Ch.4 - Chemical Quantities & Aqueous ReactionsWorksheetSee all chapters
All Chapters
Ch.1 - Intro to General Chemistry
Ch.2 - Atoms & Elements
Ch.3 - Chemical Reactions
BONUS: Lab Techniques and Procedures
BONUS: Mathematical Operations and Functions
Ch.4 - Chemical Quantities & Aqueous Reactions
Ch.5 - Gases
Ch.6 - Thermochemistry
Ch.7 - Quantum Mechanics
Ch.8 - Periodic Properties of the Elements
Ch.9 - Bonding & Molecular Structure
Ch.10 - Molecular Shapes & Valence Bond Theory
Ch.11 - Liquids, Solids & Intermolecular Forces
Ch.12 - Solutions
Ch.13 - Chemical Kinetics
Ch.14 - Chemical Equilibrium
Ch.15 - Acid and Base Equilibrium
Ch.16 - Aqueous Equilibrium
Ch. 17 - Chemical Thermodynamics
Ch.18 - Electrochemistry
Ch.19 - Nuclear Chemistry
Ch.20 - Organic Chemistry
Ch.22 - Chemistry of the Nonmetals
Ch.23 - Transition Metals and Coordination Compounds

Solution: The quantity of dissolved oxygen (DO) in natural waters is an essential parameter for monitoring survival of most aquatic life. DO is affected by temperature and the amount of organic waste. An earlie

Problem

The quantity of dissolved oxygen (DO) in natural waters is an essential parameter for monitoring survival of most aquatic life. DO is affected by temperature and the amount of organic waste. An earlier method for determining DO involved a two-step process:

(1) The water sample is treated with KI, O2(aq) + 4 KI (aq) + 2 H2SO4(aq) ⟶2 I2(aq) + 2 H2O(l) + 2 K2SO4(aq) 
(2) The I2 is titrated with sodium thiosulfate, I2(aq) + 2 Na2S2O3(aq) ⟶ Na2S4O6(aq) + 2 NaI(aq) 

A 50.0-mL water sample is treated with KI, and then 15.75 mL of 0.0105 M Na2S2O3 is required to reach the end point.

Which substance is oxidized in step 1, and which is reduced in step 2?