The Relative Formula Mass of an Unknown Acid.

The Relative Formula Mass of an Unknown Acid

Introduction

I have been provided with a solution of unknown monobasic acid. By titration with a standard solution of sodium hydroxide I am to calculate its molarity and hence the relative formula mass of this unknown acid.

(We can assume the concentration of the sodium hydroxide is 0.1M)

Procedure

Using the funnel, rinse the burette with the acid solution and fill it accurately with the same solution until the 0cm3 mark (Do not forget to rinse and fill the tip first though).

Using the pipette filler, rinse the pipette with some of the sodium hydroxide solution and carefully transfer 25.0cm3 of the solution to a clean 250cm3 conical flask (Remembering to touch the bottom of the flask with the tip of the pipette for ten seconds to fully empty the pipette).

Add 3-4 drops of phenol red indicator to the sodium hydroxide solution. (The indicator changes from pink in alkaline solution, to yellow in acid solution. The endpoint is reached when the solution first changes colour to orange.)

Run the acid solution from the burette into the flask, with continuous swirling, until the solution just loses the pink colour and goes orange. This first result may be used as a trial run because you will probably overshoot the endpoint on the first attempt. Record the final burette reading.

Refill the burette with the acid solution, again to 0cm3.

Using the pipette, transfer 25.0cm3 of the sodium hydroxide solution to another clean conical flask. Add 3-4 drops of the phenol red indicator solution.