8/17/2019 Determination of the Molar Mass of a Weak Acid by
Titration Theoretical Lab
1/2
Determination of the molar mass of a weak acid by titration
Unit 2 Module 2 Skills tested: A&I
Aim: To determine the molar mass of a tribasic (triprotic) acid
by titration
Introduction
You will determine the molar mass of an unknown acid by
titration with standardised sodium hydroxide solution. Your unknown
acid
is, H3. The solid was dissol!ed in water and phenolphthalein was
added to it and the mixture titrated a"ainst standard #a$H
solution. The reaction between the acid and #a$H is as follows:
H3 % 3#a$H #a3 % 3H&$
Procedure
'. ei"h approximately ' " of the solid acid and dissol!e it in
& cm3 of water &. *inse the burette with the
standard 0!" mol dm#$ %a' solution and fill to the +ero
mark.
3. dd & drops of phenolphthalein solution and swirl
flask.
. Titrate until the first permanent pale pink colour is seen..
*ecord your burette readin"s.
-. *epeat the entire experiment two more times usin" a fresh
accurately known mass of your acid.
. Do not write a full lab re(ort
)esults
Trial ' Trial & Trial 3
/ass of acid 0" '.1'3 '.1& '.112olume of #a$H used 0cm3 3.11
3.&1 3.1
*reatment of results
'. &1 ml of the 1. mol dm43 #a$H solution was made. hat
mass of solid sodium hydroxide would ha!e to used to prepare
1..
mol dm43 #a$H solution5
&. 6erform calculations on each of the followin" trials
4 the 7 of moles of #a$H used4 the 7 of moles of acid present in
the reaction mixture
4 the molar mass of the acid
6resent your calculations in tabular form (a su""ested format
for your table is "i!en below)
Trial 7 ' & 3
/ass of acid used0"
2olume of #a$H used0 cm3
7 mol #a$H used
7 mol acid present
/olar mass of acid 0" mol4'
3. 8ased on its a!era"e molar mass, su""est the identity of this
triprotic acid.
. 9f the acid was a diprotic acid instead of triprotic, show how
this would affect the calculations by showin" the workin" usin"
the!alues from *)IA+ , %+-.
