acid in grams that will provide 1 mol of protons () in a reaction. A gram- By definition, one gram-equivalent weight of an acid is that mass of the accurately measure with a burct the volume of your standard base that is re quired to exactly neutralize the acid present in the unknown. The technique of tions it is pink. in slightly more acidic solutions it is colorless, whereas, in more alkaline solu concentration of a sodium hydroxide solution that you have prepared. The pro point of the titration. Indicators change colors at different p values. Phenol an acid with a base. This reaction is termed neutralization, and the essential reagent is termed rirra.rion. An indicator solution is used to determine when an acid has exactly neu tralized a base, or vice versa. A suitable indicator changes colors when equivalent amounts of acid and base are present. The color change is termed the end cess of determining the concentration of a solution is called so2rzdardiza.rion. accurately measuring the volume of a solution required to react with another Next you will measure the amount of acid in an unknown. To do this, you will In this experimcnt you will use this reaction to determine accurately the 54(aq) + (aq) 21-12(1) ions with hydroxide ions to form water: solution; potassium acid phthalate (KP), primaty standard. Background Discussion: 2S-ml wide-mouth Erlenmeyer flasks, 4; approx.. in NaO; phenolphthalein 5-mL buret; buret clamp; ring stand; balance; wash bottle; weighing bottle; Materials/Chemicais: standardizing a Solution of sodium hydroxide. One of the most common and familiar reactions in chemistry is the reaction of feature of this process in aqueous solution is the combination of hydroniurn phthalein, for example, changes color from colorless to pink at a p of about 9; Objective: To become famili with the techniques of titration, a volumetric method of analysis, by Experiment: Standarditjo of NaO APChernistry
equivalent weight of a base is defined as that mass of the base in grams that will provide I rnol of hydroxide ions in a reaction or that will react with I mol of protons. A gram-equivalent weight is often referred to as an equivalent (equiv). In acid-base reactions, 1 equiv of an acid will react with I equiv of a base. Thus 1 mol of CI, which is 36.5 g, is I equivalent weight of CI. ow ever, 1 rnol of2s reacts with 2 mol of NaO, and 2 rnol of protons are transferred from the acid to the base: 4 2S 4(aq) + 2NaO(aq) Na 2SO4(aq) + 21(l) Therefore, 1 mol of2s (98. g) corresponds to 2 gram-equivalent weights. In other words, 1 gram-equivalent weight of2s or 49. g, is equal to mol of the acid (2 equiv 2 SO4 /1 mo! 2SO4). For volumetric work, normality is the most convenient method of ex pressing concentrations. Normality. N, is defined as the number of equivalents of solute in a liter of solution (which is also the number of milliequivalents of solute in a milliliter of solution). A useful mathematical relationship for nor mality is 4 4, = equivalents of solute = equiv = m liter of solution VL eqw X where m is the mass of the solute in grams, eqw is the equivalent wei (grams per equivalent), and V is the volume of the solution in liters. Because equivalent weight and molecular weight are related, normality and mnolarity are related. In general, Equivalent weight molecular weight = a where a is the number of moles of acidic hydrogen () per mole of acid that react, or the moles of per mole of base that react. Thus the normality of a solution and its rnolarity are related: N axm Normality is always equal to or greater than mnolarity. It should be obvious that a 1 M CI solution is also 1 N, because 1 gramequivalent weight of CI is exactly 1 mol of CI. owever, a I M2S solu tion is 2 N because I mol of2s is equal to 2 equiv of 2 SO4. 4 4 EXAMPLE 19.1 What is the normality of a solution that contains 2.45 g of 2 SO4 in.25 L of so lution? Solution: 2.45g =.2N (49 g/equiv)(.25o L)
