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L L ABAB 151 5:: SS OLOLUUTTIIOONNSS AANNDD TT IITTRRAATTIIOONNSS ::
D D IILLUUTTIIOONNSS AANNDD AA CICIDD -B-B ASASEE RR EEAACCTTIIOONN RR EEVVIIEEWW
P P UURRPPOOSSEE :: To prepare a solution of known concentration by dilution.
To perform an acid-base neutralization reaction. To determine the neutralizing capacity of an antacid tablet by titration.
S S AAFFEETTYY CC OONNCCEERRNNSS ::
Always wear safety goggles. Wash with soap and water if skin contacts acids or bases.
S S OOLLUUTTIIOONNSS ::
Solutions can be prepared by dissolving a solute in a solvent or by diluting a concentrated stock solution. Review the chapter in your text book dealing with preparation of solutions.
In this laboratory exercise we will use the technique of dilution to prepare a 1.0 M HCl solution from a more concentrated 6.0 M HCl solution. Remember the formula C 1 V 1 = C 2 V 2. Once we have made our dilute HCl solution we will use it to neutralize a solution containing an unknown quantity of base.
T T IITTRRAATTIIOONNSS ::
The process of determining the amount of acid required to react with a given amount of base is call a Titration. In this experiment we will add a solution of 1.0 M HCl drop by drop to an unknown quantity of base until the entire base sample has reacted. If we know how many mL’s of HCl we used, and we know the concentration, M, of the acid then we can calculate how many moles of HCl it takes to react with the base. Once we know how many moles of HCl react with our base we can determine from the balanced acid-base equation the number of moles of base we have. From the number of moles of base we can determine the mg of base present in our sample.
We will know when enough HCl has been added to completely react with our unknown quantity of base by using a pH indicator. We will use an indicator pigment that is a different color in a basic solution than when the solution is neutral or acidic. We will add Acid to our Base until the organic indicator molecule changes color. For our experiment the indicator solution, bromocresol green/methyl red will be green when the solution is basic and will turn red when the reaction is complete.
P P ROROCCEEDDUURREESS ::
ACTIONS :
I. Preparation of 1.0 M HCl by Dilution:
- Calculate in your prelab exercise problem #1 how much 6.0 M HCl solution is required to make 100 mLs of a 1.0 M HCl solution by dilution.
- Carefully measure from a stock buret^1 the calculated amount of 6.0 M HCl stock solution into a 100 mL volumetric flask.
- Add enough deionized water to the stock HCl solution to about half fill the volumetric flask and then swirl to mix the solution.^2 When the solution is mixed, continue adding water until you get close to the measurement mark on the flask. When the solution is close to the 100 mL line, add the water drop by drop just until the bottom of the meniscus touches the measurement line exactly. Mix well. 3
- Fill a clean^4 buret mounted at your workbench with your 1.0 M HCl solution. Let some HCl solution run out through the tip to evacuate any trapped air bubbles from the stopcock.
- Save the volumetric flask containing your 1.0 M HCl solution and refill your buret as needed as you perform the following acid-base titrations.
II. Titration of a 2% Lye Solution:
- Weigh out 5 0.50 grams of solid Sodium Hydroxide 6 , NaOH, into a clean, 250 mL Erlenmeyer flask.
- Add about 30 mLs of deionized water and swirl the flask to completely dissolve the sodium hydroxide.
- Add 10 drops of the pH indicator^7 solution to the flask of sodium hydroxide and mix well. The solution should be green.
- Place the flask under the clamped buret containing the 1.0M HCl. Put a piece of white paper underneath the flask to better help you see the color.
