##### Experiment 3b: Analysis of Concrete Cleaner

Aim: To determine the percentage of hydrochloric acid in a sample of diluted concrete cleaner.

Method: (Refer to sheet)

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Results:

Concrete Cleaner Information:

HCl: 33% (w/v)

Density: 1.16g

Weight of clean dry volumetric flask: 86.6g

Weight of volumetric flask + sample of concrete cleaner: 92.8g

Mass of concrete cleaner: 6.2g

The aliquot used in the experiment was ‘Bottle B’ Sodium Carbonate. It contained 21.32g of Sodium Carbonate dissolved in 2L of solution.

Titration Results:

Titration number:

Amount of Sodium Carbonate solution titred:

1

0.15mL

21.75mL

21.60mL

2

21.75mL

42.30mL

20.55mL

3

0.10mL

20.75mL

20.65mL

4

20.75mL

41.32mL

20.57mL

The 2nd 3rd and 4th titres are concordant and will be used in the calculations.

Discussion Questions:

1) The manufacturer states there is 33% w/v HCl in the concrete cleaner. This is equivalent to 330g of HCl per 1L of solution. Therefore stoichiometry can be used to determine the expected molarity of the acid.

2) The mass of concrete cleaner is the mass of the volumetric flask with the sample in it subtracted from the weight of the clean dry volumetric flask.

Therefore the mass of HCl = 92.8g – 86.6g

= 6.2g

3) The average volume of the concordant titres would be;

20.55mL + 20.65mL +20.57mL = 20.77mL

3

5) The equation for the reaction that occurred between the hydrochloric acid and the sodium carbonate is:

2HCl (aq) + Na CO (aq) –> 2NaCl (aq) + H O (l) + CO (g)

4) The sodium carbonate in bottle B contained 21.23g of sodium carbonate dissolved in a solution. Therefore using stoichiometry the amount of sodium carbonate in mol present in each conical flask can be found.

6) Using the amount of mol in sodium carbonate, stoichiometry can be employed to determine the expected mol of hydrochloric acid present in the average titre.

7)

8)

9) The percentage by mass of HCl in the concrete cleaner is assumed to be 33%. However after extracting and analyzing a sample the percentage by mass of HCl was actually found to be 28.3%. This is a difference of 4.7%.

10) Volumetric flasks should be rinsed with de-ionized water. This is because de-ionized water will later be added therefore the titration result would not be affected. The burette should be rinsed with diluted hydrochloric acid and nothing else otherwise calculation would be affected e.g. is sodium carbonate was added then molarity calculations would be higher than should be. The pipette should be rinsed with sodium carbonate alone because if water was used then the sodium carbonate would be more diluted and more solution would be needed in the titre and therefore the result would once again be affected. The conical flasks should be rinsed with de-ionized water. This is because de-ionized water will be added in the flask anyway so as long as the number of mol are calculated before the sodium carbonate is put in the conical flask.

Conclusion:

The percentage of hydrochloric acid in a sample of diluted concrete cleaner was found to be 28.3%. The reason for the difference of 4.7% could be that every time the concrete cleaner is opened, hydrochloric acid gas is lost and therefore loosing concentration.

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