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The question involves the addition of $\pu{0.15 g}$ of an already prepared sample of sodium peroxoborate which is added to $\pu{100 mL}$ of $\pu{1 M}$ $\ce{H2SO4}$ and $\pu{2 g}$ of $\ce{KI}$. The solution is left to stand for 10 minutes and is then titrated against standardized sodium thiosulfate. Starch indicator was used and a titration figure of $\pu{17.1 mL}$ was obtained. Find the moles of sodium peroxoborate.

Sodium peroxoborate, sulfuric acid and potassium iodide react as follows:

$$\ce{Na2[B2(OH)4(O2)2] + 4 KI + 3 H2SO4 -> Na2SO4 + 2 K2SO4 + B2O3 + 5 H2O + 2 I2}$$ Iodide reacts with thiosulfate as follows :

$$\ce{I2 + 2 [S2O3]^2- -> 2 I- + [S4O6]^2-}$$

I'm not sure how to use the titration value to find the result of this answer. I'm not looking for the solution I just need a point in the direction that will help me at least get a start on the problem.

What about this:

$$2 \, \text{mol} \, \ce{S2O3 ->} 1 \, \text{mol} \, \ce{I}$$

when we made the sodium thiosulfate I calculated the moles to be $0.096 \, \text{mol/L}$. Hence

$$0.096 \, \text{mol/L} \, \, \ce{S2O3->} 0.048 \, \text{mol/L} \, \, \ce{I}$$.

the relationship between iodine and peroxoborate is as follows:

$$4 \, \text{mol} \, \ce{I->} 1 \, \text{mol} \, \ce{[B_2(O2)2(OH)4]}$$

Hence

$$0.048 \, \text{mol/L} \, \, \ce{I->} 0.012 \, \text{mol/L} \, \, \ce{[B_2(O2)2(OH)4]}$$.

But I didn't use the titration value of thiosulfate in this solution?

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  • $\begingroup$ Well, you can use the amount of thiosulfate to compute the amount of iodine. You can use the amount of iodine to compute the amount of peroxyborate. There are stoichiometric relationships between all of those. $\endgroup$ – Zhe Oct 3 '17 at 20:16
  • $\begingroup$ Can I say $\text{moles} = \frac{2}{166}=0.012 \, $ mol $\ce{KI}$ $\endgroup$ – Patrick Moloney Oct 3 '17 at 20:55
  • $\begingroup$ Because I'm struggling to see why I can't just say $\frac{0.15}{307.62}= 4.88 \times 10^{-4}$ mol where $307.62$ is the $M_r$ of peroxoborate. $\endgroup$ – Patrick Moloney Oct 3 '17 at 21:00
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    $\begingroup$ How many moles of thiosulfate react with 1 mole of iodine? How many moles of peroxoborate react with 1 mole of iodine? You can't solve these types of problems without understanding the stoichiometric relationships. Nowhere in your solution do these ratios come into play... $\endgroup$ – Zhe Oct 3 '17 at 21:11
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    $\begingroup$ Amounts correlate directly, not concentrations. You are missing the core point if stoichiometry, which is that the balanced equation provided ratios between amount of substance, e.g., number of moles. That ratio does not apply to mass or concentration. $\endgroup$ – Zhe Oct 4 '17 at 21:41

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