Lab 4 – Gravimetric Analysis of a Carbonate/Bicarbonate Decomposition
Product – “Which Salt”
Chemistry B1A / by Daniel & Vaughan/Summer 2015
There are many, many kinds of questions which can be answered with a well-founded
understanding of moles and their use through balanced equations. The bread-and-butter
calculation of a chemist predicts the amount of product to be obtained based on the amounts of
reactants used. Extending that basic use to solving a problem is the focus of this lab.
The experimental
You will be examining the decomposition of an unknown carbonate or bicarbonate salt. The
unknown compound is either sodium bicarbonate, potassium carbonate, or sodium carbonate
monohydrate. Note when calculating the molar mass of a hydrate, you must include the water in
your molar mass. You should be able to determine which of these three compounds your
unknown is which by reacting the unknown salt with an excess of hydrochloric acid, which will
cause the following reactions to occur:
a. NaHCO3 +
HCl
⟶ NaCl + H2O + CO2
b. K2CO3 +
2HCl ⟶ 2KCl + H2O + CO2
c. Na2CO3⋅H2O + 2HCl ⟶ 2NaCl + 2H2O + CO2
The extra HCl, the H2O, and the CO2 will be removed by evaporation (the CO2 will mostly
disappear without any help, as it will bubble out of solution). This will leave behind only the
product salt (NaCl or KCl).
By keeping track of the mass of unknown you begin with, you should be able to predict how
much product would be made if it matched any of the salts above. The one you actually come
closest to should be the identity for that unknown.
Please take good notes in your lab notebook for this experiment.
Procedure
1. Obtain a clean, dry 125 mL Erlenmeyer flask, label with you and your partner’s initials,
unknown letter, and your lab section, weigh it on the balance and record the mass. To this
add 1 g. (±0.2 g) of an unknown salt; record the mass to 3 decimal places. Record the
unknown letter/number identifier. Record any observations made of the unknown and any
of the following reactions.
2. Back at your bench, add about 10 mL of distilled water and gently swirl your flask to
dissolve the solid. Be careful not to get solid on the walls of the container. Your
instructor will show you how to rinse and fill a buret with 1.5 M HCl. This is 1.5 molar
HCl, which is 1.5 moles of HCl solute dissolved in enough water to make one liter of
solution.
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3. Slowly add about 8 mL of the HCl solution to your unknown. Gently swirl the
Erlenmeyer flask to mix the substances. Record your observations that show a reaction is
taking place. Now slowly add more HCl, drop by drop, until you no longer see bubbles
produced. With a stirring rod and about 1 cm (~1/4”) of pH paper test your solution to be
sure it is acidic with a pH less than 7. You should not need more than a total of 11 mL of
HCl.
4. Attach the flask with a clamp to a ring stand so the flask can be heated with a hot plate.
Attach the stem of a funnel to the hose coming out of the side arm on a sink aspirator, and
clamp the funnel so that it is upside down above the Erlenmeyer flask. Be sure the hose
does not sit on the hot plate to avoid burning the hose. Place a beaker under the faucet.
(Warning, item within 17 cm (~1 foot) of the sink may get wet). Heat the Erlenmeyer
flask with a setting on the hot plate of about 1/3 of the maximum dial setting. You want a
gentle boil. If you smell an irritating odor while heating, turn on the faucet to vacuum the
gases above the flask. When the flask contents are about dry, turn off the hot plate.
5. Wait until the hot plate has cooled and your flask contents appear dry. Remove the
funnel, carefully remove the Erlenmeyer, and allow it to cool. Measure and record it’s
mass. Place the flask in the drying oven for at least 30 minutes, then cool it and record its
mass again. If the mass has changed by more than 0.020 g, repeat the oven heating.
Data
You will be doing 3 calculations for you unknown. Using the balanced equations above,
determine the theoretical yield in grams of product salt that could be made from each reactant
salt. For example, assuming your unknown is sodium bicarbonate:
g NaHCO3 reactant X
1 𝑚𝑜𝑙 𝑁𝑎𝐻𝐶𝑂3
84.01 𝑔
1 𝑁𝑎𝐶𝑙
𝑋 1 𝑁𝑎𝐻𝐶𝑂3 𝑋
58.44 𝑔 𝑁𝑎𝐶𝑙
1 𝑚𝑜𝑙 𝑁𝑎𝐶𝑙
= g product (NaCl)
This calculates the theoretical yield of NaCl produced if your unknown was NaHCO3. You will
also calculate the theoretical yield of NaCl if Na2CO3 H2O was the unknown and the theoretical
yield of KCl if K2CO3 was the unknown. You will then compare these theoretical yields with
your actual experimental yield. This should help you to decide which compound your unknown
is, sodium bicarbonate, sodium carbonate monohydrate, or potassium carbonate.
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Table 1 – Example of data table
Trial 1 – Unknown A
Mass Empty Erlenmeyer
Xxxxxxxxxxxxxxxxxxxxxxxx
Flask
Mass empty Erlenmeyer
Xxxxxxxxxxxxxxxxxxxxxxxx
Flask with Unknown A
Other things here
Xxxxxxxxxxxxxxxxxxxxxxxx
……
Percent Error if Unk A is
NaHCO3
Percent Error if Unk A is
Na2CO3
Identity of Unk A
Xxxxxxxxxxxxxxxxxxxxxxxx
Xxxxxxxxxxxxxxxxxxxxxxxx
Xxxxxxxxxxxxxxxxxxxxxxxx
Xxxxxxxxxxxxxxxxxxxxxxxx
1. Calculate the three percent errors for each unknown using the three theoretical yields and
your actual yield. Your table should have a title stating what is in the table. (e.g., Mass
analysis of unknown carbonate/bicarbonate reaction with HCl)
𝐴𝑐𝑡𝑢𝑎𝑙 𝑌𝑖𝑒𝑙𝑑−𝑇ℎ𝑒𝑜𝑟𝑒𝑡𝑖𝑐𝑎𝑙 𝑦𝑖𝑒𝑙𝑑 𝐾𝐶𝑙
% Error if Unk A is K2CO3 = |
| 𝑥 100% =
𝑇ℎ𝑒𝑜𝑟𝑒𝑡𝑖𝑐𝑎𝑙 𝐾𝐶𝑙
2. One problem with potassium carbonate is it must be heated before use because it absorbs
water from the air. If your unknown was potassium carbonate and it had water in it when you
weighted it, how would this affect your calculated yield? How could this change your
identification of your unknown. (One way to think about this is in your theoretical yield
calculation, how would water change the actual mass of potassium carbonate weighed out, and
how would that change the mass of your theoretical product.)
2. Discuss sources of error and how they would change the actual yield. Be specific.
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