Experiment M02
Preparation of benzoic acid from benzamide
Name:
Seat No.:
Date:
Grade:
Results
Colour of crystals obtained: White
Shape of crystals: Needle-like
Melting point of the crystals: 122 oC
Mass of benzamide used: 3.03 g
Mass of crystal product obtained: 1.72g
Molecular mass of benzamide (C6H5CONH2) = 121
Molecular mass of benzoic acid (C6H5COOH) = 122
1.72
Percentage yield = 122 × 100% = 56.30%
3.03
121
P.3
Experiment M02
Preparation of benzoic acid from benzamide
Interpretations
1.
Is benzamide soluble in (i) water,
(i)
2.
(ii) aqueous NaOH?
Slightly soluble
(ii) Yes
Write equations for the reactions involved in procedure b.
O
C
NH2
O
ONa
C
+
NaOH
+
NH3
benzamide
3.
After boiling for 20 minutes, what is the appearance of the reaction mixture?
Pale yellow ppt is observed.
4.
In step c, how can you tell when the acid has been added in excess?
The mixture turns blue litmus paper red.
5.
What is the purpose of washing the crystals in step d? Why is the amount of water used kept
at a minimum and kept cold?
To wash away the impurity adheres on the crystals.
Prevent the lost of the product. (Solubility decrease as the temperature of water decrease)
6.
The melting point obtained by students is usually lower than the true value. What may be
the reason(s) for this?
What improvements could be made?
As the crystals are not pure and contain some impurities.
This may be improved by recrystalize the crystals more than 1 time and wash the crystal
several times.
7.
What criteria are usually used to judge whether a preparation method is good.
1.
The yield is high.
2.
High purity.
3.
Economical.
4.
Rate of reactions is fast.
5.
Involve a few steps.
P.4