What is the Compound in Mr. Coffee Cleaner? 1

Transcription

1 What is the Compound in Mr. Coffee Cleaner? 1 A. Introduction HOW MR. COFFEE CLEANER WORKS Mr. Coffee sells a cleaner that is supposed to clean the scale that forms in coffee pots and tea kettles when water is boiled in them. The substance in this product is an acid and your task is to determine the molar mass of the acid and then attempt to identify the acid. You have probably seen scale inside coffee pots, teakettles, on hot water tanks, around the holes of a steam iron, etc. It s that rather gray (dirty white) solid that you see on surfaces that have been exposed to hot water. Scale of the type that we are talking about here forms when the water supply used for heating contains both Ca 2+ (Mg 2+ behaves in the same manner) and bicarbonate ions (HCO 3 - ). Under the influence of heat, bicarbonate breaks down as follows: 2 HCO 3 - (aq) H 2 O (l) + CO 2 (g) + CO 3 2- (aq) Then, the calcium cation reacts with the carbonate anion as follows: Ca 2+ (aq) + CO 3 2- (aq) CaCO 3 (s) In other words, the reaction forms an insoluble solid (calcium carbonate) that precipitates out of solution. The precipitate moves to the bottom of the heating container by gravity and eventually becomes baked on to the heating surface. The off-white solid that you see associated with heating surfaces is frequently calcium carbonate. There are other scales that are formed on hot surfaces, but we will not deal with them in this experiment. If you have ever worked with this scale, you know that you can chip it off with a knife or other sharp-bladed instrument. But there is an easier way. Perhaps surprisingly, you can remove this type of scale with the addition of acid: CaCO 3 (s) + 2 H + (from an acid) Ca 2+ (aq) + CO 2 (g) + H 2 O (l) The scale dissolves, the calcium ion goes back into solution and the carbon dioxide escapes as a gas. Now you know why you can remove this form of scale by boiling or soaking the container in vinegar. Vinegar contains acetic acid (CH 3 COOH, about 5% by weight) which supplies the H + ions in the above equation. Other scales, such as those made from calcium sulfate, CaSO 4, will NOT dissolve in this way. Mr. Coffee contains an acid, but it is not acetic acid. Your task is to react the powdery material that is in Mr. Coffee Cleaner with sodium hydroxide. Based on these results, you are to determine the molar mass of this acid and then with the aid of additional information, see if you can identify the acid. ACID-BASE TITRATIONS You are going to determine the amount of acid in this cleaner by a technique known as titration.

2 Mr. Coffee Acid, p. 2 of 5 For this titration you will add a strong base from a buret to neutralize all of the acid. A typical neutralization process is the reaction between the strong base sodium hydroxide (NaOH) and the strong acid hydrochloric acid (HC1). As usually performed in the laboratory, the base would be added from a buret to the acid solution until all of the acid had been neutralized. As the balanced equation shows, the reaction forms a salt and water. NaOH + HCl NaCl + H 2 O Please note that 1 mole of base neutralizes 1 mole of acid. Let us assume two points about this cleaner. In the first place, assume that the acid is monoprotic. In other words, there is only one acidic hydrogen atom in each acid molecule. In the second place, assume that this cleaner has only the pure acid in it. That is, there are no inactive or inert ingredients present in the cleaner. If we designate the acid in Mr. Coffee as HA, then the balanced neutralization reaction would be: NaOH + HA NaA + H 2 O Once you know the volume of base that has been added and the concentration (molarity) of that base, you will be able to determine the number of moles of base (recall that M = moles/volume [in liters]) that have been added to neutralize the acid. Then, use the stoichiometry of the balanced neutralization equation to determine the number of moles of acid. Now, combine the number of moles of acid that you calculated from the stoichiometry of the reaction with the weight of acid that you weighed in the beginning, and you should be able to calculate the molar mass of the acid (recall that moles = grams/molar mass). Experimental procedure PREPARING YOUR BURET 1. Take a clean, dry 200-mL beaker to the stockroom to obtain your NaOH solution. Record the molarity of the solution. Fill your buret following the following guidelines. The standard laboratory buret is constructed to deliver 0-50 ml of liquid with a reliability of ± 0.2%. or ±.02mL. Careful adherence to buret techniques is required for obtaining accurate results. SAFETY NOTE: Eye protection is essential when handling hydroxide solutions. When adding a solution to the buret, the mouth of the buret must be below face level to prevent possible splashing onto your face. Either remove the buret from the clamp or place the entire ring stand in a lower position. 2. Rinse the buret with 2 or 3 small portions (5-10 ml) of the solution. Rinsing is important since water remaining in the buret will dilute a standard solution. Fill the buret to a level about one-half inch above the zero mark. (Use a funnel or a beaker in this process.) 3. Just prior to beginning the titration, allow the liquid level to fall to the zero mark (drain the fluid into a waste beaker. The bottom of the liquid meniscus should be used as the point of

