Question Chain Thread !!! (1 Viewer)

[SkimDawg]

Re: [-- Official 2008 HSC Chemistry Pre-exam Study Thread --]

Identify your local catchment area and discuss possible sources of contamination in this catchment

Is this a first hand investigation? Ill try at least haha.
Local Catchment Area: Warrell Creek
Many tests were done with the sample of water obtained from this river as tests for anions / cations. The only anion found was chloride, which is okay as its a "salt water river". Some cations were found, however. These at first possibly could have been: barium, aluminium, silver (i think) and zinc. After this, different solutions weer added in an attempt to figure out what it is (many cations in solution form a white precipitate...). It ended up being aluminium, as the two most likely possibilites were this and zinc, and zinc doesnt react with something, blah blah blah.. I have no idea where the aluminium ions have come from, my teacher mentioned something about one of the bridges upstream.
That may have covered what your asking, but I highly doubt it lol.

anyway, next qu: Calculate the pH after 20 mL of 0.01 mol L–1 sodium hydroxide is added to 50 mL of 0.2 mol L–1 hydrochloric acid. Include a balanced chemical equation in your answer.
btw nice avatar midifile, klaxons are mad hey

 

[minijumbuk]

Re: [-- Official 2008 HSC Chemistry Pre-exam Study Thread --]

Woot, I'm doing this question only because it takes less than 1 minute to do xD

NaOH_(aq) + HCl_(aq) --> NaCl_(aq) + H₂O_(aq)

Molar ratio: 1:1 --> 1:1

moles of HCl present = 0.05 x 0.2
= 0.01 mol

moles of NaOH added = 0.02 x 0.01
= 0.0002 mol

Therefore HCl is in excess, having 0.01 - 0.0002 mol extra
= 9.8x10^-3mol of HCl

Which means 9.8x10^-3mol of H⁺
[H⁺] = 9.8x10^-3 / (0.02 + 0.05)
= 0.14M

pH = -log₁₀(0.14)
= 0.85 (2 dec. plc)


GRR I always forget to give another question...

How about...

Justify two reasons of refluxing in esterification.

 

[midifile]

Re: [-- Official 2008 HSC Chemistry Pre-exam Study Thread --]

Is this a first hand investigation? Ill try at least haha.
Local Catchment Area: Warrell Creek
Many tests were done with the sample of water obtained from this river as tests for anions / cations. The only anion found was chloride, which is okay as its a "salt water river". Some cations were found, however. These at first possibly could have been: barium, aluminium, silver (i think) and zinc. After this, different solutions weer added in an attempt to figure out what it is (many cations in solution form a white precipitate...). It ended up being aluminium, as the two most likely possibilites were this and zinc, and zinc doesnt react with something, blah blah blah.. I have no idea where the aluminium ions have come from, my teacher mentioned something about one of the bridges upstream.
That may have covered what your asking, but I highly doubt it lol.

anyway, next qu: Calculate the pH after 20 mL of 0.01 mol L–1 sodium hydroxide is added to 50 mL of 0.2 mol L–1 hydrochloric acid. Include a balanced chemical equation in your answer.
btw nice avatar midifile, klaxons are mad hey

Yeah.. I love them.. =]

I think you are about right with the catchment. We didnt do a first hand investigationbut just learnt about it (warragamba dam). You could also add in stuff about runoff, animals dying in the water and stuff like that.

NaOH + HCl --> H₂O + NaCl
n(H⁺) = cV_L
= 0.2 x 0.05
= 0.01 mol
n(OH^-) = 0.01 x 0.02
= 0.02
Therefore H⁺ ions are in excess by 0.0098

c(H⁺) = n/V_L
= 0.0098/0.7
= 0.14 mol/L

pH = -log(0.14)
= 0.85

EDIT: Too slow =[

 

[aussiechick007]

Re: [-- Official 2008 HSC Chemistry Pre-exam Study Thread --]

How about...

Justify two reasons of refluxing in esterification.

- to allow the reaction to be done at higher temperatures, and thus getting the product faster.
- to prevent loss of the reactants and products with low boiling points to the environment.

next...

Analyse how knowledge of the composition and properties of acids has led to changes in the definition of acids.

 

[SkimDawg]

Re: [-- Official 2008 HSC Chemistry Pre-exam Study Thread --]

Justify two reasons of refluxing in esterification.

