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Solutions question

2005 · Shift 0 · Q5
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Solutions question

2005 · Shift 0 · Q5

JEE MainChemistrySolutionsMCQ+4 / −1
If α\alphaα is the degree of dissociation of Na2SO4Na_2SO_4Na2​SO4​, the vant Hoff’s factor (i) used for calculating the molecular mass is :
  1. A
    1 + α\alphaα
  2. B
    1 + 2α2\alpha2α
  3. C
    1 - α\alphaα
  4. D
    1 - 2α2\alpha2α
View written solutionFree

Correct answer: B

  1. Write the dissociation of the electrolyte

For sodium sulfate: Na2SO4→2Na++SO42−Na_2SO_4 \rightarrow 2Na^+ + SO_4^{2-}Na2​SO4​→2Na++SO42−​

So, on dissociation, 1 formula unit of Na2SO4Na_2SO_4Na2​SO4​ gives 3 ions in total.

  1. Use degree of dissociation α\alphaα

Assume initially there are 1 mole of Na2SO4Na_2SO_4Na2​SO4​.

  • Undissociated moles = 1−α1-\alpha1−α
  • Dissociated moles = α\alphaα

Each dissociated mole produces 3 particles, so total particles after dissociation are: (1−α)+3α=1+2α(1-\alpha) + 3\alpha = 1 + 2\alpha(1−α)+3α=1+2α

  1. Relation with van’t Hoff factor

The van’t Hoff factor is: i=actual number of particles after dissociationinitial number of formula unitsi = \frac{\text{actual number of particles after dissociation}}{\text{initial number of formula units}}i=initial number of formula unitsactual number of particles after dissociation​

Since initial number of formula units = 1, i=1+2αi = 1 + 2\alphai=1+2α

  1. Match with options

Thus the correct option is: 1+2α\boxed{1+2\alpha}1+2α​

So, Option B is correct.

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