Using the reaction below, calculate the percent yield obtained by a student when
ID: 784940 • Letter: U
Question
Using the reaction below, calculate the percent yield obtained by a student when reacting 5.0 g of 3-methoxysalicylic acid with 3.9 mL of iodomethane in the presence of 10.3g of potassium carbonate and obtaining 4.5 g of product, 2,3-dimethoxymethyl benzonate. First determine the limiting reagent and theoretical yield.
Explanation / Answer
25
Review of Calculations for Organic Reactions (#1b)
In your experiments the quantities of many r
eactants are given in mass or volume, though
chemicals react in mole ratios, because it is not
possible to measure a quantity in moles easily.
You will have to convert mass or volume (mass
= volume x density) to moles before beginning
an experiment. You will also have to recognize
stoichiometric relationships between reactants
and products, which is based on mole ratio, in order to
be able to calculate
the theoretical yield of
product you expect to isolate. You will apply th
e knowledge from this session to complete the
theoretical yield calculation in your
prelab reports and postlab reports.
In this lab you are expected to be able to
differentiate between a limiting reagent and excess
reagents, solvents and catalysts which are crucia
l for the reaction to occur and go to completion.
A
limiting reagent
is a reactant that has the lowest num
ber of moles of all reactants in the
chemical reaction and once it is completely cons
umed the reaction terminates. An excess reagent
is a reactant that has a
higher number of moles and therefore is
not used up when a reaction goes
to completion. Solvents and catalysts are not invo
lved in the determination of limiting reagent.
An example of calculations that you will be
expected to perform is shown using the
reaction of phenol with nitric ac
id. Preparation of picric acid
requires the nitration of phenol.
Given 5.00 grams of phenol and 15.0 mL concentr
ated nitric acid (15.9 M), one can determine
the
MAXIMUM
theoretical amount of picric acid formed.
The first step in this process is to
determine which reagent is the
limiting reagent
. To do this you simply
calculate the number of
moles of each available reagent. When calculati
ng limiting reagents, there are a few important
details. For instance, you should realize that if
you use a reagent as a solv
ent in a reaction, it is
never the limiting reagent. Also if you are
using a reagent that
is a catalyst, it will
not
be the
limiting reagent. Another very impo
rtant detail is the stoichiometry
of a reaction. For instance, in
the reaction shown you will
notice that 3 moles of nitric acid
are used for every one mole of
phenol and you only form one mole of product.
From the calculations above, notice that there is
less phenol than nitric
acid. Thus you can only
form 0.0531 moles of picric acid. So
once all the phenol has been us
ed up in the formation of the
picric acid, there will be some nitric acid left
which will not have any substrate to react with and
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