- IV In this experiment your solution of NaO* will be titrating it against a very pure sample potassium hydrogen of known weight. Potassium viated as KP) has only one replaceable hydrogen. Its is shown :.below. is a monoprotic acid with the hydrogen bonded to oxygen and has a molecular weight of 24.2; hence I 4O4, KC It standardized by phthalate, of hydrogen phthalate (hereafter abbre acid acidic structure equiv of KP weighs 24.2 g. 1; C 1 1 C C C O C C C C 7 K KP base :alence In of a base against and acid are present at the equivalence point the tittion KP, an equal In point. equivalents words, at the number of other of equiv Equivalents NaO = equivalents KP [2] :Equation èquation [2) can be expressed in terms as follows: of Equation [1] by rearranging the X V fl eqw number of equivalents so that in of KP eqwofkhp [31 Lf one measures the volume (in of base to neutralize a weight (in grams) of it is possible to calculate the normality the base because the equivalent weight is k.nowri.of KP, liters) of KP required known. (N)
3.5 number sum known weight of an impure sample. - can be used to determine the amount of KP or any other acid present ifl a Once the normality of the NaO solution is accurately known, the base g3,çl)(2o4.2 g/equiv) N = (l.7g) Solution: normality of an NaO sotutioti if 35.75 rnl of it rcqued to neui,2ample 19.2 The scatter about the mean or median that is, the deviations from the ons, the more precise the measurements. mean or medianare measures of precision. Thus the smaller the devia 3.+4. are 1., 3., 4., and 5., then example, if an experiment s results are 1, 3, and 5, then m 3. If results average of the two middle results for an even number of results (m). For Median: the midpoint of the results for art odd number of results and the 1+3+5 3 For example, if an experiment s results are 1, 3, and 5, then of results of results Mean: arithmetic mean or average (j4 where which can be treated by statistics. Indeterminate errors: random errors, which are incapable of discovery but and eliminated. Error: difference between the true result and the determined result. reproducibility. Accuracy: correctness of a measurement, closeness to the true result. Precision: internal consistency among one s own results that is, are necessa.ry. Determinate errors: errors in method of performance that can be discovered culate the standard deviation. Before we illustrate how to do this, however, we will define some of the terms above as well as some additional ones that As a means of estimating the precision of your results, it is desirable to cal Standard Deviation -.J466NNaQ (24.2 giequiv) N a3l (L7g
ElS (U C (U rn x Z 4 >< Cb fd ID >< cu I(D(fl D En q og ID <ID Z El (U El El II ID (1) It) l El -. El < + < c. El. El fl (D- D- ui L) El o < [J3 I J Ci El I IC J1 L 1 I D I j+ I + CU tn ), b I El C Ci b! El Ui _ jp) El U1f!, bb + o + III o IL II IIIIIID II 5)WC J1 Uibj II PJ j rj IP El -. CU 1 to - II t)) S F ) Cii < to (fl ç ITt ZI 5 Er
The results of this experiment would be reported as 3 ± 2. The relative standard =.7, or 7% 2 deviaon is = 2 EXAMPLE 8.4 5 31=2 number of observations 11 31=2 13 31= x j = deviation for the data. The deviations from the mean are 3 1+3+5 from the mean, the standard deviation, and the relative standard deviation SOLUTION: The mean is as follows: =3 An experiment s results are 1, 3, and 5. Calculate the mean, the deviations to sun-i over the members. mean, and N = number of members in the set of data. The symbol Z. means using the formula sum of the squares of the deviations from the mean where s = standard deviation from the mean, x = members of the set, y = sure precision. the data in Example 8.2 and hence are more precise, since the deviationsare smaller. Thus, the average deviation and relative average deviation mea Standard deviation (s) is a better measure of pre.sion and is calculated Obviously, the data in Example 8.3 are internally more consistent than 1.
