Question about Volumetric Analysis

[subhradeep mahata]

Homework Statement

0.9546 g of a sample of Rochelle salt (NaKC4H4O6.4H2O), on ignition, gave NaKCO3, which was treated with 41.72 mL of 0.1307 N H2SO4. The unreacted H2SO4 was then neutralized by 1.91 mL of 0.1297 N NaOH. Find the percentage purity of the salt in the sample. Use equivalent concept.

Relevant Equations

meq of H2SO4 reacted = total meq - unreacted meq=meq of salt
unreacted meq of H2SO4 = meq of NaOH

Using the given formula, meq of H2SO4 reacted= meq of salt = 5.205
The math is correct and I have verified it.
Now, I am pretty sure that I am making a mistake in calculating the n- factor of the salt.
According to the question, C₄⁺⁶=4C⁺⁴
The n- factor is coming out to be 10, which is pretty weird (now, I might be wrong here, maybe there's nothing weird in it. Please also tell me about it's limits)
Now, on doing further calculations, we get the weight as 0.1467g, and the percentage purity is coming out to be 15.3%.
But the answer according to my book is 76.87%.
Can you tell me where I went wrong?

 

[BvU]

the math is correct and I have verified it

Good. So how many mole of Rochelle stuff have reacted ? How many grams is that ?

According to the question, C4⁺⁶=4C⁺⁴

Don't see it in the problem statement ...

0.1467g

check and verify that too !

 

[mjc123]

0.1467g is exactly 1/5 of the right answer. I suspect the problem is with your n-factor of 10. I don't know how you got that, and frankly I don't care, as I was never taught n-factors. But it does look as if you are calculating it for the redox reaction, on the assumption that all the carbons in the Rochelle salt end up as carbonate. This is not true. (Where would the extra Na and K come from?)
Can you write a balanced equation for the reaction? Assuming no K or Na is created or destroyed, it must be something like
NaKC₄H₄O₆.4H₂O → NaKCO₃ + stuff...
i.e. 1 mole of Rochelle salt gives 1 mole of carbonate.
You correctly calculated the meq of carbonate; can you now proceed to the right answer?