---
title: "A student investigates the reaction between persulfate ions and iodide ions at constant temperature.  \\[ \\text{S}_2\\text{O}_8^{2-}\\text{(aq)}+2\\text{I}^-\\text{(aq)}\\rightarrow \\text{I}_2\\text{(aq)}+2\\text{SO}_4^{2-}\\text{(aq)} \\]  The experimentally determined rate law is  \\[ \\text{rate}=k[\\text{S}_2\\text{O}_8^{2-}][\\text{I}^-] \\]  At \\(t=0\\text{ min}\\), the concentrations are \\([\\text{S}_2\\text{O}_8^{2-}]=1.00\\times10^{-2}\\text{ M}\\) and \\([\\text{I}^-]=1.00\\text{ M}\\). Even if all the persulfate reacts, \\([\\text{I}^-]\\) decreases by no more than \\(2.0\\%\\). The student observes that \\([\\text{S}_2\\text{O}_8^{2-}]\\) decreases to \\(5.00\\times10^{-3}\\text{ M}\\) after \\(20.0\\text{ min}\\). Which prediction for \\([\\text{S}_2\\text{O}_8^{2-}]\\) at \\(t=60.0\\text{ min}\\) is most nearly correct, and why?"
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date_modified: "2026-08-21T08:40:55+00:00"
---

# A student investigates the reaction between persulfate ions and iodide ions at constant temperature.

\[
\text{S}_2\text{O}_8^{2-}\text{(aq)}+2\text{I}^-\text{(aq)}\rightarrow \text{I}_2\text{(aq)}+2\text{SO}_4^{2-}\text{(aq)}
\]

The experimentally determined rate law is

\[
\text{rate}=k[\text{S}_2\text{O}_8^{2-}][\text{I}^-]
\]

At \(t=0\text{ min}\), the concentrations are \([\text{S}_2\text{O}_8^{2-}]=1.00\times10^{-2}\text{ M}\) and \([\text{I}^-]=1.00\text{ M}\). Even if all the persulfate reacts, \([\text{I}^-]\) decreases by no more than \(2.0\%\). The student observes that \([\text{S}_2\text{O}_8^{2-}]\) decreases to \(5.00\times10^{-3}\text{ M}\) after \(20.0\text{ min}\). Which prediction for \([\text{S}_2\text{O}_8^{2-}]\) at \(t=60.0\text{ min}\) is most nearly correct, and why?

A student investigates the reaction between persulfate ions and iodide ions at constant temperature.

\[
\text{S}_2\text{O}_8^{2-}\text{(aq)}+2\text{I}^-\text{(aq)}\rightarrow \text{I}_2\text{(aq)}+2\text{SO}_4^{2-}\text{(aq)}
\]

The experimentally determined rate law is

\[
\text{rate}=k[\text{S}_2\text{O}_8^{2-}][\text{I}^-]
\]

At \(t=0\text{ min}\), the concentrations are \([\text{S}_2\text{O}_8^{2-}]=1.00\times10^{-2}\text{ M}\) and \([\text{I}^-]=1.00\text{ M}\). Even if all the persulfate reacts, \([\text{I}^-]\) decreases by no more than \(2.0\%\). The student observes that \([\text{S}_2\text{O}_8^{2-}]\) decreases to \(5.00\times10^{-3}\text{ M}\) after \(20.0\text{ min}\). Which prediction for \([\text{S}_2\text{O}_8^{2-}]\) at \(t=60.0\text{ min}\) is most nearly correct, and why?

- **A.** \(0\text{ M}\), because the persulfate is consumed by a constant amount during each \(20.0\text{ min}\) interval.
- **B.** \(6.25\times10^{-4}\text{ M}\), because \(60.0\text{ min}\) corresponds to four successive half-lives.
- **C.** \(1.25\times10^{-3}\text{ M}\), because the nearly constant \([\text{I}^-]\) makes the persulfate undergo three approximately equal half-lives.
- **D.** \(2.50\times10^{-3}\text{ M}\), because each successive half-life doubles as \([\text{S}_2\text{O}_8^{2-}]\) decreases.

*The answer key and step-by-step explanation are available to logged-in users at https://nerd-notes.com/ubq/120045/*