1. Define standardization and state how you would go about doing it. Before beginning the experiment in the laboratory, you should answer the following mean, and the relative deviation from the average deviation from the men th â clevation from the ) 6 find the mean, the If an experiment s results are 1.1 1 4 molarity and normality of the NaO solution? 11. If 5. ml of NaO solution is required to react completely with 1.24 g K, what are the solution? 1. What is the normality of a solution that contains 1.89 g of2c22 22 in 1 ml of 9. Why is it necessary to rid the distilled water of carbon dioxide in this experiment? 8. What is parallax and why should you avoid it? 7. What are equivalence points and end points and how do they differ? 6. Define rnolarity and normality. ow do you distinguish between them? 5. Why do you weigh by difference? 4. Why should your NaO solution NOT be stored in a glass stoppered, glass container? 3. Why is it necessary to remove air from the tip of the buret before starting a titration? 2. Define titration. AP Chemistry \ \ \/ Prelab Questions: Standardization of NJaO ),_-
Ci) p);- 3 op -C (U Ut (U Ci) ti) CU Ut (U Ut ri-i >< ru Ui Q CI) V)p3 I-C IDCncL If) r U) tic tic -) o \D i (U 1 (I ID -t Di C, I- o 1 tnt -,,o Ui -t (DID P IDU) U) (. til U) X DC i:r. d o I-I-Jrii o ti::t) 11 r1 U) til cx ID ID 1ji. (U ID I\-) cd (IC Ui N) N) II ii (ii 1 x U DC ID ID I-h -I (U -I ci 1 1 ci 1 I I N N N) C- I- C- 1.- bbci o vi 1 N C- 1 N I-- a + N C- a + N) C- ID 1 (U (U I n o 1 ID -I ID 1 - (Ju ID, -t Cl) (DID I--. ID ::;i 9)c I
FIgure 19.1 adng a Whtc paper not leak and that the stopcock turns freely. without leaving any drops adhering to the sides. Make sure that the buret does tamed by observing the position of the lowest point on the meniscus on the edges are in line with the lowest part of the meniscus and take the reading by Record the weights and label the iasks in order to distinguish among them. Add to each sample about 1 ml of distilled water that has been freed cus at their upper surfaces. In the case of water or water solutions, this menis cus is concave (see Figure 191), and the most accurate buret readings are ob flasks; accurately weigh to four sigant flgures.* Do not weigh the flasks. weighing bottle if you don t already know) between.4 estimating to the nearest tenth of a marked division (.1 nil). A simple way and.6 g each of pure potassium acid phthalate into 25-mi. Erlenmeyer JJ 3 Reading a Buret All liquids, when placed in a buret, form a curved menis of doing this for repeated readings on a buret is illustrated in Figure 19.1. alkaline solution tcrsis to cement a glass stopper so firmly that it is difficult to remove. 7 J4L- - Se Procedure: / preparation of Approximately o.ioo N Sodium ydroxide Weigh from a weighing bottle (your lab instructor II shov ou how to use a (rcpare about 4 45 ml. of C 2-free water by boiling for about 5 min 1I Standardization of Sodium ydroxide Solution To avoid parallax errors when taking readings, the eye must be on a level graduated scales. with the meniscus. Wrap a strip of paper around the buret and hold the top edges of the strip evenly together. Adjust the strip so that the front and back 1O-mL portionsof distilled water. The water must run freely from the buret.. eparation of a Buret for Use gnse with at 1et five rubber Stopper should used for a bottle containing sodium hydroxide solution, A strongly transfer to a 1-pt bottle fitted with a rubber stoppe J change its normality C wa bjltd to rcmove c rb a Onuioxldc, which would react with the sod-n hydrox.lde and
Rinse the previously cleaned buret with at least four 5-mL portions of the any hanging drop from the buret tip by touching it to the side of the flask and Record the buret reading. Repeat this procedure with the othertãles. From the data you obtain in the e6-titrations, calculate the normality solution to stand for at least 1 mm so the buret will drain properly. Remove (t dure. entire solution change throughout the titrant causca a color change at uously until one drop of help in a pink color appears where the drops of the base come in contact with the solu towel below the flask will Swirl the flask conun A sheet of white paper or in against to pink. The solution should remain pink when it is swirled. Allow the titrated be added drop by drop. It is most