-
Allow the reaction mixture to cool and then transfer it to a 250 cm3 beaker. Add an excess of
dilute HCl with a dropper, swirling continuously until a copious amount precipitate is obtained.
- Filter the resulting solution under suction, wash the crystals thoroughly with a minimum amount of
ice cold water.
- Recrystallise the crude solid from boiling water, filter to obtain pure crystals of the product.
- Dry these crystals thoroughly between filter paper until no further sign of dampness is seen on the paper. Inspect the shape of the crystals using a hand lens. Measure the mass of the product.
- Using the melting point apparatus, determine the melting point of your product.
Results : Gas evolved = _________________ammonia________________________
Test for gas = __________________pH paper________________________
Yield (mass in gram) = _______________0.88 g______________________
Colour of crystals obtained = ______________white___________________
Shape of crystals obtained = ____________irregular shape______________
Questions :
- Is benzamide soluble in (a) water & (b) aqueous sodium hydroxide ?
Benzamide is not soluble in water, but soluble in aqueous sodium hydroxide.
- Write equations for the reactions involved.
- What is the function of the glass tubing in step 2 ?
When the solution is boiling, releasing the pressure.
- After boiling for 15 minutes in step 2, what is the appearance of the reaction mixture ? Explain.
The crystal will be dissolved completely. Because the NaOH react with the benzamide and give the salt form of benzoic acid.
- How can you tell when an excess amount of HCl has been added in step 3 ?
Neutralize the NaOH and convert the salf form of benzoic acid to molecular form of benzoic acid.
- What is the purpose of washing the crystals in step 4 ?
Why should the water used be kept cold and at a minimum amount ?
Wash away the impurities. Is should kept cold and at minimum amount because should keep a low solubility of crystal to the solvent.
- The melting point obtained in the experiment is usually lower than the literature value.
What are the reasons for this ? What improvements could be made ?
The impurities in the crystal. Recrystallization can improve the purity of the product.
- Calculate your percentage yield.
No. of mol of benzamide = 3 / (77+12+16+14+2)
No. of mol of benzamide = 0.02479 mol.
No. of mol of benzoic acid = 0.88 / (77+12+16x2+1)
No. of mol of benzoic acid = 0.007213 mol.
Percentage of yield = 0.007172/0.02479 x 100% = 29%
- What criteria are usually used to judge whether a preparation method is good ?
Comment on the suitability of the above experiment to be employed as a preparatory method for
benzoic acid.
The percentage of yield is usually used to judge whether a preparation method is good. The above experiment is not quite suitable to be employed as a preparatory method for benzoic acid, because the yield is too low.
