CHEMISTRY

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[Announcements] [Introduce Yourself] [ICT] [Prasekolah] [LINUS] [KSSR] [UPSR] [PMR] [SPM] [STPM] [School] [Classroom] [Clubs & Uniformed Bodies] [JPN] [SPA] [SICT] [SPP] [SPS] [SPPK] [SPSPK] [SPI] [SPsK] [JNJK] [PPD] [Program] [Computer & Internet] [Let's Hear It From You] [Membergroup]

[Siti Norhayati Mohd Mokhtar]

i am really confused how to calculate the concentration of sodium thiosulphate. i know that we have to use the M1V1=M2V2 formula. i am a little bit confused about the value of V2. Is V2 the total volume of reacting mixture ( sodium thiosulphate + H2SO4) or the total volume of dilute sodium thiosulphate itself  ? Do we need to calculate the concentration of sodium thiosulphate before or after adding sulphuric acid?

[Wong Hee Ting]

V2 refer to the volume of sodium thiosulphate + volume of water

[Siti Norhayati Mohd Mokhtar]

but, from the practical book , the V2 is the volume of reacting mixture..

[Wong Hee Ting]

The formula M1V1 = M2V2 is use to calculate the Dilution of sodium thiosulpahte. We need to manipulate the concentration of sodium thiosulphate, that's why V2 is volume of sodium thiosulphate + volume of water.

[Siti Norhayati Mohd Mokhtar]

But in acid also contains water..when the diluted sodium thiosulphate reacts with acid, does the concentration changes?? By the way, thank you for your concern.. I actually agree with your opinion, but the practical book stated that the M2 is based on the reacting mixture.. I'm confuse

[Wong Hee Ting]

Cikgu Siti, the acid is the constant variable. volume and concentration are fix.

[Siti Norhayati Mohd Mokhtar]

The concentration of  sulphuric acid is still constant after mixing with sodium thiosulphate because of the constant volume for every experiment..the question is.. The concentration of sodium thiosulphate.. Maybe u can refer to practical book.. So you can understand the confusing part..

[Wong Hee Ting]

Let's look at the situation:

Example:
 volume & Concentration of sulphuric acid: 5 cm3 of 1.0 mol dm–3  {fix for every experiment}

Set I:
50 cm3 of 0.2 mol dm–3 Na2S2O3 solution [no water add in]
so the concentration of sodium thiosulphate is 0.2 mol dm–3

Now set II,
40 cm3 of 0.2 mol dm–3 Na2S2O3 solution + 10 cm3 of water
So what is the concentration of the sodium thiosulphate?

M1V1 = M2V2
(0.2)(40) = M2 (40+10)
M2 = 0.16 mol dm-3

* we cannot assume v2 is the volume of (sodium thiosulphate + H2SO4).
* reacting mixture in the practical book confusing.

[Patrick Chai Teck Khiong]

Actually, this question also confusing me when I do the PEKA..But i had asked my GC from my school:
She said : for example the set I mentioned by Mr. WOng, concentration of Na₂S₂O₃ (no water add) not 0.2M, is 0.18M(after adding acid) because sulphuric acid contain water.
GC finalize that we need follow practical book from KPM..Hahahaha

[Siti Norhayati Mohd Mokhtar]

Ahahah.. Agree with ur calculation.. I calculated the M2 just like yours..secara logik.. If we calculate the m2 after adding H2SO4.. Tiada lagi sodium thiosulphate salt.. Already become the 4 products.. Na2SO4, SO2,S and H2O.... Waaaaa.. Why must we follow the wrong concept in the text book just because KPM?

[Patrick Chai Teck Khiong]

Cikgu Siti, I totally agree with u..
The question is what our teachers nid to do? Follow the text book or ourselves?

[Nazatul Fazillah Ariffin]

i agree with mr wong that V2 is the volume of sodium thiosulphate after adding water. But in my practical book the volume of sodium thiosulphate for set I is 45cm3..then
Set II:  40 cm3 of sodium thiosulphate + 5cm3 of distilled water.
Set III: 30 cm3 of sodium thiosulphate + 15 cm3 of distilled water.
Set IV: 20 cm3 of sodium thiosulphate + 25 cm3 of distilled water.
Set V: 10 cm3 of sodium thiosulphate + 35 cm3 of distilled water.
So, the V2 for all sets is 45cm3 not 50cm3. The value is different.
So, is there any correction in the practical book that i don't know?

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