VCE Chemistry Question Thread

[RazzMeTazz]

I believe the gases (CO2 and H2O as formed from the combustion reaction) were passed through CaCl2 and NaOH. The water reacts with the CaCl2, and the mass of CaCl2 increased by 0.30g, which means that there was 0.30g of water produced. Same with NaOH - 0.73g increase in NaOH, so 0.73g of CO2 produced)

I think generally it does become a hydrate not too sure about that one though

Ohh, I understand now

Thanks!

[Eiffel]

When determining oxidant and reluctant reactions, what do you do if the element appears twice or more on one side? E.g. with +1 and say 0/+2 oxidation number and then RHS as maybe 0 or +3?

[KingDrogba]

What are 3 sources of error in gravimetric analysis and then 3 in volumetric analysis?

[sunshine98]

What are 3 sources of error in gravimetric analysis and then 3 in volumetric analysis?

For gravimetric analysis:
•   Insolubility: No substance is completely insoluble , a small amount will always remain in solution.
•   Loss of precipitate: small amounts of precipitate will be lost during transfers .
•   Incomplete precipitation occurs
•   Rinsing: Incomplete rinsing of precipitate , leaving adsorbed ions.
•   Drying: Incomplete drying of precipitate
•   Decomposition: precipitate decomposes during drying
•   More than one substance precipitates
   
For volumetric analysis:
- parallax errors: misreading  of meniscus
-rinsing burette with water , instead of the titrant
- rinsing pipette with water , instead of the aliquot.

Hope this helps 

[KingDrogba]

So im trying to calculate the SO2 percentage by mass in wine:

We used 20ml aliquots of wine, and titrated it with 0.005261M Iodine and got an average titre of 10.89mL. Using the equations given plus a redox equation being
SO2 + I2 + 2H2O > HSO4- +2I + 3H+

how do i work this out?

[IndefatigableLover]

When 'determining the purity of a substance' (that is for a titration), what are we meant to be doing in terms of answering the final answer of the substance?

[Eiffel]

how does this have 180 H atoms?

Fe2(SO4)3.9H20 i am getting molar mass of 461.9gmol

[Eiffel]

how much water must be added to a 2l solution of a strong acid of ph 1.5 in order to decrease ph to 3.0. answer : 61L

why does c1v1 = c2v2 not work for this?

[grannysmith]

how much water must be added to a 2l solution of a strong acid of ph 1.5 in order to decrease ph to 3.0. answer : 61L

why does c1v1 = c2v2 not work for this?

Convert the pH into concentration. 10^-1.5 = 0.03 M. You want the concentration to be 10^-3.0 = 0.001 M. Now you can use C₁V₁ = C₂V₂.
Plugging in values, V₂ = 63L. Now, substract the initial 2L and hence the answer is 61L.

Edit: Just realised that you mentioned a decrease in pH from 1.5 to 3.0, which is not quite true :p pH increases but the acidity decreases.

[Eiffel]

its what the question says :O

[grannysmith]

Say we've got two rounded values 0.00980 and 0.00555. They're both to 3 sig figs. If I was to subtract the second figure from the first, I would do so using the non-rounded figures, yes? If so, would the resultant figure be to 5 decimal places? Consequently, should I be using the non-rounded figure or the rounded figure for subsequent questions?

Also, given the following equation:
CH₃COOH_(aq) + KOH_(aq) --> CH₃COOK_(aq) + H₂O_(l)

Would the ionic equation just simple H⁺_(aq) + OH^-_(aq) --> H₂O_(l) ?

[lzxnl]

Say we've got two rounded values 0.00980 and 0.00555. They're both to 3 sig figs. If I was to subtract the second figure from the first, I would do so using the non-rounded figures, yes? If so, would the resultant figure be to 5 decimal places? Consequently, should I be using the non-rounded figure or the rounded figure for subsequent questions?

Also, given the following equation:
CH₃COOH_(aq) + KOH_(aq) --> CH₃COOK_(aq) + H₂O_(l)

Would the ionic equation just simple H⁺_(aq) + OH^-_(aq) --> H₂O_(l) ?

Use non-rounded figures in calculations, always

Also, the ionic equation isn't the neutralisation of hydrogen and hydroxide ions. In an ethanoic acid solution, most of the ethanoic acid is undissociated. Therefore, a much more accurate ionic equation would be reaction of hydroxide with ethanoic acid to form ethanoate and water.

[chocolate.cake.1]

Hi

I read that for AAS, the concentration of metal ion and absorbance are proportional at low concentrations.
Why are they not proportional at high concentrations?

Please let me know if I am going out of the VCE course... I tend to overthink things a lot...

Thanks

[thushan]

handballs to lzxnl*

[keltingmeith]

It's because of Beer. Shouldn't drink that stuff and do chemistry, folks!! -cough-

The AAS uses something called the "Beer-Lambert Law" to determine the concentration of a substance. The Beer-Lambert Law states that absorbance and concentration are proportional to each other, basically meaning that if one is high, so is the other.
Now, the rule of thumb is that the Beer-Lambert law loses linearity (that is, changes equation) at absorbance greater than 1. The reasons for that, however, are beyond VCE (and me, tbh). What does this have to do with high concentration? Well, absorbance and concentration are proportional. So, the higher the concentration, the higher the absorbance. The higher the absorbance, the more likely it's greater than 1.

So, the chemical reason: can't tell you, maybe lzxnl knows.
The reason I'm telling you: at high concentrations, our maths stops working, and the other maths is too hard.