we prepared a standard solution of Na2CO3 in a standard volumetric flask, which is as well solution of definite concentration since the substance we utilized, for instance sodium carbonate is a primary standard. Thereafter, we discussed those substances which cannot be employed as primary standard.
In this chapter, we shall describe such solutions and how to prepare them and standardize them that is, determine their accurate concentration. We will also learn how to prepare an approximate 1.0 molar solution of hydrochloric acid.
If we remind experiment la wherever a distinction was made between certain substances whose concentration can accurately be determined and prepared and certain other substances whose concentrations can't be accurately prepared. Such substances whose concentration can't be accurately prepared are as well significant and we must hence be able to prepare them and determine their accurate concentration.
These is done via preparing an approximate concentration and then standardize them, that is, react them, which in practice means titrating them, through a standard solution whose accurate concentration is known.
1. Measure accurately 20cm3 of pure concentrated HCl via the utilize of a dry measuring cylinder or a burette. Caution. Be careful not to spill the acid on your person or on the bench. Acids are extremely corrosive and 'eat' anything it comes in contact through Refer to chapter 1 on safety and regulations.
2. Pour the acid into a 250cm3 volumetric flask containing about 100cm3 of distilled water gradually but gently. Caution. This process of adding acid to water is the preferred process. Never add water into acid. We could cause an explosion. This is since the dissolution of acid in water is exothermic, that is, it liberates (the beaker becomes hot) energy which might be so great as to cause an explosion.
Shake the mixture and permit the mixture to cool to room temperature. Make up to the 250cm3 mark via more distilled water from the wash bottle. Them, shake thoroughly. Stopper and let to cool to room temperateness.
Observations and Results:
1. As the acid was being steadily added, there was hissing sound accompanied via several observation of coloureless vapour.
2. In addition, the volumetric flask was getting hotter.
Treatment of Results:
The concentration of the pure HCl utilized is 11.7 molar. If 20.0cm3 V1 of 11.7 molar of C1 of HCl acid is diluted to 250cm3 V2 , then the new concentration C2 will be C1V1 = C2 V2
11.7 x 20 cm3 = C2 x 250cm3
C2 = 11.7 x 20 moles dm-3
= 0.936 moles dm-3
Standardization of an approximate 1.0 molar solution of hydrochloric acid by standard solution of 0.5M Na2CO3 solution.
Solution of Na2CO3 prepared
in experiment IA
Solution of HCI prepared in
Methyl orange indicator
Conical flask 250cm3
White tile or white
Sheet of paper, clamp
1. Put up a clamp with the 50cm3 capacity burette. Rinse the burette with the solution of the acid.
2. Fill the burette by the acid solution through the aid of a glass funnel. Fill it more than the 50cm3 mark, take away the funnel and release the tap of the burette such that some of the solution will drop out into a waste beaker. Stop the release when the liquid reaches the 50cm3 marks, this will expel any air bubble trapped within the body of the liquid
3. Rinse the pipette with the 0.5 molar Na2CO3 solution. Pipette out 25cm3 of the solution from the volumetric flask into a clean 250cm3 conical flask. Add 2 or 3 drops of the methyl orange indicator. Place the conical flask containing the Na2CO3 solution on the white tile or sheet of paper.
4. Run the acid from the burette into the base until the colour of the solution transforms to pink.
5. Record the reading of the burette.
6. Repeat steps 2, 3, 4, and 5 two more times.
7. Prepare a table as shown in section 4.5 of this unit and find the average volume of the acid used in the titration. R
The values used here are hypothetical
Volume of acid
The average volume of the acid used = 24.40 + 24.40 / 2
We are carrying out this calculation with no previous introduction on problem solving. I would suggest you note the method of calculation for now. More detailed explanation would be given in the next unit. Equation of the Reaction
2HCl + Na2CO3 -> 2NaCl + CO2 + H2O
From the equation, 2 moles of HCl react through 1 mole of Na2CO3. Using the formula we are familiar through
CAVA/ CB VB = nA/nB , where
CA = Concentration = unknown
VA = Volume of the acid obtained from titration = 24.40cm3
CB = Concentration of the Na 2CO3 = 0.5moldm-3
VB = Volume of the Na2CO3 =.25cm3
nA = No. of moles of acid from the balanced equation
nB = No. of moles of Na2CO3 from the balanced equation
.'. CA x 24.40 / 0.5 x 25 = 2 / 1
CA = 0.5 x 25.2/24.40
= 25/ 24.40 moldm-3
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