Buffers: Solutions which are resistant to change in pH upon the addition of smal

Question

Buffers: Solutions which are resistant to change in pH upon the addition of small amount of strong acid or strong base. Buffer stabilises the pH of by providing sources to neutralize either H+ or OH-. E.g.: A mixture of weak acid and its conjugate base in appreciable amounts eg. CH3COOH / CH3COONa. Appreciable amounts typically means that the concentrations of an acid and its conjugate base are within an order of 10 times. Is a solution made by diluting 0.01mole of CH3COOH (Ka = 1.8 x 10-5) in 100 mL of H2O a buffer?

Explanation / Answer

Step 1)

Let’s first calculate concentration of acetic acid.

[CH3COOH] = moles / volume in L

Mol of acids = 0.01 mol , volume in L = 0.100 L

[CH3COOH]= 0.01 mol / 0.100 L = 0.1 M

Step 2)

Use ICE to determine concentration of acid and its conjugate base

   CH3COOH(aq) + H2O (l) ---- > CH3COO- (aq) + H3O+ (l)

I    0.1                                                     0                     0

C -x                                                       +x                    +x

E 0.1 – x                                                    x                      x

Ka = [CH3COO-][H3O+] / [CH3COOH]

1.8 x 10^-5 = x2 / ( 0.1 – x)

We can use 5 % approximation and neglect the value of x in the bracket.

1.8 x 10^-5 = x2 / 0.1

x ^2 = 1.8E-5 x 0.1

x = 0.001342

Step 3 )

Determine the concentration of conjugate base ( CH3COO- )

Since x is the equilibrium concentration of CH3COO-

[CH3COO-] = 0.001342 M

Step 4 ) Determine the equilibrium concentration of Acetic acid

[ CH3COOH] = 0.1 ( original) – x = 0.1000- 0.0013416 = 0.0987 M

[CH3COOH]/[CH3COO-] = 0.0987 / 0.001342 = 73.55

73.55 is much higher than the order 10 so its not the buffer solution.

Related Questions

Navigate

• Browse (All)
• Browse B
• Subjects

Integrity-first tutoring: explanations and feedback only — we do not complete graded work. Learn more.