Someone already did a question very similar to that, and i gave an answer to it as well.
The Reactants are volatile and yet need to be heated to reach their activation energy. Refluxing cools the reactant gases and condenses them returning them to the reaction mixture for continued heating. The gases would otherwise escape before they reacted. It also allows the reaction to be brought about at a higher temperature than would otherwise be possible. The alternative of performing the reaction in a closed vessel could lead to a dangerous build-up of pressure and the possibility of an explosion.

 

[SkimDawg]

HAHAHAH, cant get much funnier, answered by a person who posted another question that has been answered.

Analyse how knowledge of the composition and properties of acids has led to changes in the definition of acids.
Early definitions of acids were based on their properties, including their effect on indicators and their reactions with other substances. Acidic substances such as vinegar and lemon juice were known to taste sour and react with bases. In the 17th century, Robert Boyle sed this knowledge of properties to define acids as all sour-tasting substances.
Most acids such as CH3COOH, H2SO4, and H2CO3 were known to contain oxygen and this was ued by Antoine Lavoisier as the basis of his definition in 1779. He thought that all oxides dissolved in water to form acids.

However, many oxygen-containing substances have since been found to be basic, such as sodium oxide and calcium oxide. Also, some acidic substances were known not to contain oxygen, such as HCL and HCN. Acids were shown to react with metals forming salts, e.g ZnCl2. Humphry Davy, in the early 1800s, used these observations to define acids such as substances that contain replaceable hydrogen.

In 1884, Svante Arrhenius proposed a theory to explain the properties of acids that were known by at that time. Acids were known to conduct electricity and react with metals to produce hydrogen gas. These observations suggested that ions were involved. Arrhenius defined acids as substances that ionise in solution to produce hydrogen ions. Strong acids ionise fully and weak acids slightly.

Arrhenius defined bases as substances that produce hydroxide ion in water but carbonates were shown to be basic without hydroxide, Because this current theory had inconsistencies like this, Johannes Bronsted and Thomas Lowry in 1923 redefined acids as proton donors and bases as proton acceptors. They elaborated to say that acids give up protons to form conjugate bases and that a strong acid will form a weak conjugate base. They came to this conclusion by observing that the solvent seemed to play a role in acid behaviour. The strength of an acid appeared to be due to the nature of the solvent. They also observed that different salts had different pH's.

forgot the next qu again:Calculate the theoretical mass of ethanol required to heat 200mL of water from 21 degrees C to 45 degrees C
Sorry if im hammering you guys with numerical calculations haha, its something i wanna nail before thurs

 

[danz90]

Re: [-- Official 2008 HSC Chemistry Pre-exam Study Thread --]

- to allow the reaction to be done at higher temperatures, and thus getting the product faster.
- to prevent loss of the reactants and products with low boiling points to the environment.

next...

Analyse how knowledge of the composition and properties of acids has led to changes in the definition of acids.

Lavoisier initially suggested that oxygen was the acidifying principle in acids. However, later research discovered that not all acids contained oxygen. Davy then suggested that hydrogen was the acidifying principle in acids. However, this was flawed, since although all acids contain hydrogen, not all substances containing hydrogen were acidic.

Svante Arrhenius then came to a conclusion that acids were substances that released H+ ions when dissolved in water. His theory of acids was only plausible for acids in solution, and could not explain gaseous neutralisation:
NH_{3 (g)} + HCl_(g) ---> NH₄Cl_(s)

The Bronsted-Lowry theory of acids incorporates the Arrhenius theory, however specifically states that an acid is a substance that donates a proton (H+) to a corresponding base. This theory can be applied to all known acids.

Mehh.. I hate all that historical shit.
Nicer question now, lol

A student was given a sample of an unknown soluble salt. She suspected the sample contained barium ions. Describe the procedures she may have used to confirm her suspicion. Include at least one precaution taken to minimise risk.

 

[Zippora]

It's a shit one to use because it's deliquescent.

Undermyskin, I challenge you:
If you have a pH 3 acetic acid solution and a pH 3 HCl solution, predict which one will require more alkaline solution to neutralise it in a titration.

the same amount?

 

[danz90]

the same amount?

well in order for there to be pH3, each acid solution must have the same [H₃O⁺] . So I would agree with above poster in saying Equal volume.

but then the confucious thing is, CH3COO- ions then undergo hydrolysis to form OH- ions in solution. (maybe this could mean u would need less to neutralise the acetic)..

