A mixture of equal parts of a carboxylic acid, a phenol, and a neutral substance
ID: 978524 • Letter: A
Question
A mixture of equal parts of a carboxylic acid, a phenol, and a neutral substance is to be separated by extraction from an ether solvent. Bicarbonate converts carboxylic acids (but not phenols) to ions. Dissolve about 0.18 g of the mixture (record the exact weight) in 2 mL of t-butyl methyl ether or diethyl ether in a reaction tube (tube 1). Then add 1 mL of a saturated aqueous solution of sodium bicarbonate to the tube. Use the graduations on the side of the tube to measure the amounts, since they do not need to be exact. Mix the contents of the tube thoroughly by pulling the two layers into a Pasteur pipette and expelling them forcefully into the reaction tube. Do this for about 3minutes. Allow the layers to separate completely and then draw off the lower layer into another reaction tube (tube 2). Add another 0.15 mL of sodium bicarbonate solution to the tube, mix the contents as before, and add the lower layer to tube 2.
Use the above procedure to obtain the answer to the question below.
A. Calculate the minimum volume of 10%(m/v) sodium bicarbonate needed to react with all of the carboxylic acid. Show your work. (Hints: How much of the mixture is carboxylic acid? How many moles of the acid are present? What is the mole ratio for the reaction?)
Explanation / Answer
The molar mass of the carboxylic acid (RCOOH) is required, let it be M g/mol.
Amount of RCOOH in the mixture
= 0.18/3
= 0.06 g
No of moles of RCOOH
= 0.06/M moles
The reaction is: RCOOH + NaHCO3 = RCOONa + H2CO3
According to the reaction, 1 mole of NaHCO3 reacts with 1 mole of RCOOH.
So, (0.06/M) moles of NaHCO3 reacts with (0.06/M) moles of RCOOH.
(0.06/M) moles of NaHCO3
=( 0.06/M) * molar mass of NaHCO3
=( 0.06*84/M) g of NaHCO3
The bicarbonate solution is a 10%(m/v) solution, i.e., 1 g bicarbonate is dissolved in 10 mL water.
Volume of sodium bicarbonate needed to react with all of the carboxylic acid
= 10*(0.06*84/M) mL
= (50.4/M) mL
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