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experiment_6___identification_of_a_compound_by_mass_relationships.pdf

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CHEM 403
Exp 6
Experiment 6:
IDENTIFICATION OF A COMPOUND BY MASS RELATIONSHIPS*
In the previous experiment, you learned how to find the formula of a compound by analysis
for the elements it contains. When chemical reactions occur, there is a relationship between the
masses of the reactants and products that follows directly from the balanced equation for the
reaction and the molar masses of the species that are involved. In this experiment, you will use this
relationship to identify an unknown substance. Your unknown will be one of the following
compounds, all of which are salts:
NaHCO3
Na2CO3
KHCO3
K2CO3
In the first part of the experiment, you will be heating a weighed sample of your compound in a
crucible. If your sample is a carbonate, there will be no chemical reaction that occurs, but any small
amount of adsorbed water will be driven off. If your sample is a hydrogen carbonate, it will
decompose by the following reaction, using NaHCO3 as the example:
2NaHCO3(s) → Na2CO3(s) + H2O(g) + CO2(g)
(1)
In this case, there will be an appreciable decrease in mass, since some of the products will be driven
off as gases. If such a mass decrease occurs, you have strong evidence that your sample is a
hydrogen carbonate, either NaHCO3 or KHCO3.
In the second part of the experiment, you will treat the solid carbonate in the crucible with
HCl, hydrochloric acid. There will be considerable effervescence as CO 2 gas is evolved; the
reaction that occurs, again using Na2CO3 as our example, is:
Na2CO3(s) + 2H+(aq) + 2Cl−(aq) → 2NaCl(s) + H2O(l) + CO2(g)
(2)
(Since HCl in solution exists as ions, the equation is written in terms of ions.) You will then heat
the crucible strongly to drive off any excess HCl and any water that is present, obtaining pure, dry,
solid NaCl as your product.
To identify your unknown, you will need to find the molar masses of the possible reactants
and final products. For each of the possible unknowns there will be a different relationship between
the mass of the original sample and the mass of the chloride salt that is produced in Reaction 2. If
you know your sample is a carbonate, you need only be concerned with the mass relationships in
Reaction 2, and should use as the original mass of your unknown the mass of the carbonate after
it has been heated. If you have a hydrogen carbonate, the overall reaction your sample undergoes
will be the sum of Reactions 1 and 2.
From your experimental data, you will be able to calculate the ratio of the mass of the solid
chloride to the mass of either the original hydrogen carbonate or the mass of the anhydrous
carbonate in your sample of unknown. From your calculation of the relative masses of solid
chloride to solid hydrogen carbonate or solid carbonate in Equations 1 and 2 you can calculate
*Adapted from Slowinski, E. J., Wolsey, W. C. Chemical Principles in the Laboratory 9th ed.
CHEM 403
Exp 6
what the theoretical ratio of those masses should be. Your observed value should match one of the
theoretical values and thus allow you to identify your unknown compound.
Procedure
Obtain an unknown from the instructor. Weigh a clean, dry crucible, and record its mass.
Place approximately 0.5 g of your unknown into the crucible, and record the total mass of the
sample with the crucible.
Place the crucible on a clay triangle supported by an iron ring. Light your Bunsen burner
away from the crucible, and adjust the burner so that you have a small flame. Holding the burner
in your hand, gently heat the crucible as you move the burner back and forth. Heat the crucible,
gently and intermittently, for a few minutes. Gradually increase the flame intensity, to the point
where clay triangle glows red. Heat for 10 minutes. Allow the crucible to cool for 10 minutes, and
then weigh it with the sample, recording the mass. At this point, the sample in the crucible is a dry
carbonate, since the heating process will convert any hydrogen carbonate to carbonate.
Place the crucible back on the clay triangle. Add about 25 drops of 6 M HCl, a drop at a
time, to the sample. As you add each drop, you will probably observe effervescence as CO2 is
produced. Let the action subside before adding the next drop, to keep the effervescence confined
to the lower part of the crucible. You do not want the product to foam up over the edge. When you
have added all the HCl, the effervescence should have ceased, and the solid should be completely
dissolved. Heat the crucible gently for brief periods to complete the solution process. If all of the
solid is not dissolved, add 6 more drops of 6 M HCl and warm gently.
Heat the crucible, gently and intermittently, for about 10 minutes, to slowly evaporate the
water and excess HCl. If you heat too strongly, spattering will occur and you may lose some
sample. When the sample is dry, gradually increase the flame intensity, finally getting the bottom
clay triangle to a red heat. Heat at full flame strength for 10 minutes, then allow the crucible to
cool for 10 minutes. Weigh it and the contents, recording the mass.
