Scenes A to D represent tiny portions of 0.10 M aqueous solutions of a weak acid

Question

Scenes A to D represent tiny portions of 0.10 M aqueous solutions of a weak acid HA (red and blue; Ka = 5.5x10^-5), its conjugate base A- (red), or a mixture of the two (only these species are shown):

a) which scene(s) show(s) a buffer?

(it could be one or more than one)

b) What is the pH of each solution

Scene A=

Scene B=

Scene C=

Scene D=

c) Arrange the scenes (by letter) in sequence, assuming that they represent stages in a weak acid-strong base titration.

Which one is 1st, 2nd, 3rd, and 4th?

d)Which scene represents the titration at its equivalence point?

A, B, C, or D?

Explanation / Answer

a)

A buffer is a combination of conjugate base + acid, therefore all options with a conjugate + acid could act as a buffer. This example are A and D

b)

Apply pH equation

pka = -log( 5.5x10^-5) = 4.26

a)

4:4 ratio

pH = pKa+log(A-/HA) = 4.26 + log(4/4) = 4.26

b)

8:0 ratio

pH = 7 (since ther eis no reaction with water)

c)

pH = 7, no free H+ ions

d)

5:3 ratio so

pH = pKa+log(A-/HA) = 4.26 + log(5/3) = 4.48

c) Arrange the scenes (by letter) in sequence, assuming that they represent stages in a weak acid-strong base titration.

C --> A --> D --> B

d)Which scene represents the titration at its equivalence point?

Equivalence poitn means moles of acid = moles of base

therefore

no H+ should be present, the only case this is true is B

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