HSC 2012-2015 Chemistry Marathon (archive) (1 Viewer)

[Menomaths]

re: HSC Chemistry Marathon Archive

Random derp moment.

Here's my answer to Heroic's question: The reaction between Sodium Hydrogen Carbonate and Acetic acid will produce CO2 which will lower the pH, so the neutralization point will not be accurate.

 

[HeroicPandas]

re: HSC Chemistry Marathon Archive

Random derp moment.

Here's my answer to Heroic's question: The reaction between Sodium Hydrogen Carbonate and Acetic acid will produce CO2 which will lower the pH, so the neutralization point will not be accurate.

U can argue that with Sodium Hydrogen Carbonate + HCl, and it'll be wrong

 

[Menomaths]

re: HSC Chemistry Marathon Archive

wasn't question asking bout actc acd/

 

[HeroicPandas]

re: HSC Chemistry Marathon Archive

wasn't question asking bout actc acd/

u are the man, i've lost everything

 

[Menomaths]

re: HSC Chemistry Marathon Archive

Why would NaHCO3 + HCl be wrong? I'm pretty sure that produces CO2 too?

 

[HeroicPandas]

re: HSC Chemistry Marathon Archive

"..will produce CO2 which will lower the pH, so the neutralization point will not be accurate"

Would this explanation be suitable for a titration between NaHCO3 and HCl?

 

[Frie]

re: HSC Chemistry Marathon Archive

The answer says C, but i'm pretty sure its B, though i'm not sure how to draw polymers from monomers.

How would one do this question?

 

[girlworld_club]

re: HSC Chemistry Marathon Archive

i would also say b as condensation is a reaction between two functional groups.

 

[someth1ng]

re: HSC Chemistry Marathon Archive

The production of CO2 isn't a problem in titration - Na2CO3 is a PRIMARY standard and produces CO2 when it reacts with acid.

The problem with using NaHCO3 to make a primary solution is that it ISN'T a primary standard. You cannot make a primary solution with a non-primary standard.

 

[Frie]

re: HSC Chemistry Marathon Archive

NaHCO3 can be used as a primary standard?

The criteria that is necessary for a primary standard:

- Easy to purify and store in pure form
- Is not affected by moisture in the air, doesn't react so the mass stays the same
- Water soluble, so it can be easily dissolved and made into a solution with known concentration
- Not as important, but reasonably high molecular mass, this minimises a chance of error.

Anhydrous NaHCO3 the criteria, hence it can be used as a primary standard.

Substances which can't be used as primary standards are like nitric acid or hydrochloric acid as they are volatile and also sulfuric acid and sodium hydroxide because they absorb water/moisture from the air.

 

[someth1ng]

re: HSC Chemistry Marathon Archive

2NaHCO3-->Na2CO3+H2O+CO2

You won't know the composition of NaHCO3 - how much Na2CO3 is there etc. Hence, you cannot say NaHCO3 is pure.

And you can't have anhydrous NaHCO3 because NaHCO3 is hydrated. We use anhydrous Na2CO3 has the primary standard.

 

[someth1ng]

re: HSC Chemistry Marathon Archive

Calculate the pH of a 1x10^-8M solution of HCl.

 

[Menomaths]

re: HSC Chemistry Marathon Archive

Calculate the pH of a 1x10^-8M solution of HCl.

LOL seems like 8 pH.

It's diluted in water, which has [H] of 1x10^-7
[H] of HCL is 1x10^-8
1x10^-7+1x10^-8=1.1x10^-7
-log(1.1x10^-7)=6.96 (2 dp)

 

[HeroicPandas]

re: HSC Chemistry Marathon Archive

nvm

 

[someth1ng]

re: HSC Chemistry Marathon Archive

Explain why ionic solids tend to be insoluble in water.

 

[abdog]

re: HSC Chemistry Marathon Archive

A beaker initially contained 250ml of 0.05mol per litre copper sulphate solution. A piece of zinc metal is put into it and after a while, 0.325g of copper was deposited.

Calculate the concentration of copper sulphate remaining in the beaker.

 

[AB940]

re: HSC Chemistry Marathon Archive

A beaker initially contained 250ml of 0.05mol per litre copper sulphate solution. A piece of zinc metal is put into it and after a while, 0.325g of copper was deposited.

Calculate the concentration of copper sulphate remaining in the beaker.

n(Cu) = 0.325/63.55
= 5.114x10^-3 moles
Mol ratio of solid copper : CuSO4 = 1:1
therefore n(CuSO4 'lost') = 5.114x10^-3 mol

initial amount (using n=cV) was 0.250 x 0.05 = 0.0125 mol
final amount = 0.0125 - 5.114x10^-3
= 6.886x10^-3 mol

therefore final concentration = 6.886x10^-3/0.250
=0.0275 mol/L

 

[abdog]

re: HSC Chemistry Marathon Archive

n(Cu) = 0.325/63.55
= 5.114x10^-3 moles
Mol ratio of solid copper : CuSO4 = 1:1
therefore n(CuSO4 'lost') = 5.114x10^-3 mol

initial amount (using n=cV) was 0.250 x 0.05 = 0.0125 mol
final amount = 0.0125 - 5.114x10^-3
= 6.886x10^-3 mol

therefore final concentration = 6.886x10^-3/0.250
=0.0275 mol/L

With this are we not just calculating the concentration of just Copper?

Edit: Nvm I got it. Thanks!

 

[Youi_]

re: HSC Chemistry Marathon Archive

Compare the strength of the following acids:

0.1 M HCl
0.1 M H2SO4
0.1 M Acetic acid

 

[petermik]

re: HSC Chemistry Marathon Archive

Compare the strength of the following acids:

0.1 M HCl
0.1 M H2SO4
0.1 M Acetic acid

HCL and H2SO4 are stronger acids and are most likely to fully ionise in water however Acetic Acid is a weak acid as it only partially ionises in water. Distinguishing the Ph levels between Hydrogen Chloride and Sulfuric Acid H2So4 will be higher in Ph as it is diprotic whilst Hcl is monoprotic in which it will have have as many Hydrogen Ions and the calculations would be Ph= -log(2x0.1) and for HCl would be the regular calculation resulting in 1 ph for hcl and ph of ~.7 for H2so4. Therefore h2so4 is stronger then hcl


What is needed to prepare a buffer solution?