1. Calculate the approximate volume of HCl you will use in the back titration st
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Question
1. Calculate the approximate volume of HCl you will use in the back titration step if the mass of ASA is 325 mg per tablet. (The procedure is as described in the experiment below using 0.10 M HCl and 0.10 M NaOH)
2. Find the appropriate equations that you will need for the statistical analysis section. Include descriptions of all variables, and under which circumstances you will use each equation.
EXPIREMENTAL PROCEDURE:
Preparation of Carbonate Free NaOH
Prepare a solution of sodium hydroxide as follows:
Boil 1.5 L of distilled water and allow to cool.
Roughly weigh 5 g of NaOH pellets (MW 39.997 g/mol) on a watch glass.
Sodium Hydroxide is very caustic. Ensure that you clean up spills immediately.
Rinse off surface Na2CO3 with a small quantity of distilled water from a wash bottle. Discard the rinsings. Do not allow the pellets to sit in the liquid or you will dissolve too much of the
NaOH and your solution will be too weak.
Dissolve the pellets by adding them to 5 mL boiled distilled water in a small beaker. This will give a 50% solution of NaOH in which Na2CO3, is practically insoluble. If some undissolved Na2CO3 remains, allow this to settle and carefully decant 4 to 5 mL of the liquid or remove it with a pipette into a little distilled water that has been freshly boiled and cooled. Avoid transferring any of the carbonate in this step.
Transfer the solution to a 1 litre plastic storage bottle, and add approximately 995 mL of the boiled water (enough to almost fill the bottle), stopper the bottle well and mix thoroughly by inversion. The solution will now be about 0.1M.
Always store you solution in a plastic bottle. Before storage, squeeze the bottle slightly as you put on the cap to expel as much air as possible. This will minimize the amount of CO2 that is absorbed into the solution.
Standardization of NaOH using Potassium Hydrogen Phthalate
Rinse your burette with about 5 mL of the carbonate-free 0.1 M NaOH solution prepared above. Ensure that the burette tip is rinsed as well. Discard the solution.
Fill the burette with NaOH solution and remove any air bubbles from the tip or sides of the burette. Adjust the solution level to approximately 0 mL.
Remove the drop at the end of the burette by touching it to the side of a waste beaker.
Read the starting volume accurately. The burette should be read to a precision of 1/5 of the smallest division i.e. to a precision of 0.02 mL.
Using a weighing boat or weighing paper, weigh with precision a quantity of 0.7 to 0.9 g of pre- dried potassium hydrogen phthalate (stored in the desiccator) to the nearest 0.1 mg and rinse into a erlenmeyer flask.
Calculate your rough titre (volume of NaOH that must be added to reach the equivalence point) based on the mass of the standard.
Add to each sample 50 to 75 mL of boiled distilled water. Stopper the flasks and swirl to dissolve the solid.
Add 3 drops of phenolphthalein
Add 75% of the calculated volume of the NaOH (from step 7) from the burette into the conical flask with rapid agitation. Rinse the sides of the flask with distilled water.
Add the NaOH dropwise, mixing well between each drop until a faint pink colour is seen which persists for 30 seconds. (This colour may fade after a few minutes owing to the uptake of carbon dioxide. If it doesn’t fade within a few minutes, then you have over titrated your solution). If you have some difficulty seeing the pale pink colour of the indicator, a doubled quantity (6 drops) may be employed without serious error being introduced. Ensure that any drops on the end of the burette tip are added to the solution, and accurately read the final volume on the burette.
Repeat this standardization 2 more times using new samples of potassium hydrogen phthalate for each trial.
A blank titration must be carried out and any blank titre should be subtracted from all titrations. This is done by titrating 5 mL of 0.5 M sodium chloride with the NaOH solution. Treat the solution as above. (It will only take a drop or two of the NaOH solution if any at all.)
Calculate the molarity of the NaOH solution by taking the mean of your three trials. The deviation of these values from the average should not exceed 0.5% relative error if you are to proceed to the next part of the experiment.
