Prelab Assignment Experiment 3: Answers
1. In this experiment the intermediate product, sodium benzoate, is converted to benzoic acid
by reaction with hydrochloric acid. Why can acetic acid not be used for this purpose?
The simplest definition of an acid is that it generates protons when dissolved in water (we
shall see some slightly more sophisticated definitions later in the course). We measure the
strength of an acid by the value of Ka, which is the equilibrium constant for the dissociation
reaction:
HA + H2O ⇄ H3O+ + A-
𝐾𝑎 =
[H3 O+ ][𝐴− ]
[HA]
Benzoic and acetic acids are both organic acids, so we expect them to be weak; that is, to
generate only a little H+ in solution. The Ka values for acetic and benzoic acids are rather
similar, roughly 1.76 x 10-5 and 6.4 x 10-5 respectively at room temperature. These values tell
us that benzoic acid is the slightly stronger acid, so it dissociates more than acetic acid in
solution.
In the experiment we want to prepare undissociated, crystalline, benzoic acid. To do this, we
react benzoate ion with protons according to the scheme given in the manual. However, the
reaction that makes benzoic acid can go both ways (forwards or backwards), with the acid
that we form dissociating again to regenerate the benzoate ion. We need to suppress the
dissociation as much as possible, since when the acid dissociates it stays in solution and
therefore passes through the Buchner funnel and is lost. We can do this by using a strong
mineral acid, such as HCl, to supply a large excess of hydronium ion and force the
equilibrium to the left (think Le Chatelier if you've come across this…). Acetic acid won't
work properly because it will generate too low a concentration of hydronium ions, so the
amount of undissociated benzoic acid present when acetic acid is used is very low.
2. Calculate the molecular weights of benzoic acid and ethyl benzoate.
Benzoic acid: C7O2H6 → 84 + 32 + 6 = 122 g mol-1
Ethyl benzoate: C9O2H10 → 108 + 32 + 10 = 150 g mol-1
3. In a particular reaction 20.0g of ethyl benzoate was hydrolysed and 13.5 g of benzoic acid
produced. Calculate the percentage yield of benzoic acid.
Begin by calculating how many moles of ethyl benzoate are present at the start of the
reaction. Then check to see how many moles of benzoic acid can theoretically be made from
this amount of starting material. Next calculate how many grams of acid this is, and finally
calculate the percentage yield.
Moles of ethyl benzoate = 20g /150 g mol-1 = 0.133
This is also the number of moles of acid produced, as the reaction stoichiometry is 1:1
Hence the theoretical yield is 0.133 mol x 122 g mol-1 = 16.26g
and the percentage yield = 13.5/16.26 x 100 = 83%