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ChemLab113-0029 - The sharp pH decrease that marks the...

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Unformatted text preview: The sharp pH decrease that marks the equivalence point results fiom a marked contrast in the strengths of two acids, namely H2C03 and HCl. HCl produces H30+ ions in water by the same sort of dissociation reaction we have seen for H2C03. HCl (aq) + H20 (1) => H30+ (aq) + Cl' (aq) However, HCl is an example of a strong acid, which dissociates completely in water, while H2C03 is a weak acid, which dissociates to a much smaller extent. All HCl molecules dissociate in an aqueous solution, while most H2C03 molecules remain undissociated, Therefore, a strong acid is much more effective than a weak acid in producing H3O+ ions. Before the equivalence point is reached in your titration the strong acid reacts with the carbonate to produce the weak acid H2C03, which decreases the pH, but not sharply. Beyond the equivalence point the strong acid no longer reacts with carbonate, and the pH decreases sharply. The sharpest pH change occurs at the equivalence point, where the transition occurs between pH values largely determined by H2C03 and pH values largely determined by HCl. Directions for Preparing the Acid Solution Take the 1 liter brown bottle and its cap out of your lab deer. Check to make sure that the cap is clean. Remove the label from the bottle, wash it with tap water, and dry it on the outside. Put a new label on it, reading your name and 0.1 M HCl. Take the bottle to one of the distilled water taps and half-fill it with distilled water. Obtain approximately 20 mL of 6 M HCl from the reagent bottle on your bench top in a 100 mL beaker. (There should be a mark indicating 20 1111. on the beaker). Use a graduated cylinder to measure out 17 mL of 6 M HCl solution fi'om the 100 mL beaker, and pour it into the brown bottle. Rinse the graduated cylinder several times with distilled water, and pour the rinses into the brown bottle. Take the brown bottle to a distilled water tap and fill it to the bottom of the large curved portion at the top of the bottle. Cap the bottle and shake it well for three minutes. (CAUTION: The 6 M HCl is highly corrosive; wash your hands well after handling it.) Directions for Preparing a Buret for Titration Begin the preparation of your buret by checking it as in Experiment 1 and then emptying it if it is found to be clean, unclogged and to have no leaks or air bubbles. The preparation is completed by rinsing the buret three times with your 0.1 M HCl solution, then filling the buret with that solution, and adjusting the liquid level to slightly below zero. (Please note that you do not fill the buret for each rinsing step.) Pour the acid solution into the buret using a small beaker. If the rinsing isn't done, the solution will become diluted by mixing with water that remains on the inside surface of the buret. Effective rinsing can be done with relativelysmall volumes of solution, and your instructor will demonstrate how that is done. It is important that you not waste solution, or you will not have enough to carry out all the titrations in this block of experiments. 41 ...
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