Preparation of 0.1 M Hydrochloric Acid
Add 8.6 mL of concentrated hydrochloric acid (11.6 M) to 500mL (use a measuring cylinder) of tap water in a glass storage bottle and make up to 1 litre by adding approximately 490mL water. Concentrated acid should always be added to water, never the other way around.
Mix well by inverting the sealed bottle several times.
Standardization of 0.1 M Hydrochloric Acid
Rinse the 25 mL pipette 3 times with small portions of HCl.
Pipette 25 mL of the hydrochloric acid into an erlenmeyer flask.
Add 2 -3 drops of phenolphthalein to the solution in the erlenmeyer flask and titrate with standardized 0.1 M NaOH as described above.
Repeat the titration until 3 titrations differ by no more than 0.5%.
Calculate the concentration of the HCl.
Preparation of Acetyl Salicylate Solution
Weigh with precision 3 aspirin tablets and place each tablet in a separate 100 mL volumetric flask. (The flasks must be clean and dry. Remember not to dry the flasks in an oven.)
Label each flask clearly.
Add approximately 75 mL of 0.1 M NaOH to each flask and shake well until the tablet is dissolved. A small quantity of white residue may not dissolve.
Fill the flask to the 100 mL calibration mark with 0.1 M NaOH.
Shake well to mix contents.
Back titration of the excess NaOH
Empty the burette and wash well with tap water, then rinse with distilled water.
Rinse the burette with HCl before filling.
Use the same procedure as previously used with the NaOH.
Pipette 25 mL of the acetyl salicylate solution into an erlenmeyer flask. If there is any residue on the bottom of the flask, be careful not to disturb it or suck any of it into the pipette.
Add 2 - 3 drops of phenolphthalein and titrate with the hydrochloric acid until a faint pink colour persists.
Repeat the titration 2 more times. Your results should agree within 0.5%.
Calculate the initial quantity of NaOH (in 25 mL), the amount of NaOH in excess and determine the quantity of acetyl salicylic acid in each tablet.
Calculate the ASA as a %wt.
Analysis of aspirin powder
A number of aspirin tablets have been crushed and ground into a homogenous mixture. You will be given 1 portion of this powder to analyze for the concentration of aspirin.
Precisely weigh an amount of aspirin powder roughly equal to the weight of an aspirin tablet.
Analyze the ASA as described above.
Calculate the amount of ASA as a %wt concentration.
Explanation / Answer
Let us understand the experiment with steps
a)The concentration of NaOH solution prepared (carbonate free) = 0.1 M
b) concentration of HCl solution = 0.1M
c) One aspirin tablet in each 100mL flask, in which 0.1M NaOH solution is added
d) The aspirin tablet will get neturalized by some volume of NaOH.
e) The rest or left NaOH (unreacted NaOH ) is calculated by back titration wit 0.1M HCl
The chemistry question is
Calculate the approximate volume of HCl you will use in the back titration step if the mass of ASA is 325 mg per tablet. (The procedure is as described in the experiment below using 0.10 M HCl and 0.10 M NaOH)
Answer: the mass of ASA = 325mg / per tablet
ASA is a monoprotic acid, let us represent it as HA, the appropriate equation will be
HA + NaOH ---> NaA + H2O
the molecular formula of ASA = C9H8O4
The molecular weight is =180.157 g / mole
Moles of ASA taken form each tablet = Mass of ASA / mol wt = 0.325 / 180.157 = 0.0018 moles
So moles of NaOH required = 0.0018 moles
initial moles of NaOH present = molarity X volume = 0.1 x 0.1 = 0.01 moles
Moles of NaOH left = 0.01 - 0.0018 = 0.0082
moles of HCl required = 0.0082
Volume of HCl required =moles / molarity = 0.0082 / 0.1 = 0.082 L = 82mL
Reaction of NaOH with HCl will be
NaOH + HCl --> NaCl + H2O
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