VCE Chemistry Question Thread

[lzxnl]

Hey guys.

Say we've got a strong base (NaOH) being titrated with a strong acid (HCl) and they're concentrations are identical at 0.1M. The equivalence would be at a pH of 7. Would the choice between methyl red and bromothymol blue as an indicator make any difference?

Because I'm thinking that methyl red would be less suitable given that it has a pH range between 4.2-6.3. If it is used as the indicator, would this mean that the end point occurs after the equivalence and so the titre volume would be higher than the actual value?

It's such a sharp endpoint it shouldn't matter. But I'd go bromothymol blue if I had it

[RazzMeTazz]

Similar to the term: acid/base conjugate pairs, is there a term such as this but in reference to redox reactions, so conjugate redox pairs?

I feel like I have heard this before, but I'm not sure.

Thanks

[lzxnl]

Similar to the term: acid/base conjugate pairs, is there a term such as this but in reference to redox reactions, so conjugate redox pairs?

I feel like I have heard this before, but I'm not sure.

Thanks

You can sort of define a term like that, but whereas an acid has one conjugate base only, a species may have multiple redox pairs.

For instance, Fe(s) can be oxidised to Fe(II) or Fe(III). Similarly, I^- can be oxidised to I₂ or even I₃^-

[KingDrogba]

Why cant lawn fertilizer be analysed using Gas Chromatography?

[Eiffel]

Why cant lawn fertilizer be analysed using Gas Chromatography?

because it cannot be readily vaporised.

I have a question, it will need to be done visually. 2 chromatograms both using TLC. the first one has a solvent front of 6.3, yellow 3.9 and red 4.7.

second chromatogram has solvent front 10.2, ref 7.6 and yellow 3.9

asks about if red, yellow or neither are the same component.
We can knock out yellow due to different Rf, however red although producing same rf have different solvent front and hence means not similar conditions and hence cannot be concluded as same, however the answer says red is.

[lzxnl]

because it cannot be readily vaporised.

I have a question, it will need to be done visually. 2 chromatograms both using TLC. the first one has a solvent front of 6.3, yellow 3.9 and red 4.7.

second chromatogram has solvent front 10.2, ref 7.6 and yellow 3.9

asks about if red, yellow or neither are the same component.
We can knock out yellow due to different Rf, however red although producing same rf have different solvent front and hence means not similar conditions and hence cannot be concluded as same, however the answer says red is.

TLC only really tells you if two things are different. It provides evidence that two things may be the same, but often you need other evidence as well.
In your case, as long as the solvent is the same, the temperature/pressure are all the same, then a different distance travelled by the solvent front does not constitute different conditions.

[fightingchance]

Im a little stuck. Any help would be greatly appreciated.

The calcium content of bottled water can be measured using the technique of AAS. As part of the process, the instrument was calibrated with standards that contained calcium ion concentrations 0.10,0.30,0.50,0.75 and 1.00 mg/L
Calculate the mass of CaCl2 that would need to be dissolved in 1000ml to make up the 1.00mg L standard.

(what i did was covert the v to L and use the n=cv formula, then multiply that by the molar mass of CaCl2. Answer in the book did not match though. Using the jacaranda book)

[keltingmeith]

Im a little stuck. Any help would be greatly appreciated.

The calcium content of bottled water can be measured using the technique of AAS. As part of the process, the instrument was calibrated with standards that contained calcium ion concentrations 0.10,0.30,0.50,0.75 and 1.00 mg/L
Calculate the mass of CaCl2 that would need to be dissolved in 1000ml to make up the 1.00mg L standard.

(what i did was covert the v to L and use the n=cv formula, then multiply that by the molar mass of CaCl2. Answer in the book did not match though. Using the jacaranda book)

n=CV can only be used if your concentration is in mol/L. What is the concentration used for the AAS?

[scarletmoon]

Is m=c*v a thing?

[keltingmeith]

Is m=c*v a thing?

It depends on how they define concentration - in fact, as you hit unit 4, you'll find that defining things as a concentration of mass against volume actually makes life easier (in particular, for calorimetry). In all instances, they should say if the concentration is Molarity (aka, mol/L) or something else weird (today, a student showed me mg/mL) - if for whatever reason they don't define what their concentration is, assume it's mol/L. There's also molality (mol/kg), but I don't think that'll ever pop up in VCE.

[mahler004]

It depends on how they define concentration - in fact, as you hit unit 4, you'll find that defining things as a concentration of mass against volume actually makes life easier (in particular, for calorimetry). In all instances, they should say if the concentration is Molarity (aka, mol/L) or something else weird (today, a student showed me mg/mL) - if for whatever reason they don't define what their concentration is, assume it's mol/L. There's also molality (mol/kg), but I don't think that'll ever pop up in VCE.

IIRC it's one of those things that's explicitly not in the syllabus.

Im a little stuck. Any help would be greatly appreciated.

The calcium content of bottled water can be measured using the technique of AAS. As part of the process, the instrument was calibrated with standards that contained calcium ion concentrations 0.10,0.30,0.50,0.75 and 1.00 mg/L
Calculate the mass of CaCl2 that would need to be dissolved in 1000ml to make up the 1.00mg L standard.

(what i did was covert the v to L and use the n=cv formula, then multiply that by the molar mass of CaCl2. Answer in the book did not match though. Using the jacaranda book)

Remember that not all concentration measurements are molar concentration measurements. It's often more convenient for some things to express concentrations as mass/volume units (instead of mol/volume units.) When you've been getting basic stoich into your head it's tempting to convert every mass you see into mols, but it can be misleading - such as in this case.

It's really a trick question. How much mass of CaCl2 do you need to make a 1mg/L standard?

In all instances, they should say if the concentration is Molarity (aka, mol/L) or something else weird (today, a student showed me mg/mL) - if for whatever reason they don't define what their concentration is, assume it's mol/L.

If you think mg/ml is weird, never study biochemistry

[keltingmeith]

If you think mg/ml is weird, never study biochemistry

Yeah, look - I haven't studied bio since year 7, I don't plan on starting now.

[lzxnl]

Yeah, look - I haven't studied bio since year 7, I don't plan on starting now.

I did the subject last sem. I have no idea why the hell I decided to overload that semester for that subject in retrospect. It's so much rote learning.

[fightingchance]

n=CV can only be used if your concentration is in mol/L. What is the concentration used for the AAS?

mg L-1.

[cosine]

I'm having a bit of trouble with titration
We are trying to determine the amount of Nitrogen as ammonium in a fertiliser

We have a sample of fertiliser, weighed it and added de-ionised water to dissolve it. Once it was dissolved, sodium hydroxide was added to it and then it was heated until all the ammonium had turned into vapour. Then we added methyl red indicator and titrated it. I don't understand what titrating the solution actually does in terms of determining the Nitrogen in the fertiliser. If someone could help me that would be awesome