CHEM 403 – Exp 6
Name:______________________________ Section: _________
Data & Calculations
Unknown # __________
Mass of crucible
__________ g
Mass of crucible and unknown
__________ g
Mass of unknown
__________ g
Mass of crucible and unknown after heating
__________ g
Loss of mass of sample
__________ g
Based on the loss of mass, this sample is a:
(circle your choice)
carbonate
hydrogen carbonate
Mass of crucible and solid chloride
__________ g
Mass of solid chloride
__________ g
Experimental Mass Ratio
mass of chloride : mass of hydrogen carbonate
(if your sample is a hydrogen carbonate)
__________:1
mass of chloride : mass of dry carbonate
(if your sample is a carbonate)
__________:1
Comparison to Theoretical Mass Ratios (from prelab assignment)
If your unknown in a hydrogen carbonate, compare your experimental mass ratio to the two
hydrogen carbonate theoretical mass ratios; if your unknown is a carbonate, compare your
experimental mass ratio to the two carbonate theoretical mass ratios.
Identity of unknown based on match to theoretical mass ratio: _____________
Discuss your results. How well did your experimental mass ratio match one of your theoretical
mass ratios? Are you confident in your identification of your unknown?
CHEM 403 – Exp 6
Name:______________________________ Section: _________
Prelab Assignment
Atomic masses: Na ________; K ________; H ________; C ________; O ________; Cl ________
Molar masses: NaHCO3 ___________ g
Na2CO3 ___________ g
NaCl ___________ g
KHCO3 ___________ g
K2CO3 ___________ g
KCl ___________ g
There are four theoretical mass ratios between the unknown compounds and the amount of chloride
salt formed as a product. These can be calculated prior to the experiment using Reactions 1 and 2.
2NaHCO3(s) → Na2CO3(s) + H2O(g) + CO2(g)
Na2CO3(s) + 2H+(aq) + 2Cl−(aq) → 2NaCl(s) + H2O(l) + CO2(g)
(1)
(2)
Considering the overall reaction from the original hydrogen carbonate in Reaction 1 to the chloride
salt in Reaction 2, complete the following mole ratios and mass ratios for the two possible
hydrogen carbonates.
1 mole NaHCO3 → __________ moles NaCl
__________ g NaHCO3 → __________ g NaCl
mass ratio __________:1
(NaCl to NaHCO3)
1 mole KHCO3 → __________ moles KCl
__________ g KHCO3 → __________ g KCl
mass ratio __________:1
(KCl to KHCO3)
Considering only Reaction 2, complete the following mole ratios and mass ratios for the two
possible carbonates.
1 mole Na2CO3 → __________ moles NaCl
__________ g Na2CO3 → __________ g NaCl
mass ratio __________:1
(NaCl to Na2CO3)
1 mole K2CO3 → __________ moles KCl
__________ g K2CO3 → __________ g KCl
mass ratio __________:1
(KCl to K2CO3)
CHEM 403
Exp 6
Experiment 6:
IDENTIFICATION OF A COMPOUND BY MASS RELATIONSHIPS*
In the previous experiment, you learned how to find the formula of a compound by analysis
for the elements it contains. When chemical reactions occur, there is a relationship between the
masses of the reactants and products that follows directly from the balanced equation for the
reaction and the molar masses of the species that are involved. In this experiment, you will use this
relationship to identify an unknown substance. Your unknown will be one of the following
compounds, all of which are salts:
NaHCO3
Na2CO3
KHCO3
K2CO3
In the first part of the experiment, you will be heating a weighed sample of your compound in a
crucible. If your sample is a carbonate, there will be no chemical reaction that occurs, but any small
amount of adsorbed water will be driven off. If your sample is a hydrogen carbonate, it will
decompose by the following reaction, using NaHCO3 as the example:
2NaHCO3(s) → Na2CO3(s) + H2O(g) + CO2(g)
(1)
In this case, there will be an appreciable decrease in mass, since some of the products will be driven
off as gases. If such a mass decrease occurs, you have strong evidence that your sample is a
hydrogen carbonate, either NaHCO3 or KHCO3.
In the second part of the experiment, you will treat the solid carbonate in the crucible with
HCl, hydrochloric acid. There will be considerable effervescence as CO 2 gas is evolved; the
reaction that occurs, again using Na2CO3 as our example, is:
Na2CO3(s) + 2H+(aq) + 2Cl−(aq) → 2NaCl(s) + H2O(l) + CO2(g)
(2)
(Since HCl in solution exists as ions, the equation is written in terms of ions.) You will then heat
the crucible strongly to drive off any excess HCl and any water that is present, obtaining pure, dry,
solid NaCl as your product.
To identify your unknown, you will need to find the molar masses of the possible reactants
and final products. For each of the possible unknowns there will be a different relationship between
the mass of the original sample and the mass of the chloride salt that is produced in Reaction 2. If
you know your sample is a carbonate, you need only be concerned with the mass relationships in
Reaction 2, and should use as the original mass of your unknown the mass of the carbonate after
it has been heated. If you have a hydrogen carbonate, the overall reaction your sample undergoes
will be the sum of Reactions 1 and 2.
From your experimental data, you will be able to calculate the ratio of the mass of the solid
chloride to the mass of either the original hydrogen carbonate or the mass of the anhydrous
carbonate in your sample of unknown. From your calculation of the relative masses of solid
chloride to solid hydrogen carbonate or solid carbonate in Equations 1 and 2 you can calculate
*Adapted from Slowinski, E. J., Wolsey, W. C. Chemical Principles in the Laboratory 9th ed.
