Procedure: Setting Up: Add 1.0 g of p-aminobenzoic acid and 10 mL of absolute et

Question

Procedure: Setting Up: Add 1.0 g of p-aminobenzoic acid and 10 mL of absolute ethanol to the round-bottom flask containing a stir bar. Stir the mixture until the solid is completely dissolved. Add 1 mL of concentrated [18M] sulfuric acid dropwise to the ethanolic solution of p-aminohenzoic acid, equip the flask with a condenser, and set up the apparatus for heating under reflux using the heating mantle. Reaction: Heat the mixture under gentle reflux for 30 min, stirring all the while. If any solid remains in the flask at this time, remove the flask from the heat source and allow the mixture to cool for 2-3 min. Add an extra 3 mL of ethanol and 0.5 mL of concentrated H2SO4, to the reaction flask and resume heating under reflux. After the reaction mixture becomes homogeneous, continue heating it under gentle reflux for another 30 min. For the experiment to succeed, it is important that all of the solids dissolve during the reflux period. Work-Up and Isolation: Allow the reaction mixture to cool to room temperature and then pour it into a beaker containing 30 mL of water. Basify the mixture to a pH of about 8 by slowly adding 10% aqueous sodium carbonate with stirring. Be careful in this step as frothing occurs during the neutralization. You should calculate beforehand the approximate volume of 10% aqueous sodium carbonate that will be required to neutralize the total amount of sulfuric acid you used. Collect the crude benzocaine that precipitates by vacuum filtration. Use three 10-mL portions of cold water to rinse the solid from the beaker and wash the filter cake. Air-dry the product. Purification: Weigh the crude product and recrystallize from methanol/water pair of solvents: Place the solid in a 50 mL Erlenmeyer flask. Add the minimum amount of hot methanol to dissolve it. Keep on a steam bath. Slowly add hot water to the solution using a disposable pipette just until the solution becomes cloudy and fails to clear up when it is swirled. To the hot solution on the steam bath add hot methanol, drop wise, until the turbidity or crystals just disappear. Allow the solution to cool to room temperature and then cool it in an ice-water bath for 10-15 min to complete crystallization. Isolate the crystals by vacuum filtration, wash them with 5-10 mL of cold water, and allow them to air-dry. Weigh the crystals and calculate the percentage yield.

If you end up adding 1.5 mL of concentrated sulfuric acid (18 M), how many mL of 10% aqueous sodium carbonate would be required to neutralize the reaction mixture?

A. 1.5 mL

B. 3 mL

C. 15 mL

D. 29 mL

Explanation / Answer

1.5 mL of concentrated sulfuric acid (18 M)

10% aqueous sodium carbonate

Reaction equation-

H2SO4 (aq) + Na2CO3 (aq) --> H2CO3 (s) + Na SO4 (aq)

Find the molarity of 10% aqueous sodium carbonate

10% aqueous sodium carbonate means 10 g of sodium carbonate in 100ml water

Molarity = moles/L

Convert 10 g of sodium carbonate to moles

The molar mass of sodium carbonate = 106 g/mol

Number of moles in 10g = 10g/106g/mol = 0.094 moles

Molarity = 0.094 moles/0.1L = 0.94M

Now calculate volume of Na2CO3 needed to neutralize 1.5 ml 6M H2SO4-

We calculate it by applying equation-

M1V1 = M2V2

6M x 1.5 = 0.94 x V2

V2 = 6M x 1.5ml/0.94 M = 28.72 ml

So D) 29 ml is correct option.

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