important that the flask be swirled constantly the color disappears more slowly, at which time the sodium hydroxide should the standardization should be repeated until agreement is reached. The average II ing drop from the buret tip by touching it to the side of the beaker used for the rrieniscus is at the zero mark or slightly lower. Allow the buret to stand for at wash down the sides of the flask with a stream of water from the wash bottle. of the sodiqjri hroxide solution to four significant figures as in Example 19.2. some of the liquid into an empty beaker. Make sure that the lower part of the Slowly add the sodium hydroxide solution to one of your flasks of potas fill the buret with the solution and remove the air frord the tip by running out card each portion. Do nor return any of the washings to the bottle. Completely approximately.1 N sodium hydroxide solution that you have prepared. Dis solution. washings. Record the initial buret reading. throughout the entire titration. The end point is reached when one drop of the The ee determinations should agree withtn 1. percent. If they do not, completely dissolved. Add to each flask two drops of phenolphthalein indicator tion. This coloration disappears with swirling. As the end point is approached, from carbon dioxide by boiling, and warm gently with swirling until the salt is least 3 s before reading the exact position of the rneniscus. Remove any hang sium hydrogen phthalate solution while gently swirling the contents of the flask, as illustrated in Figure 19.2. As the sodium hydroxide solution is added, sodium hydroxide solution turns the entire solution in the flask from colorless Figure 1 9.2 Titration proce ;- rk ui2t(-n r-f vntir of the e acceptable determinations is taken as the normality of the sodium taper each time you turn it Lcvel of meniscus
1, 4(aq) Na 498 2C32O4 + 22 DataJQbser-patjons Calculations: Ques lions: numbers. mass You should include the following infoation as data in your notebook for four trials: d A solution contains.63 g of oxalic acid, C,O 2,Q, 25 ml.at the molarity of this solution? What is the molaiit-y of the I-LSO? 25 2CQ3(aq) + 2S 4(aq),O(l) + 2) + Na t experiment. In such a standardization it was fod that ao. required 23.5 rnl of a sulfuric acid solution to reach the end point for the reaction., Sodium carbonate is a reagent that may be used to standardize acids the same way that you have used -g sple of sodium carbonate 2C324 + 2NaC Na what is the rroiarity of the rrialonic acid solution? ml of the NaQ solution were required to neutralize completely 13.15 rnl of the malonic acid solution, A solution of rnalonic acid, 2C324, was standardized by titration with.1 M NaO solution. If 2.76 1. Wtite the balanced chemical equation for the reactiofl of KP with NaO. Show all calculations! (For the mass of KJ-IP, volume of NaC and N of NaO, you may show one trial as a sample!) Be sure to show any foulas used and substitute your Standard deviation Average N For each of the four trials - of KP used, volume of NaC used, N of NaC. Should you discard any of your trials? Why or why not? Be sure to include any observations about each trial that you feel are needed. These measurements must include unceainty ( 1- values) and unii Mass of bottle + KP before removing sample Mass of bottle + KP after removing sample Final buret reading Initial buret reading
of your lab results? 6 fl- 4 f1 d. using a sample of KP that was contaminated with NaCI to c. not boiling the distilled water to remove the carbon dioxide b. leaving an air bubble in the tip of the buret when titrating a. rinsing the buret with distilled water before titrating affected by the following errors. this experiment to make it easier to achieve the goal? Did any unexpected occurrences happen in this lab? Can you think of any improvements that could be made in the design of your data support this statement? Explain how the value of your normality (N) of the NaO would be What was the goal of the experiment? Was the goal achieved? ow does What does your error analysis say about the accuracy andjor precision Conclusion: In general, certain questions should be addressed in each conclusion. You can also address specific questions. In the conclusion for this experiment, you should also be sure to address the following specific question. standardize your NaO