CHEM 403
Exp 6
what the theoretical ratio of those masses should be. Your observed value should match one of the
theoretical values and thus allow you to identify your unknown compound.
Procedure
Obtain an unknown from the instructor. Weigh a clean, dry crucible, and record its mass.
Place approximately 0.5 g of your unknown into the crucible, and record the total mass of the
sample with the crucible.
Place the crucible on a clay triangle supported by an iron ring. Light your Bunsen burner
away from the crucible, and adjust the burner so that you have a small flame. Holding the burner
in your hand, gently heat the crucible as you move the burner back and forth. Heat the crucible,
gently and intermittently, for a few minutes. Gradually increase the flame intensity, to the point
where clay triangle glows red. Heat for 10 minutes. Allow the crucible to cool for 10 minutes, and
then weigh it with the sample, recording the mass. At this point, the sample in the crucible is a dry
carbonate, since the heating process will convert any hydrogen carbonate to carbonate.
Place the crucible back on the clay triangle. Add about 25 drops of 6 M HCl, a drop at a
time, to the sample. As you add each drop, you will probably observe effervescence as CO2 is
produced. Let the action subside before adding the next drop, to keep the effervescence confined
to the lower part of the crucible. You do not want the product to foam up over the edge. When you
have added all the HCl, the effervescence should have ceased, and the solid should be completely
dissolved. Heat the crucible gently for brief periods to complete the solution process. If all of the
solid is not dissolved, add 6 more drops of 6 M HCl and warm gently.
Heat the crucible, gently and intermittently, for about 10 minutes, to slowly evaporate the
water and excess HCl. If you heat too strongly, spattering will occur and you may lose some
sample. When the sample is dry, gradually increase the flame intensity, finally getting the bottom
clay triangle to a red heat. Heat at full flame strength for 10 minutes, then allow the crucible to
cool for 10 minutes. Weigh it and the contents, recording the mass.
CHEM 403 – Exp 6
Name:______________________________ Section: _________
Data & Calculations
Unknown # __________
Mass of crucible
__________ g
Mass of crucible and unknown
__________ g
Mass of unknown
__________ g
Mass of crucible and unknown after heating
__________ g
Loss of mass of sample
__________ g
Based on the loss of mass, this sample is a:
(circle your choice)
carbonate
hydrogen carbonate
Mass of crucible and solid chloride
__________ g
Mass of solid chloride
__________ g
Experimental Mass Ratio
mass of chloride : mass of hydrogen carbonate
(if your sample is a hydrogen carbonate)
__________:1
mass of chloride : mass of dry carbonate
(if your sample is a carbonate)
__________:1
Comparison to Theoretical Mass Ratios (from prelab assignment)
If your unknown in a hydrogen carbonate, compare your experimental mass ratio to the two
hydrogen carbonate theoretical mass ratios; if your unknown is a carbonate, compare your
experimental mass ratio to the two carbonate theoretical mass ratios.
Identity of unknown based on match to theoretical mass ratio: _____________
Discuss your results. How well did your experimental mass ratio match one of your theoretical
mass ratios? Are you confident in your identification of your unknown?
CHEM 403 – Exp 6
Name:______________________________ Section: _________
Prelab Assignment
Atomic masses: Na ________; K ________; H ________; C ________; O ________; Cl ________
Molar masses: NaHCO3 ___________ g
Na2CO3 ___________ g
NaCl ___________ g
KHCO3 ___________ g
K2CO3 ___________ g
KCl ___________ g
There are four theoretical mass ratios between the unknown compounds and the amount of chloride
salt formed as a product. These can be calculated prior to the experiment using Reactions 1 and 2.
2NaHCO3(s) → Na2CO3(s) + H2O(g) + CO2(g)
Na2CO3(s) + 2H+(aq) + 2Cl−(aq) → 2NaCl(s) + H2O(l) + CO2(g)
(1)
(2)
Considering the overall reaction from the original hydrogen carbonate in Reaction 1 to the chloride
salt in Reaction 2, complete the following mole ratios and mass ratios for the two possible
hydrogen carbonates.
1 mole NaHCO3 → __________ moles NaCl
__________ g NaHCO3 → __________ g NaCl
mass ratio __________:1
(NaCl to NaHCO3)
1 mole KHCO3 → __________ moles KCl
__________ g KHCO3 → __________ g KCl
mass ratio __________:1
(KCl to KHCO3)
Considering only Reaction 2, complete the following mole ratios and mass ratios for the two
possible carbonates.
1 mole Na2CO3 → __________ moles NaCl
__________ g Na2CO3 → __________ g NaCl
mass ratio __________:1
(NaCl to Na2CO3)
1 mole K2CO3 → __________ moles KCl
__________ g K2CO3 → __________ g KCl
mass ratio __________:1
(KCl to K2CO3)

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