HSC 2012-2015 Chemistry Marathon (archive) (2 Viewers)

[AnimeX]

re: HSC Chemistry Marathon Archive

A student wished to determine the percentage of calcium carbonate in a shell found at the beach. The clean dry shell, which weighed 1.306g, was placed in a small beaker and 10mL of 5mol/L of hydrochloric acid was added. When the shell had completely dissolved, the resulting solution was transferred to a volumetric flask and the volume made up to 25mL with distilled water. A 10mL sample from this solution required 11.2mL of 1mol/L sodium hydroxide for complete neutralisation.

a) Write a balanced equation for the reaction of calcium carbonate with hydrochloric acid. [1]
b) Calculate the number of moles of NaOH present in the 11.2mL of 1mol/L NaOH solution. [1]
c) How many moles of acid remained in the beaker after the reaction with the shell (before the dilution was made)? [2]
d) How many moles of acid reacted with the shell? [1]
e) What mass of calcium carbonate was present in the shell? [2]
f) What was the percentage of calcium carbonate in the shell? [1]​

[woops read q wrong and attempted to find the pH of the final solution at part a.... so what was the use of the dilution and NaOH in this question? I did this wrong D:]

a) CaCO3 (aq) + 2HCl(aq) --> CaCl2 (aq) + H2O (l) + CO2 (g)
b)n(NaOH) = cv = 1x0.0112 = 0.0112 mol
c) n(HCl) = cv = 10x0.010=0.1mol,
n(CaCO3) = 1.306/ 40.08+12.01+16x3 mol =0.013 (2dp) [here do I need to keep exact value or use rounded value??]
therefore 0.1-2x0.013=0.074 mols of acid remain
d) 0.026 mols
e) m(Ca) = 1.306x40.08/ 40.08+12.01+16x3 = 0.523g (3dp)
f) (0.523/1.306) x100=39.95%

 

[bedpotato]

re: HSC Chemistry Marathon Archive

When an acid is defined as strong, it means the acid ionises completely in solutions, such as HCL. Concentrated is used when an acid has a high concentration, such as 2molL-1, regardless of the degree of ionisation.

I don't think you should say that either.

Also, this sounds incomplete: "Concentrated is used when an acid has a high concentration". Okay, but a high concentration of what? The term 'concentration' refers to the concentration of solute in a solution. If an acid is 'concentrated', the concentration of solute is high, meaning there's a large amount of acid molecules present in the solution. And diagrams would probably help explain your answer a bit better. You don't have to include them.

Question: During this course, you encountered various chemical processes that use a catalyst. Identify the features of the catalysts that make them useful for two chemical processes.

 

[bedpotato]

re: HSC Chemistry Marathon Archive

[woops read q wrong and attempted to find the pH of the final solution at part a.... so what was the use of the dilution and NaOH in this question? I did this wrong D:]

a) CaCO3 (aq) + 2HCl(aq) --> CaCl2 (aq) + H2O (l) + CO2 (g)
b)n(NaOH) = cv = 1x0.0112 = 0.0112 mol
c) n(HCl) = cv = 10x0.010=0.1mol,
n(CaCO3) = 1.306/ 40.08+12.01+16x3 mol =0.013 (2dp) [here do I need to keep exact value or use rounded value??]
therefore 0.1-2x0.013=0.074 mols of acid remain
d) 0.026 mols
e) m(Ca) = 1.306x40.08/ 40.08+12.01+16x3 = 0.523g (3dp)
f) (0.523/1.306) x100=39.95%

This isn't right, but how did you get the bolded part? :s
If you want to see the solution, scroll up

 

[Menomaths]

re: HSC Chemistry Marathon Archive

I don't think you should say that either.

Also, this sounds incomplete: "Concentrated is used when an acid has a high concentration". Okay, but a high concentration of what? The term 'concentration' refers to the concentration of solute in a solution. If an acid is 'concentrated', the concentration of solute is high, meaning there's a large amount of acid molecules present in the solution. And diagrams would probably help explain your answer a bit better. You don't have to include them.

Question: During this course, you encountered various chemical processes that use a catalyst. Identify the features of the catalysts that make them useful for two chemical processes.

Magnetite- provides a favourable surface for the reaction in the Haber Process
Sulfuric Acid- Good dehydrating agent so used in Esterification

 

[AnimeX]

re: HSC Chemistry Marathon Archive

Magnetite- provides a favourable surface for the reaction in the Haber Process
Sulfuric Acid- Good dehydrating agent so used in Esterification

1) Fe3O4 - might also be good to add, probs explain what it does [speeds up rate of reaction/ more successful collisions]
2) should probably mention concentrated and how it favours the esterification process, ie dehydration of water shifts equilibrium to left/right...

 

[Menomaths]

re: HSC Chemistry Marathon Archive

1) Fe3O4 - might also be good to add, probs explain what it does [speeds up rate of reaction/ more successful collisions]
2) should probably mention concentrated and how it favours the esterification process, ie dehydration of water shifts equilibrium to left/right...

