|In general, we say that a reaction will go to completion (be quantitative)
if one of the reactants is completely consumed by the reaction. There
are several ways that a reactant may be consumed. This will occur if
(a) a precipitate is formed, (b) a weakly dissociated substance such as water
or a weak acid is formed, and (c) a gas (volatile substance) is formed.
|In this experiment you will allow sodium bicarbonate (baking soda)
to react with hydrochloric acid for the purpose of obtaining a high yield
of sodium chloride.
|You will use an accurately measured mass of NaHCO3 and
enough dilute HCl to completely react with it. You will then isolate
the NaCl from the other products and determine its mass. The theoretical
yield can be calculated by using the mole and mass ratios obtained from the
balanced equation for the reaction. The percentage yield can then be
determined by comparing the experimental yield with the theoretical yield.
|1. To prepare and determine the percentage yield of sodium
2. To gain an understanding of mass relationships in chemical reactions.
|Materials: Evaporating dish, watch glass, balance, sodium bicarbonate,
hydrochloric acid, wire guaze, ring stand, wash bottle.
|Trial 1||Trial 2|
|Mass of evaporating dish, watch glass, and NaHCO3||________ g||________ g|
|Mass of evaporating dish and watch glass||________ g||________ g|
|Mass of NaHCO3 used||________ g||________ g|
|Mass of evaporating dish, cover, and NaCl 1st||________ g||________ g|
|2nd||________ g||________ g|
|3rd||________ g||________ g|
|Mass of salt obtained (experimental)||________ g||________ g|
|1. Calculate the theoretical mass of NaCl that should
have been obtained. Show mathematical calculations in your report.
2. Determine the percentage yield.
| The reaction which you used to prepare the salt in this experiment
should have proceeded to completion. This type of reaction is also used
to prepare weakly dissociated acids. It can be seen from the equation
that the reactants are a stong acid and the salt of a weak acid. One
product of such a reaction is a weak acid. We can assume that when
a strong acid reacts to form a weak acid that ions of the strong acid are
removed from the solution. Hence, the reaction proceeds essentially
to completion. In this reaction, the weak acid formed as a product
was carbonic acid. H2CO3. However, carbonic acid decomposes
into CO2 and H2O.
|One of your objectives in this experiment was to compare the experimental
and calculated yield of product. The following example is provided to
assist you in making the required calculations. The theoretical yield
is calculated as follows:
|From the balanced equation it can be seen that 1 mole of NaHCO3
produces 1 mole of NaCl. Assume that 2.00 grams of NaHCO3 were
|Moles of NaHCO3 =
2.00 g = 0.0238
molecular mass 84.01 g/mole
|Since the NaHCO3: NaCl mole ratio is 1:1, then 0.0238 moles
of NaHCO3 would form 0.0238 moles of NaCl. The mass of this
amount of NaCl would be:
|grams of NaCl = n X mm = 0.0238 mol
X 58.44 g/mol = 1.39 grams of NaCl
|The theoretical yield of NaCl expected is 1.39 grams.
|Assume also that the student obtains 1.30 grams of NaCl. Their percentage
yield would then be
what they actually obtained = 1.30 grams * 100%
what they should have obtained 1.39 grams