 

[SkimDawg]

Re: [-- Official 2008 HSC Chemistry Pre-exam Study Thread --]

A student was given a sample of an unknown soluble salt. She suspected the sample contained barium ions. Describe the procedures she may have used to confirm her suspicion. Include at least one precaution taken to minimise risk.

Add NaOH, white precipitate forms. She may have used sulfate to precipitate two ions, barium and chloride, then used Ag+ to separate chloride from the salt. If there is still a precipitate, then barium is a possibility of being apart of the salt. One precaution taken to minimise risk? Where safety glasses, even wear gloves maybe (incase of the salt being dangerous).

 

[danz90]

forgot the next qu again:Calculate the theoretical mass of ethanol required to heat 200mL of water from 21 degrees C to 45 degrees C
Sorry if im hammering you guys with numerical calculations haha, its something i wanna nail before thurs

q = 0.200 * (4.18*10³) * 24

q=20.064kJ

DeltaH_c = q/n

1367 = 20.064/n

n= 20.064/1367
n(ethanol) = 0.0147mol

m(ethanol) = 0.0147*46.1

therefore, m(ethanol) required = 0.68g (2 sig. fig.)

Next calculation:
2.5L of HBr(g) and 1.6L of NH_{3 (g)} measured at 25C and 100kPa are mixed. They react to produce ammonium bromide, a white solid.

(a) Demonstrate that this is an acid-base reaction, using Bronsted-Lowry definitions.

(b) Calculate the mass of ammonium bromide formed in this reaction.

 

[SkimDawg]

Next calculation:
2.5L of HBr(g) and 1.6L of NH_{3 (g)} measured at 25C and 100kPa are mixed. They react to produce ammonium bromide, a white solid.

(a) Demonstrate that this is an acid-base reaction, using Bronsted-Lowry definitions.

(b) Calculate the mass of ammonium bromide formed in this reaction.

a) HBr + NH3 -----> NH4Br

HBr is an acid, NH3 is a base. As HBr combines with ammonia, it gives up a hydrogen atom, defining it by the Bronsted-Lowry definitions as a proton donor. The base, ammonia, receives this hydrogen atom (as well as the bromide), defining it by the Bronsted-Lowry definitions, as well, as a proton acceptor.
Ill edit this soon, the 2nd part looks extremely gay.

b)ok
moles HBr = 2.5/24.79 = 0.10mol
moles NH3 = 1.6/24.79 = 0.0645mol

0.10HBr + 0.0645NH3 --------> NH4Br

moles NH4Br = 0.0645mol
mass of NH4Br = (14 + 4 + 79.90) x 0.0645
= 6.31g
hopefully fixed

Next qu: Outline the procedure you would use to prepare a standard solution of sodium hydrogen carbonate from solid sodium hydrogen carbonate. Calculate the mass of solid sodium hydrogen carbonate required to make 250 mL of 0.12 mol L−1 solution.

 

[midifile]

a) HBr + NH3 -----> NH4Br

HBr is an acid, NH3 is a base. As HBr combines with ammonia, it gives up a hydrogen atom, defining it by the Bronsted-Lowry definitions as a proton donor. The base, ammonia, receives this hydrogen atom (as well as the bromide), defining it by the Bronsted-Lowry definitions, as well, as a proton acceptor.
Ill edit this soon, the 2nd part looks extremely gay.

b)ok
moles HBr = 2.5/24.79 = 0.10mol
moles NH3 = 1.6/24.79 = 0.0645mol

0.10HBr + 0.0645NH3 --------> NH4Br

moles NH4Br = 0.1645mol
mass of NH4Br = (14 + 4 + 79.90) x 0.1645
= 16.1g
I guarantee this is wrong haha, i dont know how to find the moles of the products.