3 Mr. Coffee Acid, p. 3 of 5 reference when reading the buret. Avoid parallax error by keeping your eyes level with the mark being read. Touch off any drop clinging to the tip. The following rules apply to all titrations and are included to assist you in your titration. Use an Erlenmeyer flask for all titrations. Position the tip of the buret about 1 inch down inside the neck of the flask. Place piece of white paper under the flask. Control the stopcock with your left hand and continuously swirl the solution in the flask with your right hand. (Left-handers reverse.) Add the titrant (NaOH) continuously, slowing the flow rate as the equivalence point is reached. Repeatedly stopping the flow and shaking the flask wastes time. Near the equivalence point, add drops singly. Fractional drops may be added by washing them off with a stream from the wash bottle. (Be sure your wash bottle contains distilled water.) Rinse down the walls of the flask. TITRATION OF MR. COFFEE CLEANER 4. Weigh a sample of the Mr. Coffee Cleaner that is between and grams into a 125-mL Erlenmeyer flask. To do this, use the following procedure. Remember to record all weighings in your notebook. 1) Fold a piece of weighing paper into a weighing boat as demonstrated by your instructor. 2) Take your "boat" to the stockroom and obtain a sample of Mr. Coffee Cleaner. 3) Carefully take your boat with the acid to the balance room and find its mass. Pour the acid into your 125-mL Erlenmeyer flask. 4) Reweigh the paper and record this value. The mass of your sample is the difference between these weights. Be sure to weigh to at least 3 decimal places! 5. Add about 25 ml of distilled water and 3 drops of phenolphthalein indicator to this solution. Record the initial buret reading in your notebook. Begin titrating with the NaOH solution. The molarity of this solution will be very close to M; you should have been told the exact molarity when you got your solution; record this value in your notebook. 6. As you add the base to this solution you will begin to see the formation of a pink color just at the point where the drop of base meets the acid solution. As you swirl the flask this pink color will disappear. As more base is added it will take more swirling and more time for the pink color to disappear. When all of the acid has been neutralized by the base the pink color will remain. "Sneak" up on this point by S-L-O-W additions of base. The entire solution should turn pink, but this pink should only last for about 30 seconds and after that the solution should turn colorless. If the solution turns hot pink you ve gone too far, you ve added too much base. If you have followed these directions correctly, you will have used between ml of the NaOH to titrate your sample. Record the final buret volume reading and determine the volume of NaOH delivered. When the equivalence point is reached, record the reading and double-check it. Always record ±2 decimal places for each volume reading. With practice, the reading can estimated to ± 0.02 ml. For example write ml, rather than 24.5 ml, since the buret

4 is capable of more than ±0.1 ml accuracy. Mr. Coffee Acid, p. 4 of 5 7. Refill the buret with NaOH, empty and rinse the Erlenmeyer flask and weigh a new sample. Make sure all your data is entered in your notebook. Follow this procedure until you have done at least 2 good trials (no hot pink). Then do the calculations. When finished with calculations, rinse the buret with distilled water and return it to the stockroom. Chemical Waste Faintly pink titration solutions are neutral and can be poured down the drain. Excess titration (NaOH) should be poured into the waste container provided on the center island in the lab. Calculations Do the following calculations in your notebook. 1. From the volume of base (V b ) added and the molarity (concentration) of the base (M b ), calculate the number of moles of base (n b ) used. n b 1000 ml ( mol) = M ( ) ( ) b mol L Vb L 1 L 2. Use the stoichiometry of the balanced equation to find the number of moles of acid (n a ) that reacted with the base (see page 2). 3. Calculate the molar mass of the acid (MM a ) from MM g ( ) a mol = m n a ( g) ( mol) 4. Identify the acid in Mr. Coffee Cleaner from the table. Your report will consist of purpose, conclusion, data, calculations, and answers to the questions on the next page. Questions Acid Ascorbic Citric Malic Oxalic Phosphoric 82.0 Salicylic Succinic Sulfamic 97.1 Molar Mass (g/mol) 1. If you had started the titration from 1.00 ml on the buret instead of zero, but forgot that you did so, how would that mistake affect your results? 2. Why is the exact amount of water that you add to the Mr. Coffee cleaner in the Erlenmeyer flask not important? 3. If the acid in Mr. Coffee cleaner were diprotic (contains 2 acidic hydrogen atoms per molecule) what answer would you have obtained for your molar mass? Hint: Use the following balanced equation. H 2 A + 2 NaOH Na 2 A + 2H 2 O

5 Prelab: Mr. Coffee Acid, p. 5 of 5 To test your understanding of the calculations, work the following problem before continuing with the experiment. A pure sample of a monoprotic acid was titrated with M NaOH solution. If grams of the acid required ml of the base for complete neutralization, what was the molar mass of the acid? In Experiment 1, the buret had an average correction value of ml. 1. What is the corrected volume of base? Answer 2. Use the formula in the Calculations section to find the moles of base. 3. How many moles of acid were in the sample (n a )? n b 4. Calculate the molar mass of the acid (MM a ). n a 5. Identify the acid in Mr. Coffee Cleaner from the table. MM a Chemical