|Room temperature:||3CaHPO4(aq) + 3NaHCO3(aq) ® 3H2O(l) + 3CO2(g) + Na3PO4(aq) +Ca3(PO4)2(s) (1)|
|High temperature:||NaAl(SO4)2(aq) + NaHCO3(aq) ® CO2(g) + Na2SO4(aq) + Al(OH)SO4(aq) (2)|
The purpose of todays experiment is to make some
observations of the behavior of baking soda and baking powder, to
determine the percentage sodium bicarbonate in a commercial
double acting baking powder, and to compare the relative amounts
of carbon dioxide gas produced in the two stages of gas
The analysis of baking powder for its sodium bicarbonate content will be accomplished with the apparatus shown below. Two experiments are required to measure the amount of CO2 available at room temperature and the amount produced at high temperature. In one experiment, a weighed amount of baking powder, and a vial containing water are placed in the flask. The flask is tightly stoppered. The tubing from the flask is run into a calibrated container which is filled with water and placed in an inverted position in a tub of water. The flask is swirled, causing the water in the vial to mix with the baking powder. Reaction (1) occurs, and the carbon dioxide gas displaces some of the water in the calibrated volume. At this point, the baking powder sample still contains unreacted NaHCO3 because the baking powder sample does not contain enough CaHPO4 to react with all of the sodium bicarbonate, and reaction (2), which normally occurs at high temperature, does not occur under these conditions. To determine the total amount of CO2 gas available from the baking powder, a second experiment is conducted using the same mass of baking powder as before, and placing vinegar, an aqueous solution of acetic acid, CH3CO2H, in the vial instead of water. The vial contains more than enough acid to react with all of the sodium bicarbonate in the sample, and all of the available CO2 is released. The reaction is:
In summary, we have:
Volume of CO2 obtained with water in the vial = Volume of CO2 available at room temperature
Volume of CO2 obtained with vinegar in the vial = Total volume of CO2 available
Figure 1. Apparatus
The volume of CO2 measured with vinegar in the vial is directly proportional to the total amount of sodium bicarbonate in the baking powder sample. The relationship between the mass of NaHCO3 and the volume of CO2 is established by means of a calibration graph. A sample calibration graph is shown in Figure 2, and consists of a plot of volume of CO2 versus mass of NaHCO3. The calculation of the percentage NaHCO3 in a baking powder sample is illustrated in Example 1, below.
Example 1: A 1.75 g sample of baking powder produced 70. mL of CO2 gas when vinegar was placed in the vial in the apparatus in Figure 1. What is the percentage NaHCO3 in the sample?
Solution: With vinegar in the vial, all of the NaHCO3 reacts to produce CO2. From the calibration graph (Figure 2), we see that 70 mL of gas is produced by 1.4 g of NaHCO3. Therefore, the sample contains 1.4 g of NaHCO3. The percentage NaHCO3 is given by:
|Percentage NaHCO3 =||
|Percentage NaHCO3 =|
Things for each group to borrow and return on the same day.
|8 oz jar|
Observe, and record in your notebook the result of each
1. Add about 1g solid sodium bicarbonate to 50 mL of water. And stir.
2. Dissolve about 0.5 g of cream of tartar in 50 mL of water, add 1g of sodium bicarbonate and stir.
3. Add 1 g of commercial baking powder to 50 mL of water and stir.
4. Heat the solution prepared in 3., above, on a hot plate.
Part 2, The calibration graph.
The first step in analyzing the baking powder for sodium bicarbonate is to obtain the data needed for the construction of a calibration graph. Weigh out 0.2 g of pure NaHCO3. Record the mass to 0.01 gram. Transfer the NaHCO3 completely to the filter flask shown in Figure 1. The flask must be dry. Fill the plastic vial to within about 1 cm of the top with vinegar and carefully place the vial upright in the flask without spilling any vinegar. This is best done with the forceps found in your drawer. Place the stopper tightly on the flask. Fill the plastic tub with water. Fill the calibrated volume (jar, in this case.) with water and place it in the water as shown. Insert the end of the tube coming from the flask into the jar. You will probably have to hold the jar to keep it from tipping. Swirl the flask to cause the vial to tip over, thereby causing the vinegar to react with the NaHCO3 and releasing carbon dioxide. The CO2 will displace water from the jar. Occasionally swirl the flask to speed up the reaction.
Continue the experiment until bubbles cease emerging from the tube. Remove the tube from the jar, and raise or lower the jar in the water until the water level inside the jar is the same as the level outside. Read the volume of the CO2 gas on the mL scale on the side of the jar, and record it in your notebook (note that there are two scales on the jar, "mL" and "oz",use "mL"). Repeat the procedure with 0.3 g, 0.5 g, 0.7 g and 0.8 g of pure NaHCO3.
Part 3, Analysis of baking powder.
Part 3a, Total CO2 available: Repeat the calibration procedure using 1.9-2.0 g of baking powder in place of the pure NaHCO3. Again, the flask must be completely dry before adding the baking powder. The volume of CO2 produced is the total amount available. Do this experiment in duplicate.
Part 3b, CO2 available at room temperature: Repeat the procedure in Part 3a, above, using exactly the same mass of baking powder except, this time, fill the vial with deionized water instead of vinegar. The volume of CO2 measured here is the amount available at room temperature. Again, do the experiment in duplicate.
Include the following:
1. All observations.
2. The data for the calibration graph and the baking powder analysis.
3. The brand of the baking powder.
4.The calibration graph.
5. The complete calculation of: the percentage NaHCO3 for each trial in Part 3a, and the average percentage.
Answer each of the following
1. Explain each of the observations made in Part 1.
2. What is the average percentage of NaHCO3 in the baking powder?
3. What fraction of the total amount of CO2 released is released at room temperature?
to Pernona's Homepage