Since the color change is near but not exactly at the

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Unformatted text preview: n. Since the color change is near but not exactly at the equivalence point, the point at which the color change occurs is called the e ndpoint. Indicators are chosen so the endpoint is very close to the equivalence point. It is important to keep a titration well mixed, so the titrant and analyte can contact each other and react rapidly. Either manual swirling of the flask or mechanical stirring can be used. You will use manual swirling in this experiment. Remember to constantly swirl in order to ensure complete mixing of the solutions. The most common type of titration is the acid- base titration. In this experiment, you will determine the concentration of acetic acid, HC2H3O2 in commercial vinegar. In this titration, aqueous NaOH is the titrant, and vinegar is the analyte. We assume that the strong base and the weak acid react completely according to the net equation: (1) HC2H3O2(aq) + OH−(aq) → C2H3O2−(aq) + H2O(l) The balanced equation shows 1:1 stoichiometry, so we can write: (2) moles HC2H3O2 reacting = moles OH− added Or more generally: (3) moles of acid reacting = moles of base reacting Moles of base can be calculated from molarity times volume (molesb as e = Mb as e × Vb as e). As a reminder on concentration units: molarity is defined as the number of moles of solute in a liter of solution (M = mol/L). This is numerically equal to the number of millimoles of solute in a milliliter of solution (M = mmol/mL). It is often convenient to use this second definition of molarity in titrations and other work where small quantities are involved. There are 1000 mmol in 1 mol and 1000 mL in 1 liter. For example, 10.2 mL of 0.100 M NaOH solution contains 1.02 mmol of NaOH. (4) 10.2 mL solution × 0.100 mmol NaOH 1 mol solution www.webassig n.net/ebooks/wsug encheml1/lab_10/manual.html 2/5 4/15/13 Lab 10 - Titr ations = 1.02 mmol NaOH Molarity of the acid can be calculated from moles divided by volume (Macid = molesacid / Vacid ). In this experiment, a carefully measured volume of vinegar (Vacid ) is placed into a flask and the ma...
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