its conjugate base, so this assumption is ok) n benzoate anion pH = pkat SA 4.20 must be a conj. acid base for benzoic acid benzoic acid pair! ALWAYS pH 4.20+ log / 1.6x10-4 moles/or I when using H-H. 12.0x10"moles / UTOT pH = 4.20 + log(0.8) : 4.20 +(-0.0969) IPH = 4.10) Is this a buffer? CAN use moles in H-H (but NEVER in Ka can) VTOT (total volume) terms Cancel you YES

Answer 1

In a Buffer solution

1. There should be a weak acid (like benzoic acid or acetic acid) and its conjugate base (i.e. benzoate ion or acetate ion)

2. Or, there should be a weak base (like NH3) and its conjugate acid (i.e. NH4+).

3. Buffer of weak acid (NH4+) and weak base (CH3COO-) is also possible.

3. All weak acid and base used in buffer solution are stronger than water.

4. A buffer solution can resist the change of pH when a small quantity of acid or base is added.

5. Any kind of strong acid (like HNO3)or base (KOH, NaOH) can not be used in buffer solution.

6. In many cases though a strong acid or base added but after some reaction with other moieties in the system it produces some conjugate acid or base of those weak base or acid present in the system. Thus a buffer solution formed. [It is explained in the attached document]

These are some points which can help you to know any given solution is a buffer solution ir not.

for an example are let a strong acid Hel (20 ml 0·15 (M) ) added to a solution of NH3 (25 ml 0.26 (M)) then; He should react with Ntb. Hi + NH3 - NHCl After 100% reaction NHA el produced - 200 x 0.15 moles. - 3x10-3 moles. NH₃ left in system = 25 8 x 0.20 - (3x103) = (5 - 3) x 10-3 moles - 2x10-3 mole So, now system has Nth (a weak base) and Nhace which contains what (conjugate acid). It is a buffer solution llon 100 serris Now