- Record the initial level of HCl in the buret on the report sheet (Box IIA)
NOTES :
(^1) Burest and Volumetric flasks are more accurate than graduated cylinders so we use them when our measurements need to be more precise. (^2) If you fill up the volumetric flask and then attempt to mix the solution there will not be enough room in the flask for agitation and the resulting solution may not be of uniform concentration. (^3) This solution is now 1.0M HCl which will be used for your acid- base titrations. (^4) If the buret has been recently washed and so is wet on the inside, you should rinse the buret with 5 mLs of your 1.0 M HCl solution. This will insure that the drops on the inside of the wet buret are 1.0M HCl and not water. If you were to pour your 1.0M HCl into a buret with water in it, the concentration would be diluted by the water and not be 1.0 M any more. (^5) Either weigh the solid directly into a tared Erlenmeyer flask, or onto a tared weighing paper and then pour it into the flask. Never put chemicals directly on the balance pans. (^6) Sodium Hydroxide is also called Lye, or Caustic Soda. It is the main ingredient in drain cleaner. It is harmful to your skin. If you come in contact with it wash with soap and copious amounts (lots) of water. (^7) We are using a mixture of bromocresol green and methyl red. This indicator will be green when basic but red when below pH 3.8.
- Add the 1.0M hydrochloric acid dropwise to the antacid solution with constant mixing.^8 As the titration proceeds, each drop of acid will turn the solution a red color, which then vanishes^9 as you mix it. A red color which persists for over 2 minutes indicates the end (endpoint) of the titration.
- When the endpoint has been reached, record the level of HCl left in the buret (Box IIIB) and determine the total number of mLs of HCl you used to neutralize your base. Record the total number of mLs HCl used on your report sheet (Box IIIC).
- Insert the number of mLs HCl used (from Box IIIC) into equation #2 on the report sheet. Calculate^10 the total number of mg’s CaCO 3 titrated and report it in Box IIID.
- Compare the total number of calculated mg’s of CaCO 3 (from Box IIID) with the number of mg’s reported by the manufacturer (Box IIIE). Determine the percent of theoretical yield according to the given equation and report it in Box IIIF.
- Repeat the procedures of Part III with another brand of antacid tablet.
- In the conclusion section of the report sheet compare the brands of antacid you used. Discuss any sources of error in your experimental technique.
L L ABAB 151 5:: SS OLOLUUTTIIOONNSS AANNDD TT IITTRRAATTIIOONNSS :: NN AMAMEE __________________________
PP (^) RREE LL (^) AABB EE (^) XXEERRCCIISSEESS (^) :: DD ATATEE (^) ____________________________
- You need 100 mLs of a 1.0 M HCl solution. You have a stock solution that is 6.0 M HCl. How can you prepare the 100 mLs of the dilute solution you need from the concentrated stock solution available? (hint: Remember C 1 V 1 = C 2 V 2 ) - Show your calculations. - Tell how much of the stock solution to use and - Tell exactly what to do with it to make the dilute solution.
- Complete and balance the following Acid-Base equation showing the reaction of stomach acid with Lye.
____HCl + _____NaOH Æ
- Complete and balance the following Acid-Base equation showing the reaction of stomach acid with a Tums antacid tablet.
____HCl + _____CaCO 3 Æ
- If it requires 25.0 mL of 1.0M HCl to react with an antacid tablet, how many mgs of CaCO 3 must have been present in the tablet? Complete the following conversion scheme to find out.
3 3 3 3 3
25.0mL HCl L HCl mol HCl mol CaCO g CaCO mg CaCO 1 mL HCl L HCl mol HCl mol CaCO g CaCO
⎛ ⎞ ⎛ ⎞ ⎛ ⎞ ⎛ ⎞ ⎛^ ⎞ ⎜ ⎟ ⎜ ⎟ ⎜ ⎟ ⎜ ⎟⎜ ⎟= ⎝ ⎠ ⎝ ⎠ ⎝ ⎠ ⎝ ⎠ ⎝ ⎠
mg CaCO (^3)
L L ABAB 151 5:: SS OLOLUUTTIIOONNSS AANNDD TT IITTRRAATTIIOONNSS NN AAMMEE ______________________________________
RR (^) EEPPOORRTT :: PP AARRTTNNEERR __________________DD ATATEE ______
Titration of an Antacid:
2% NaOH Antacid
_________brand
Antacid
_________brand Beginning Buret Reading (in mL’s)
IIA. IIIA.
