탄자니아에서의 화학 수업교안(carrying out a titration)2 -/Indicator2

5.5 Procedures for carrying out titration

(a) Clean the pipette and burett. Mount the burette on a burette(or retort) stand.

(b) Pour the acid into the burette

(c) Pipette 25cm3 of base and transfer the solution into a conical flask. Add two to three drops of an appropriate indicator into the conical flask.

(d) Repeat part (c) to obtain three more conical flasks.

(e) Place one conical flask on a white tile(or paper) below the burette. Slowly run the acid from the burette into the flask drop by drop, while shaking the flask until the color of the indicator changes completely. This is the end point. Allow a few seconds for the acid to drain down from the sides of the burette.

(f) Record the final burette reading. This is Pilot titration. It gives you a rough estimate of volume of the acid.

(g) Repeat step ( c ) three more times using the basic solution in the other three conical flasks to obtain consistent volumes within the range of ± 0.2cm3.

(h) Calculate the average volume of acid used.

(i) Calculate the concentration of the unstandardized solution using the average volume and the formula :

MaVa/MbVb = na/nb (a=acid, b=base) Ma= Molarity of acid Va= Volume of acid Mb= Molarity of base Vb= Volume of base na= moles of acid nb= moles of base

for example

ex1. 25cm3 (ml) of 0.2M sodium hydroxide solution react with 30cm3(ml) of hydrochloric acid solution of unknown concentration. Calculate the molarity of the hydrochloric acid.

ans) HCl + NaOH → NaCl + H20

from the information given:

(Mb=0.2, Vb = 25cm3, Ma= x, Va= 30cm3, na =1 , nb =1)

by using the formula :MaVa/MbVb = na/nb,

Ma=MbVbna / Vanb

x= 0.038M

ex2. 25cm3 of 0.2M sodium hydroxide solution react with 25.03cm3 of sulphuric acid solution of unknown concentration. Calculate the molarity of the hydrochloric acid.

ans) H2SO4(aq) + 2NaOH(aq) → Na2SO4(aq) + H2O(l) 1 mole 2moles

 

Ma= x M ( Molarity of sulphuric acid)

Va= 25.03cm3 ( Volume of sulphuric acid)

Mb= 0.2 M(Molarity of base)

Vb= 25cm3(Volume of base)

na= 1 ( moles of acid)

nb= 2 (moles of base)

 

Using the formula:

MaVa/MbVb = na/nb

Ma=MbVbna / Vanb

= 0.2×25×1/25.03×2 = 0.1M

∴the molarity of sulphuric acid is 0.1M

5.6 Application of volumetric analysis

volumetric analysis finds useful application in the following areas of our daily lives. (In water authorities, In medicine, in agriculture, in industries)

 

@ Application of volumetric analysis include determination of amount of salts in water or urine; concentration of virus or bacteria in blood samples;

acidity of stomach fluids, fruits juices and vinegar.

It is also used to find percentage purities of various chemicals manufactured in industries; amount of antacid tablets that can neutralized stomach acids and composition of different substances in soil solution.