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

At Experiment 1

Specimen results

Mass of watch glass = 12.32 g

Mass of watch glass plus sodium carbonate= 15.02 g

Mass of sodium carbonate = (15.02-12.32)=2.70g

The 250cm3 solution in the measuring flask contained 2.70g of the carbonate.

One litre(1000 cm3 or 1dm3) of the solution contained 2.70×1000/250 =10.80g.

Relative molecular mass of Na2CO3 =106

One mole of Na2CO3 has a mass of 106g.

10.80g of Na2CO3 = 10.80/106g mol-1=0.102 mol.

0.102 mol of Na2CO3 were present.

The concentration of the solution=0.102mol dm-3 (0.102M)Na2CO3.

 

@ You may produce dilute solutions of sulphuric and hydrochloric acid by diluting concentrated acids.

Q1. How to make 0.1 M Sulphuric acid (Ans) Carefully add 5.5 cm3 of concentrated sulphuric acid slowly with stirring to 200 cm3 of distilled water and then dilute to a total volume of 1000cm3(ml)

 

## CAUTION! Always add acid to water and not water to acid.

 

Q2. How to make 0.1 M Hydrochloric acid. (Ans) 8.5 cm3 of concentrated hydrochloric acid is added to water and made up to 1000cm3. (1L=1000cm3)

 

5.4 Carrying out a titration

 

Titration is the process of determining the concentration of a substance in a solution. Titration is performed by adding to a known volume of the solution a standard reagent of known concentration in standard measured amounts until a reaction of definite and known proportion is completed.

 

A colour change is observed at the reaction’s end-point.

The end-point is the stage where the acid and base have reacted completely.

 

@ Titration is followed by calculations to determine the unknown value.

 

@ In an acid-base titration the acid is put into a burette and a known volume of base into a conical flask. A few drops of an indicator solution are added to the flask to indicate the end-point.

The common acid-base indicators and there colour in acidic, neutral and alkaline media are shown in table.

Table

Indicator	Colour in different media
	Acid	Neutral	Base
Litmus paper	Red	blue/red	blue
Metyl orange	Pink	Orange	Yellow
Phenolphthalein	colourless	colourless	Pink

When carrying out a titration, a suitable indicator has to be chosen according to the properties of the particular acid and base for titrating. below table gives appropriate indicators for different acid-base titrations.

 

Table: Indicators suitability

Titration	Example	Indicator used
Strong acid and strong base	Sulphuric acid and sodium hydroxide	Any Indicator
Strong acid and weak base	Hydrochloric acid and sodium carbonate	Methyl orange
Weak acid and strong base	Oxalic acid and sodium hydroxide	Phenolph- thalein
Weak acid and weak base	Oxalic acid and sodium hydrogen- carbonate	No suitable indicator