Ending Buret Reading (in mL’s)
IIB. IIIB
Total mLs HCl used (in mL’s)
IIC. IIIC
Calculated Mass of Base Present
(from equations below)
IID. (from equation 1)
g’s
IIID (from equation 2)
mg’s
(from equation 3)
mg’s
Theoretical Mass of Base Present
(from actual data)
IIE.
0.50 g’s
IIIE
mg’s (from the bottle label)
mg’s (from the bottle label)
Percent of Theoretical
Experimental (Box D) X 100 = % Theoretical (Box E)
IIF. IIIF
Equations to Calculate Mass of Base Present in a Titrated Sample: (see footnote 10
in the procedures section for hints.)
mL HCl L HCl mol HCl mol NaOH g NaOH 1 mL HCl L HCl mol HCl mol NaOH
g NaOH
3 3 3 3 3
mL HCl L HCl mol HCl mol CaCO g CaCO mg CaCO 1 mL HCl L HCl mol HCl mol CaCO g CaCO
⎛ ⎞ ⎛ ⎞ ⎛ ⎞ ⎛ ⎞ ⎛^ ⎞ ⎜ ⎟ ⎜ ⎟ ⎜ ⎟ ⎜ ⎟⎜ ⎟= ⎝ ⎠ ⎝ ⎠ ⎝ ⎠ ⎝ ⎠ ⎝ ⎠
mg CaCO 3
3 3 3 3 3
mL HCl L HCl mol HCl mol CaCO g CaCO mg CaCO 1 mL HCl L HCl mol HCl mol CaCO g CaCO
⎛ ⎞⎛ ⎞ ⎛ ⎞⎛ ⎞ ⎛^ ⎞ ⎜ ⎟⎜ ⎟ ⎜ ⎟⎜ ⎟⎜ ⎟= ⎝ ⎠⎝ ⎠ ⎝ ⎠⎝ ⎠ ⎝ ⎠
mg CaCO 3
Conclusion and Error Analysis:
L L ABAB 151 5:: SS OLOLUUTTIIOONNSS AANNDD TT IITTRRAATTIIOONNSS :: NN AMAMEE __________________________
RR (^) EELLAATTEEDD EE XEXERRCCIISSEESS :: DD ATATEE (^) ____________________________
- A team of students is performing an acid base titration. They proceed to add 1.0M hydrochloric acid, drop by drop, to a solution of sodium hydroxide base in an Erlenmeyer flask. After adding 22 mLs of acid they realize that they forgot to add the drops of pH indicator to their basic solution. They decide to add the pH indicator now. They enter drops of pH indicator into the Erlenmeyer flask and the solution immediately turns red. What conclusions can you make from this result?
- A chemistry student measured out 1.7 mLs of a 6.0 M HCl stock solution and then diluted it to 100 mLs in a volumetric flask. What it the final concentration of the dilute solution? Show your work.
= M HCl
- A 5.00 g sample of solid Sodium Hydroxide, NaOH, is placed in a flask to which enough water is added to total 25 mLs of solution. What is the concentration of this solution in m/v %? Show your calculations.
= m/v %
- A drug manufacturer reports that its antacid tablets contain 500 mgs of Calcium Carbonate, CaCO 3 , as the active ingredient. Use the following equation to predict how many mLs of a 1.0 M HCl solution it should take to completely neutralize this amount of base in a titration.
3 3 3 3 3 3
500 mg CaCO g CaCO mol CaCO mol HCl L HCl mL HCl 1 mg CaCO g CaCO mol CaCO mol HCl L HCl
⎛ ⎞ ⎛^ ⎞ ⎛^ ⎞⎛ ⎞⎛ ⎞ ⎛ ⎞ ⎜ ⎟ ⎜ ⎟ ⎜ ⎟⎜ ⎟⎜ ⎟ ⎜ ⎟= ⎝ ⎠ (^) ⎝ ⎠ ⎝ ⎠⎝ ⎠⎝ ⎠ ⎝ ⎠
mL HCl
