Macalester CHEM 111 - Chemistry 111 Laboratory Experiment 8

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1Chemistry 111 Laboratory Experiment 8: Stoichiometry in Solution – Standardization of Sodium Hydroxide Opening Comments This is a skill-building experiment, in which you will have to work carefully and use equipment skillfully in order to achieve good results. Learning good titration technique will be helpful in subsequent chemistry labs, not only as a specific skill, but also as it illustrates the excellent results that can be achieved if sound technique is employed. Students should work individually on this experiment: that is, each of you will be carrying out your own titration. Standardization Many analytical procedures involve comparison of an unknown with a standard. A standard is usually a solution whose composition (concentration) is known very precisely (and accurately!). Unfortunately, NaOH is not a suitable primary standard. Solid NaOH is highly hygroscopic (it absorbs water from the air) and thus it cannot be accurately weighed. It also absorbs carbon dioxide from the air, forming sodium carbonate and thereby reducing the amount of sodium hydroxide present. This means that a NaOH solution of known molarity cannot be directly prepared without resorting to an air-and-water-free environment. We can only prepare a solution of NaOH that has the approximate concentration we will need. In cases like this we need to use an acid as the primary standard to determine the exact concentration of the NaOH solution. The primary standard we will use is potassium hydrogen phthalate (KC8H5O4, often abbreviated KHP). It is available as a pure, stable, crystalline solid that can be accurately weighed. A KHP sample of known mass (and, therefore, known moles) can be titrated with the NaOH solution to determine very precisely the concentration of the NaOH. This procedure is called standardizing the NaOH solution. Once this concentration is determined, the NaOH solution can serve as a standard for further work. Standardization of Base Using KHP The amount of the base (NaOH) in a solution can be determined by measuring very accurately the amount of solution required to react with a known amount of acid (KHP). One mole of KHP contains one mole of H+ that can react with one mole of sodium hydroxide, which contains one mole of OH-. The net ionic reaction for this weak acid-strong base reaction is provided on the next page. Notice that early in a titration of KHP with OH-, not much OH- has been added, so it is the limiting reagent and KHP is left over (the solution is acidic). At the equivalence point stoichiometric amounts of KHP and OH- have been mixed. The product KP- is a weak base (the solution is basic). Just past the equivalence point, KHP is limiting, and so OH- is left over (the solution is basic). The point at which equivalent amounts have been mixed is determined using an indicator that changes color as the solution changes from acidic to basic. Phenolphthalein is the most commonly used indicator (and the one we will use in this experiment), although many other indicators would also work for this titration.2COOHCOO- K++ OH-COO-COO- K++ H2OThe Reaction of KHP and Hydroxide Ion Accuracy and Precision of Measurements This week you will make two sorts of measurements. Using a very precise balance, you will accurately weigh out specific amounts of solid KHP. All weighings should be done to ± 0.0001 g, the limit of our analytical balances. Using a somewhat less precise buret, you will accurately measure the volume of your NaOH solution required to react with your KHP. The volume of liquid in a buret should be recorded to ± 0.01 mL, by estimating the value between the marks. The precision with which you measure is critical, and relates to the number of digits you record in your laboratory notebook. For example, when measuring volumes using a buret, you should record volumes with two places to the right of the decimal point, for example, 12.01 mL (12.0 mL would not be precise enough). In order for this to be valid, though, you must interpolate (estimate your position between the buret gradations) well enough that your estimate of the last number is off by ± 1 digit, at most. In other words, in this experiment, you need to be able to reliably differentiate between 12.04 and 12.01 mL. (12.04 ± .01 needs to be reproducibly distinguishable from 12.01 ± 0.01!) The accuracy with which you measure is also critical and has to do with how close the value you report is to the actual value. For example, if the amount really is 12.01 mL, but you have misread your buret and write down 12.21 mL instead, your very precise measurement is useless, because it is not accurate. When several determinations are made (as you will do here), the overall precision is related to how consistent the data are. For example, if you do three titrations and from them calculate the three molarities 0.1020 M, 0.1022 M, and 0.1018 M, the precision of your work is very high (approximately ± 0.2%). In this experiment you will quantify your precision so that it can be compared with other people's work. Don't worry if it takes several tries to get a good set of titrations - practice yields proficiency! Because the objective of our experiment is to make the concentration of our sodium hydroxide solution accurately and precisely known, great care must be taken not to contaminate it. Keep it tightly capped when not in use. Do not allow droplets of solution to evaporate around the mouth of the storage container, or carbon dioxide to enter and react with the solution.3Colorimetric Titration The method you will use to determine the molarity of your NaOH solution is called a colorimetric titration. The basic idea in colorimetric titration is to put one solution (in this case your NaOH solution) into a buret and add it slowly to another (in this case your KHP dissolved in water, containing an indicator dye) until a sudden color change occurs. The buret is then read and a calculation done to determine exactly how much solution was used. This point, when the indicator dye changes color, is called the end point. The color change you will observe is due to the indicating dye undergoing a sudden chemical reaction. In this experiment we will use an acid-base indicator called phenolphthalein. This substance has the interesting property of changing color as the hydrogen ion concentration changes. Phenolphthalein is colorless in acidic or neutral solutions and pink in basic ones


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