"During this course, you encountered various chemical processes that use a catalyst. Identify the features of the catalysts that make them useful for two chemical processes."

It says identify the features that make them useful, doesn't say explain what they do.
This is why mark values should also be provided.

 

[bedpotato]

re: HSC Chemistry Marathon Archive

"During this course, you encountered various chemical processes that use a catalyst. Identify the features of the catalysts that make them useful for two chemical processes."

It says identify the features that make them useful, doesn't say explain what they do.
This is why mark values should also be provided.

Haha, sorry. It was only worth 2 marks. But the sample answer says "provides surface for particles to collide". So you probably don't have to include "speeds up rate of reaction"

 

[Menomaths]

re: HSC Chemistry Marathon Archive

Haha, sorry. It was only worth 2 marks. But the sample answer says "provides surface for particles to collide". So you probably don't have to include "speeds up rate of reaction"

Nws, seemed like a 2 marker anyway

 

[HeroicPandas]

re: HSC Chemistry Marathon Archive

Q: Compare the chemical and physical properties of ethane and ethene 3 marks

 

[Menomaths]

re: HSC Chemistry Marathon Archive

Here's some questions for the rote learners:

1)Describe how to qualitatively check for the presence of nitrates and phosphates

2)Describe how to quantitatively measure the amounts of nitrates and phosphates

 

[Menomaths]

re: HSC Chemistry Marathon Archive

Q: Compare the chemical and physical properties of ethane and ethene

And the mark value is...?

 

[HeroicPandas]

re: HSC Chemistry Marathon Archive

Here's some questions for the rote learners:

1)Describe how to qualitatively check for the presence of nitrates and phosphates

2)Describe how to quantitatively measure the amounts of nitrates and phosphates

hahhahahhah

 

[Menomaths]

re: HSC Chemistry Marathon Archive

^ hahhahahhah

It's a good question though lol

 

[bedpotato]

re: HSC Chemistry Marathon Archive

Here's some questions for the rote learners:

1)Describe how to qualitatively check for the presence of nitrates and phosphates

2)Describe how to quantitatively measure the amounts of nitrates and phosphates

why can't there be more calculation questions :/

 

[Menomaths]

re: HSC Chemistry Marathon Archive

why can't there be more calculation questions :/

http://community.boredofstudies.org/showthread.php?t=297690&p=6449442&viewfull=1#post6449442

There needs to be a fair balance

 

[Buwwy]

re: HSC Chemistry Marathon Archive

Here's some questions for the rote learners:

1)Describe how to qualitatively check for the presence of nitrates and phosphates

2)Describe how to quantitatively measure the amounts of nitrates and phosphates

1. Nitrates can be tested for using the brown ring test, where iron sulfate is added to the water sample inside a test tube, then sulfuric acid is slowly added. If a brown ring forms, then nitrates are present. Phosphates - ammonium molybdate test, where you acidifying sample with nitric acid then add ammonium molybdate, then slowly heat. yellow precipitate = presence of phosphates.
2. I don't have an answer, but does it have something to do with colorimetry?

 

[Menomaths]

re: HSC Chemistry Marathon Archive

1. Nitrates can be tested for using the brown ring test, where iron sulfate is added to the water sample inside a test tube, then sulfuric acid is slowly added. If a brown ring forms, then nitrates are present. Phosphates - ammonium molybdate test, where you acidifying sample with nitric acid then add ammonium molybdate, then slowly heat. yellow precipitate = presence of phosphates.
2. I don't have an answer, but does it have something to do with colorimetry?

1)Yup, 2/2

Yes, it has something to do with colorimetry.

 

[rednellav]

re: HSC Chemistry Marathon Archive

1)Yup, 2/2

Yes, it has something to do with colorimetry.

A wild-question appears

Wild-question used content that im not rote learning

its super effective!

rote learning student fainted

 

[superSAIyan2]

re: HSC Chemistry Marathon Archive

Q: Compare the chemical and physical properties of ethane and ethene 3 marks

Ethane and ethene have similar physical proeprties as they are both non-polar hydrocarbons with similar molecular mass. They have similar MP/BP, both are water insoluble and cannot conduct electricity
However their chemical properties differ greatly due to their functional groups. Ethane is saturated and relatively unreactive whereas unsaturated ethene is highly reactive due to the presence of the C=C double bond, which is a site of high electron density. This allows ethene to participate in addition reactions by opening up the double bond.

Question: Assess current developments in the use of biopolymers. 5M or Explain how O2 and O3 have different physical and chemical properties despite both being allotropes of oxygen. 4m

 

[bedpotato]

re: HSC Chemistry Marathon Archive

2)Describe how to quantitatively measure the amounts of nitrates and phosphates

Is it by measuring dissolved oxygen and biochemical oxygen demand? Or is that for eutrophication? Or am I completely off?