4s
Counting to Weighing (Stoichiometry)
OBJECTIVES
•
•
•
To explore the relationship between the number of atoms or molecules and their mass.
To determine the amount of reactant consumed by measuring mass lost due to CO2 formation.
To apply the concept of limiting reactant.
INTRODUCTION
In the first three labs, we studied properties of matter related to mass and volume measurements. This lab
will extend the mass relationships further by investigating a chemical reaction. We will react baking soda
(solid sodium bicarbonate, NaHCO3) with vinegar (an aqueous solution of acetic acid, CH3COOH) and
study the relationships between the masses of reactants and products of the reaction.
The reaction between baking soda and vinegar produces three products, sodium acetate, carbon dioxide
and water. Write the balanced equation for this reaction below:
Chemical reactions must follow the laws of conservation of energy, mass and charge. Conservation of mass
mandates that the total mass of reactants must equal the total mass of the products. In the reaction of baking
soda and vinegar, we must be able to account for all of the mass originally present as acetic acid and
sodium bicarbonate in the sodium acetate, carbon dioxide and water produced during the reaction. We
begin by balancing the equation representing the reaction. A balanced equation has equal numbers of each
kind of atom on the reactant and product sides.
Which of the following represent properly balanced chemical reactions (circle all that apply)?
In this lab, we will combine larger and larger volumes of vinegar (5.0% by weight of acetic acid
(CH3COOH) in water) with a constant amount of baking soda and determine the changes in mass that ensue
due to the loss of carbon dioxide. Based on the balanced equation, it is possible to predict how much mass
will be lost. Before performing the lab, let's review the calculations for predicting the mass of CO2 lost in
the first experiment.
1. How many grams of CH3COOH are present in 2.5 cm3 of vinegar?
_________
(assume the density of vinegar to be 1.00 g/cm3)
2.
How many moles of CH3COOH are present in 2.5 cm3 of vinegar ?
_________
3.
How many moles of NaHCO3 are present in 1.2 g of baking soda?
_________
(Assume the baking soda to be pure sodium bicarbonate)
4.
5.
If 2.5 cm3 of CH3COOH are combined with 1.2 g of NaHCO3,
how many moles of CO2 will be formed?
_________
Indicate the effect that each of the following would have on the mass of CO2 produced by this
reaction. If
a.
If the volume of vinegar were increased from 2.5 ml to 5.0 ml, the mass of CO2 would
double.
be the same.
drop to half.
b.
If the mass of baking soda were increased from 1.2 to 2.4 g, the mass of CO2 would
double.
be the same.
drop to half.
EXPERIMENTAL
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Vinegar (5% acetic acid)
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Baking soda (sodium bicarbonate)
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At least 7 small plastic cups (4-6 oz.)
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Balance, primary, and secondary
measurement standards
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Measuring spoons
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Permanent marker
1.
Label 5 small plastic cups (4-6 oz.): 2.50 ml. (1/2 tsp), 10 ml. (2 tsp), 17.5 ml (1 Tsp + 1/2 tsp), 25 ml
(1 Tsp + 2 tsp), and 30 ml (2 Tsp).
2.
Carefully measure a level ¼ teaspoon (1.2 g) of baking soda into each labeled cup.
3.
Obtain the mass of each cup after the baking soda has been added and indicate the value in the
spreadsheet provided.
4.
Measure the volume of vinegar required for the first cup (1/2 tsp) in a separate container and determine
its mass. Note: Be sure not to include the mass of the container.
5.
Very slowly and carefully add the vinegar to the first cup. Note: Pour small increments into the cup
containing the baking soda. This chemical reaction can cause the reactants to foam over. If this
happens, begin that trial again.
6.
Repeat steps 4 and 5 for the other four cups.
7.
Stir each cup to make sure all the reactants are thoroughly mixed.
8.
Allow the cups to stand for about 10 minutes, gently stirring occasionally. Stir to remove as many
bubbles as possible.
9.
When bubble formation cease, determine the mass each each cup plus its contents. Record this mass in
the spreadsheet Note: The balance may become front heavy with the larger volumes, so carefully hold
the pan on which the cup is resting so it doesn't spill.
RESULTS & DISCUSSION
To help clarify your understanding, it is useful to represent the results obtained in these experiments
graphically. First consider the relationship between the moles of acetic acid used in an experiment and the
moles of carbon dioxide generated.
(a)
(c)
(b)
(d)
Answer the following questions to complete your report:
1.
Which of the graphs above most closely resembles the graph of your data?
2.
Which of the graphs would be predicted based on the theoretical relationship between the moles of
limiting reactant consumed and moles of carbon dioxide formed? (explain)
3.
Is there a proportional relationship between the amount of vinegar used in the experiment and the
amount of CO2 formed?
a. for every experiment?
b. for some experiments (indicate which ones)?
c. for no experiments?
Explain your answer in detail.