Chemistry 3/4 2013 Thread

[Edward21]

Ignore that question. The question you got is out of the course. All questions you get in VCAA will use the n+1 rule

Oh thankgod, it was a checkpoints 2011 question *phew*

[Edward21]

Sodium hydrogen carbonate, NaHCO3, will decompose to sodium carbonate, Na2CO3, when heated. Decomposition formula is: 2NaHCO3(s) --> Na2CO3(s) + CO2(g) +H2O(g), when a 4.081g sample is heated containing both sodium hydrogen carbonate and sodium carbonate there was a mass loss of 0.636g. Find the percentage by mass of Na2CO3 in the original sample  I'm not 100% how to approach this question

[9_7]

Hey guys, how do we know what to rinse the equipment with in volumetric analysis? For example, for the burette, do we rinse it with water or the solution? Cheers.

[Jeggz]

Hey guys, how do we know what to rinse the equipment with in volumetric analysis? For example, for the burette, do we rinse it with water or the solution? Cheers.

You always rinse the flasks (conical and volumetric) with de-ionsed water, and in regards to the burette and pipette, you always rinse them with the solution that it contains

[deleted]

Hey guys, how do we know what to rinse the equipment with in volumetric analysis? For example, for the burette, do we rinse it with water or the solution? Cheers.

You just need to remember these.

Burette, clean with whatever you're titrating with (so the solution).
Volumetric flask, clean with distilled water.
Pipette, clean with solution.

Well I was too slow.

[Aurelian]

You always rinse the flasks (conical and volumetric) with de-ionsed water, and in regards to the burette and pipette, you always rinse them with the solution that it contains

You just need to remember these.

Burette, clean with whatever you're titrating with (so the solution).
Volumetric flask, clean with distilled water.
Pipette, clean with solution.

Well I was too slow.

Make sure you understand why though =)

[Jeggz]

That was your cue to enlighten us with your 99.95 intelligence Aurelian!
Please explain 

[9_7]

Sweet, thanks for the responses =]

[lzxnl]

Sodium hydrogen carbonate, NaHCO3, will decompose to sodium carbonate, Na2CO3, when heated. Decomposition formula is: 2NaHCO3(s) --> Na2CO3(s) + CO2(g) +H2O(g), when a 4.081g sample is heated containing both sodium hydrogen carbonate and sodium carbonate there was a mass loss of 0.636g. Find the percentage by mass of Na2CO3 in the original sample  I'm not 100% how to approach this question

So...mass loss is in the form of escaping gas. 0.636g gas has the same number of moles of CO2 and H2O, which have a combined molar mass of 44+18=62 g/mol. Therefore we have 0.636/62=0.01026 moles of CO2 and H2O forming.
By mole ratio, we have 0.02052 moles of NaHCO3. M(NaHCO3)=23+1+12+48=84g/mol, so the mass of sodium bicarbonate is 0.02052*84=1.723 grams of NaHCO3
This is 42.2% of the original sample so remaining 57.8% must have been from Na2CO3.
Someone check my working.

Let's say I have a blue solution
If i was trying to find the concentration of this solution using colorimetry I would set up several standard solutions, plot a calibration graph, etc. etc. but when i am shining a light into the blue solution, am i shining a blue light and detecting how much light is being transmitted through, or a complimentary light (e.g yellow) and seeing how much is being transmitted through?

On another note, is colorimetry specific colours or specific wavelengths, and if we are talking about those boxes with the RGB LED's, are those just referred to as colours or wavelengths?

A blue solution is something that reflects blue light but absorbs others. If you shone blue light onto the solution, you wouldn't have an absorbance. You would, as you've mentioned, shine a yellow light because the yellow light (green and red) is what is actually absorbed.

[Edward21]

So...mass loss is in the form of escaping gas. 0.636g gas has the same number of moles of CO2 and H2O, which have a combined molar mass of 44+18=62 g/mol. Therefore we have 0.636/62=0.01026 moles of CO2 and H2O forming.
By mole ratio, we have 0.02052 moles of NaHCO3. M(NaHCO3)=23+1+12+48=84g/mol, so the mass of sodium bicarbonate is 0.02052*84=1.723 grams of NaHCO3
This is 42.2% of the original sample so remaining 58.8% must have been from Na2CO3.
Someone check my working.

A blue solution is something that reflects blue light but absorbs others. If you shone blue light onto the solution, you wouldn't have an absorbance. You would, as you've mentioned, shine a yellow light because the yellow light (green and red) is what is actually absorbed.

CORRECT. I didn't know you could accumulate molar masses of compounds (eg. two gases H2O and CO2) with the same mol ratio..but there you go!

[clıppy]

I've got a prac coming up where I am determining the iron concentration in fertiliser by using colorimetry and known solutions.
A few questions I have in regards to this are:
If my percentage is higher/lower than the manufacturers standards, what could be some reasons for this?
I've been told the fertiliser is in the form of FeSO4. If this is true, what effect would this have on the results and how would I overcome it?

[barydos]

I'm always getting confused with these, but how would you label these molecules?
Like, for the first one, which carbon is labelled the "second carbon"? does the methyl take priority or the hydroxyl?
I'm having similar problems with the second one, methyl vs amino

[Limista]

(Image removed from quote.)

I'm always getting confused with these, but how would you label these molecules?
Like, for the first one, which carbon is labelled the "second carbon"? does the methyl take priority or the hydroxyl?
I'm having similar problems with the second one, methyl vs amino

The functional group takes priority.

Example 1) Number carbon atoms from the right

Example 2) Number carbon atoms from the left (as you have done)

[barydos]

The functional group takes priority.

Example 1) Number carbon atoms from the right

Example 2) Number carbon atoms from the left (as you have done)

Hey thanks, but also
for the second molecule, would it be A or C? What convention do we use here?

[Limista]

It would be A.

Convention - alphabetical order. Letter 'a' comes before 'm' in the alphabet.