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Unformatted text preview: 41 Acids and the Environment Part II: Determining Molarity Introduction Chem-Connections As we saw in the last experiment, atmospheric carbon dioxide can dissolve in water to produce carbonic acid. Many plant and animal species have evolved to live in very specific chemical environments. For aquatic species this means they can only live within a narrow pH range. Coral reefs are a dramatic example of this pH sensitivity. As the concentration of CO 2 in the atmosphere increases, the oceans become more acidic which makes it harder for coral to produce its calcium carbonate exoskeleton. The additional stress of rising temperatures has lead to a massive decline in coral reef growth. Some scientists estimate that by the year 2050 95% of the living coral in the Great Barrier Reef will have been killed, by these relatively small changes in pH and temperature 1 . In this experiment you will be titrating a sample of acidic water to determine its concentration. Titrations allow chemists to use concepts like stoichiometry, molar mass and balanced chemical equations to determine the number of moles of reactants being used in a given reaction. These methods and techniques are the scientific basis for quantifying how acidic the oceans may get when CO 2 levels in the atmosphere are increased. Using a few relatively simple measurements, scientist can make predictions like the one above. New Science The concentration of acidic and basic solutions can be quantitatively determined using a titration. In the titration of acid with base, base is added in increments until the solution becomes neutralized. To visually see the end of the titration, an acid-base indicator is often used. In this experiment the indicator used will thymol blue which changes from yellow to blue as the pH increases from 8 to 10. The primary reaction taking place is the neutralization of the acid with the base. Once the acid is neutralized by the added base, a tiny amount of excess base will start titrating the indicator itself. The color change that results from titrating the indicator signals that the titration has reached the endpoint. When half the molecules of thymol blue indicator are in the basic form (blue) and half are in the acidic form (yellow) the solution is a green color. A blue solution results when too much base is added. The concentration of the acidic solution can be determined from the titration. The reaction between the strong base and the acid is: H 3 O + (aq) + OH (aq) 2H 2 O (l) 1 G. De'ath; J.M. Lough; K.E. Fabricius. Declining Coral Calcification on the Great Barrier Reef. Science , 2009 , 116-119. Experiment 4 42 As soon as OH- is added past the equivalence point the indicator starts to get titrated resulting in a color change. The thymol blue indicator is a diprotic acid that we can represent as HInd , where Ind denotes that the acid is an indicator....
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