Hi there, hereÂ’s my bit of a contribution of a Chemistry SPA assessment. As the co-founder of this "society"
, I sincerely do hope that you guys will try your best to participate and contribute actively in this forum.
LetÂ’s share our ideas generously, and trust me, all of us will definitely benefit from this by learning and laughing at the mistakes weÂ’ve made during each SPA assessment in the long run. Remember, our ultimate aim is to see you guys excelling in SPA.
So I hope to see more and more contributions. Please, do introduce your friends from other colleges here. Peace!!
P.S And sgboy, thank you so much for being our threadÂ’s first supporter. Hydrogen gas does create a pop sound when its exposed to a burning splint! (Bravo..) Good luck for your O levels ok? ShouldnÂ’t be a problem for you, lah. WeÂ’ll get back to you once our term opens.. WeÂ’ll be moving onto QA then.
Cheerios, stcp
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LetÂ’s get down to serious stuff.
Name of School: Temasek Junior College
Subject of spa: Chemistry
Month of attempt: May
Skill: Skill C and D only
Number of attempts: 1/3
Topic on: Double Indicator Titrations
Set up: Large beaker, retort stand, solutions, conical flask, burette.
Table/Calculations:
Determining the mass of Sodium Carbonate in an aqueous solution containing sodium hydroxide and some solution I canÂ’t rememberÂ…
Typical double indicator problem… Go flip through your notes on ‘Mole Concept’ and everything will be a breeze.
You see, sodium hydroxide reacts with acid in 2 stages. In the first stage, hydrogen carbonate is formed, right? (Using phenolphthalein as an indicator) If the reaction is allowed to proceed further, using SMO, the hydrogen carbonate decomposes into carbon dioxide.
Sodium Hydroxide reacts with the acid immediately after the 1st stage. K.. Aiya, just read your notes. Can one.
(The above is testing you on Skill C)
Precautions you must mention:
(Now for Skill DÂ…)
The procedure given us was that a measure mass of something and something was dissolved in 1dm³ of water in a
larger beaker.
25cm³ of whatever was measured out using a
measuring cylinder into a conical flask.
Five drops of SMO was added and the titration continued to the final end.
The same titration procedure was repeated another
three times.
Take note of whatÂ’s written in
bold. IÂ’ll address them nowÂ…
Now, a beaker is not a suitable apparatus to prepare the standard solution.
The precision of a beaker is very poor and the accuracy of the experiment depends on the accuracy of the concentration of the standard solution. A standard flask should be used.
NextÂ…. A measuring cylinder is not a suitable apparatus to measure out accurate volume of reaction mixture.
This is because its precision is poor. Since the volume measured out is only 25cm³ the imprecision will result in a large percentage error in volume. A pipette should be used.
Excess indicator is used!!
Too much indicator will affect the accuracy of the result because an acid base indicator is a weak acid or a weak base and will react with the reaction mixture. Only 1-2 drops of indicator can be used.
You donÂ’t need to carry out the titration 4 times.
If 4 readings are inconsistent, then the final results will also not be accurate. Instead the titration should be carried out until consistent readings (within +-0.10cm³) are obtained. Ask your why... Why are you carrying the titration out 4 times... Why should be pipette be used instead... etc...
Oh yah the reliability of your data must be included tooÂ… You can follow this format.
Only the ____ and _____ titration readings are used because they are consistent. (Within +- 0.10cm³)
The total calculated mass of _____ is ______, which is less than ______. This could be due to the fact that the solid had absorbed some moisture or impurities
Okay! Now for the extra stuff you must add inÂ…
Tap water/plain water should not be used to prepare the standard solution as water may contain impurities, which will react and affect the final results. Instead, de-ionized water should be used to prepare the standard solution.
The accuracy of the experiment could be affected by the difficulty to identify the end point color of both indicators. (Human error) A date logger or a pH meter should be used to determine the end point.
The container containing the standard solution should be capped or covered at all times when not in use. This is to prevent accidental contamination of standard solution.
Precautions for marking scheme: For the paper, a beaker, some solutions and burette is issued. Don’t be foolish enough to carry out the experiment. Spend all the time wisely on the written paper. I’ve confirmed this with my chemistry teacher already. Even though the apparatus are given to you, there is NO NEED for you to carry out the experiment ‘cos you’ll only land up with inaccurate readings.
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K, just my two cents worth. Do look forward to more contributions from you guys...
