What happens when HCl is added to a buffer solution?

Well, the STRONG acid PROTONATES the base present in the buffer … and of course it moderately increases the [math]pH[/math] of the solution with respect to the buffer…

We know that the [math]pH[/math] of a buffer is given by the old buffer equation…

[math]pH=pK_{a}+log_{10}(dfrac{ ext{[Conjugate Base]}}{ ext{[Conjugate Acid}]})[/math]

Given the increase in the concentration of the acid … the logarithmic term becomes MORE negative, and [math]pH[/math] reduces…. If the conjugate base is completely protonated, i.e. we exceed the capacity of the buffer, the [math]pH[/math] rises dramatically…

What is the [math]pH[/math] of the buffer when [math] ext{[Conjugate Base]} equiv ext{[Conjugate Acid]}[/math]?

HCl is a stronger acid, so NaHCO3 acts as a base.
What happens is an acid-base reaction:

HCl + NaHCO3 -> NaCl + H2CO3

But H2CO3 is highly unstable, so it splits into H2O and CO2. Basically, you would see bubbles.

The most important thing in chemistry is to UNDERSTAND the reaction: why HCl is giving H+ and not NaHCO3? During an acid-base reaction a salt is formed, where is it? Ask questions to yourself, try to answer them.

To control and maintain the pH. Unbuffered water will change pH rather dramatically in response to tiny amounts of acid or base, even from contact with carbon dioxide in air. Often, especially when dealing with enzymes or other biological materials, this is very undesirable.

This depends what the buffer solution is.

The definition of a buffer solution is clear :

A buffer solution will only undergo a small change in pH when a small amount of strong base or acid is added to the buffer solution

The definition does not read , in part …. will not undergo any chnge in pH ….

So you add a strong acid to the HF /NaF buffer and it is to be expected that the pH will drop slightly Adding 0.2 mol HCl to the buffer you describe can hardly be described as a small amount of strong acid - but as Nadide has calculated - the change in pH is quite small

Acetic acid is weak acid ,hence it ionizes feebly producing less number of H+ and acetate ions.But HCL is strong acid ,hence ionises completely producing large number of H+ions.

Hydrogen ion is common to both acetic acid and HCl. Due to this ,concentration of H+ ion in the solution increases.

According to Le-Chatelier’s principle ,acetate ions combine with H+ ions to form undissociated acetic acid.And hence equilibrium get shifted towards left hand side. Thus due to addition of HCl to acetic acid ,ionization of Acetic Acid get suppresed.

The values of [weak acid] and [conjugate base] are lowered, but by the same amount. Thus, the ratio of [conjugate base] to [weak acid] is not changed and the pH of the dilute buffer remains the same. However, with lower values of [weak acid] and [conjugate base], it is easier for the buffer capacity to be exceeded (i.e., addition of a smaller amount of strong acid could tie up all the conjugate base in the buffer and addition of a smaller amount of strong base could tie up all the weak acid in the buffer).

HCl is a strong acid which completely ionizes to give 0.05 M H⁺ ion.

[H⁺] [OH⁻] = Kw
Hence [OH⁻] = Kw / [H⁺] = (1 × 10⁻¹⁴) / 0.05 M = 2 × 10⁻¹³ M