Question

1. Briefly explain why it is necessary to analyze more specimen of the drain cleaner .

2 Briefly explain why three titrations are carried out for the solution of each specimen of the drain cleaner.

3. A 1.9248 g specimen of a drain cleaner is dissolved and diluted to 50 mL with deionized water. A 5.00 mL aliquot is titrated with 0.1189 M HCl It requires 25.93 mL of the HCl to reach the color- less phenolphthalein end point.

A. Compute the molarity of the 5.00 mr aliquot of the solution, assuming that the alkali and HCl react in a 1: 1 mole ratio

B. Compute the number of moles of alkali that were in the volumetric flask before the aliquot was taken

C. Compute the number of grams of alkali originally in the 50 mL flask, assuming that the alkali in the drain cleaner is NaOH

D. Compute the weight alkali (expressed as wt-% NaOH) in the specimen of drain cleaner.

4- A student found the standard deviation of a set of three values ​​of the weight-% alkali of a specimen to be 1.552 x 10^-1. Compute the variance associated with this standard deviation. Report your answer to 4 significant figures using scientific notation.

Answer 1

1)  Analysis of a typical drain cleaner will illustrate the points discussed above. Drain cleaners are mixtures of sodium chloride, sodium hydroxide and aluminum metal. We will 2 analyze a typical drain cleaner for sodium hydroxide content. The analysis step will be a titration with the standard HCl prepared in Equipment.The reaction is: H₃O⁺ + OH^- = 2H₂O . Only the sodium hydroxide present in the drain cleaner reacts with the acid. The titration of a strong base with strong acid has an end point at neutral pH.The indicator phenolphthalein will be used to locate the end point of the titration. To investigate the variances due to sampling and due to the titration, two samples of drain cleaner will be titrated. For each sample, the analysis will be repeated three times, in order to estimate the titration variance.

These samples are very heterogeneous.It is very important that you do not select one type of particle over the other. Close the can and shake it, then take the sample as blindly and quickly as you can.This will give a more representative estimate of the %NaOH in the can, and will improve your results.The sodium hydroxide in the drain cleaner is also very hygroscopic. An opened can of drain cleaner should be exposed to the atmosphere for no longer than is necessary. Store the drain cleaner in a covered weighing bottle in the desiccator between uses.

Solid NaOH in the drain cleaner is very corrosive! Handle it very carefully, and avoid contact with your skin or clothes. If you do spill some, wash it off at once. Wash your hands carefully when you complete this laboratory.

2) Calculate a standard deviation by using this average as the mean, and the three sample results as the individual trials. This standard deviation is S_sampling. Report this value with your results. This value should also have one significant figure.Report your values for the mean percentage of NaOH from consideration of all nine titrations, and your values for the three different standard deviations (as well as their relative standard deviations) determined above