The Mass Percentage of Carbon in Sodium Bicarbonate

Louis Proust (1755-1826) was one of the first to observe that elements combine with one another in a definite mass ratio.   Proust's experimental work helped form the Law of Definite Proportions, sometimes called the Law of Definite Composition.   In other words, there is a precise quantity of each component required in the formation of a specific compound;  the percent composition is constant.  This, in turn,implies that in the decomposition of a specific compound, there is a precise quantity of each component produced.  In this experiment, you will indirectly determine the mass percent composition of carbon in sodium bicarbonate by determining the amount of carbon found in the carbon dioxide produced from the following chemical equation:

2   NaHCO3   +    H2SO4   --->    Na2SO4    +    2   H2O    +    2   CO2

Reacting sodium bicarbonate with sulfuric acid will produce a salt, water, and carbon dioxide gas.  In this experiment you will determine the mass of carbon produced in the carbon dioxide.  This in turn wil1 allow you to determine indirectly the mass of carbon found in the specific amount of sodium bicarbonate used, and thus calculate the mass percentage of carbon found in the bicarbonate. According to the Law of Definite Composition, the mass percentage of carbon in the bicarbonate should be constant no matter how much sodium bicarbonate is being decomposed.

The experiment should be performed twice and the results averaged.  Final1y, your average result is compared to the theoretical mass percentage of carbon in sodium bicarbonate.

Wear safety goggles and aprons in the lab at all times.  Chemicals, particularly 2 M  H2SO4 , may pose unexpected hazards, so use caution in dealing with them.  Add the sulfuric acid Slowly to the sodium bicarbonate.  Release 1 drop of the acid then swirl the contents until effervescence ceases.  Continue this process until a drop of acid added to the sodium bicarbonate does not produce an effervescent effect.  Wash your hands immediately upon contact with chemicals.
Pre-lab Questions:
1.  Which type of scientist makes use of the reaction of carbonates with acids?
        (Hint:  the study of this subject can be rocky at best!)

2.  What is the percentage of phosphorus in the compound sodium hydrogen phosphate,
       Na2HPO4 ?  (Show your work)


1. Preparation and Massing of the Pipet
          Obtain a beral pipet full of 2M sulfuric acid.  Next, place a known amount (to the nearest 0.01 gram) of sodium bicarbonate into a clean and dry modified jumbo pipet as shown in Fig. 1.  The amount of sodium bicarbonate can be any mass between 0.2Og to O.4Og.  Using a beral pipet full of 2 M sulfuric acid, insert it into the jumbo pipet as shown in Fig. 2.  Now mass this unit to the nearest 0.01 gram.

2.  Generating the Carbon Dioxide Gas
     Begin to generate tho carbon dioxide by SLOWLY releasing, dropwise, the sulfuric acid.  Continue releasing the acid drop wise in this manner until effervescence ceases.  Now mass the unit once again and calculate the mass of carbon dioxide released in the reaction.  RECORD your result.  Use your "partner's" data for the second trial and RECORD your results.


                                                                 Trial 1                      Trial 2

mass of jumbo pipet                             ______ g                    ______ g

mass of jumbo pipet with NaHCO3        ______ g                    ______ g

initial mass of unit                              ______ g                    ______ g

final mass of unit                                 ______ g                    ______ g

1.  Theoretical msss percentage of carbon in sodium bicarbonate: (show work)

2.   Calculate the mass of carbon dioxide produced for each trial.  (show work)

3.   Calculate the mass percentage of carbon present in the specific amount of sodium hydrogen carbonate used for each trial.  (Hint: you need to use the mass of CO2 and the percent of C in the CO2.  show work)

4.   Average these mass percentages and compare (percent difference) the result to that of the theoretical value.  (show work)

1.  What effects would there be in your results if you did not decompose all of the sodium hydrogen carbonate in each of your trials?

2.  A similar reaction occurs between limestone, known as calcium carbonate, and hydrochloric acid, HCl.  Write a balanced equation for this reaction.


3.  Write a balanced equation for the reaction of vinegar (acetic acid, CH3COOH) with baking soda.


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