Lol yeah its wrong =[ You cant just add together the moles of the reactants

Becuase HBr and NH₃ react to produce NH₄Br in the ratio 1:1:1, you go by the the lower number of moles (which is 0.0645mol) of the reactants

So 0.0645mol HBr reacts with 0.0645mol NH₃ to form 0.0645 mol NH₃Br

Then you can easily work out mass of the NH₃Br

 

[adnan91]

a) HBr + NH3 -----> NH4Br

HBr is an acid, NH3 is a base. As HBr combines with ammonia, it gives up a hydrogen atom, defining it by the Bronsted-Lowry definitions as a proton donor. The base, ammonia, receives this hydrogen atom (as well as the bromide), defining it by the Bronsted-Lowry definitions, as well, as a proton acceptor.
Ill edit this soon, the 2nd part looks extremely gay.

b)ok
moles HBr = 2.5/24.79 = 0.10mol
moles NH3 = 1.6/24.79 = 0.0645mol

0.10HBr + 0.0645NH3 --------> NH4Br

moles NH4Br = 0.1645mol
mass of NH4Br = (14 + 4 + 79.90) x 0.1645
= 16.1g
I guarantee this is wrong haha, i dont know how to find the moles of the products.

Next qu: Outline the procedure you would use to prepare a standard solution of sodium hydrogen carbonate from solid sodium hydrogen carbonate. Calculate the mass of solid sodium hydrogen carbonate required to make 250 mL of 0.12 mol L−1 solution.

C=n/V .12x0.25= 0.03 mol n=m/M 0.03x(23+1+12+16x3)=2.52 grams of NaHCO3

1. Obtain beaker, funnel, 250ml Volumetric flask, washing bottle with distilled water and a spatula.
2. Rinse all throughly with distilled water and dry.
3. Put beaker on electronic scale and zero. Carefully place 2.52 grams of solid NaHCO3 in the beaker with a spatula.
4. Place the funnel in the neck of the volumetric flask and transfer the 2.52 grams to the flash by 'washing out' the beaker with distilled water. Make sure all the water has passed into flask.
5. Rinse the funnel down to transfer any remaining solid.
6. Gently swirl the flask the dissolve any solid left.
7. Add distilled water unitl the just below the graduation mark.
8. Using a pipette transfer distilled water until the meniscus sits exactly on the graduation mark.
There u have it.


Question: How do u measure BOD and whats it significance.

 

[aussiechick007]

Next qu: Outline the procedure you would use to prepare a standard solution of sodium hydrogen carbonate from solid sodium hydrogen carbonate.
Calculate the mass of solid sodium hydrogen carbonate required to make 250 mL of 0.12 mol L−1 solution.

procedure:
- rinse a volumetric flask with distilled water.
- weigh the primary standard (solid sodium hydrogen carbonate) in a clean, small beaker.
- dissolve the correctly weighed amount in distilled water, and pour through a funnel into the volumetric flask.
- rinse the beaker several times with distilled water, pouring each time into the flask.
- add distilled water to the volumetric flask until the bottom of the meniscus reaches the calibration line.

calculation:
n = cv
= 0.250 x 0.12
= 0.03 moles required
n = m/M
m = nM
= 0.03 x 84.008
= 2.52 grams


Assess the impact of the recent development of a named biopolymer on society and the environment.

 

[SkimDawg]

Assess the impact of the recent development of a named biopolymer on society and the environment.

A recently developed biopolymer is poly(3-hydroxybutanoate).
The impact of this development is that it has demonstrated that it is possible to use a biological process (biotechnology) to manufacture commercial quantities of a polymer, and that it is possible to make a biodegradable polymer and to make it from a non-petrochemical source (virtually all other synthetic polymers are non-biodegradable and come from petrochemicals, that is from oil).
The impact is quite significant, first because it has shown that biotechnology can be used to synthesise purely chemical substances, not just biological ones, and secondly because it has produced a simple biodegradable polymer. Most other biodegradable polymers are complex mixtures of synthetic and natural polymers. The ability to produce polymers from non-oil sources is also of considerable significance. However the impact of developing PHB has not been as great as had originally been expected in that the polymer is quite expensive and has found only limited uses.

qu: Describe the role of a chemist employed in an industry or enterprise, and a chemical principle used by the chemist. (Choose an occupation other than teaching.)

 

[SkimDawg]

Question: How do u measure BOD and whats it significance.

The method used for measuring biochemical oxygen demand depends on the nature of the water to be tested, For municipal water supplies where BOD is expected to be low (because of protection of the catchment from contaminated run-offs), it is measured by adding nutrient to the sample and incubating the residual dissolved oxygen. The difference between the dissolved oxygen readings before and after incubation is the BOD.
Samples with high BOD such as untreated or partially treated sewerage would quickly use up all the dissolved oxygen, so they are analysed by adding oxygen to keep the sample saturated and measuring the amount of oxygen used. For these samples a suitable micro-organism as well as nutrient is added before incubation.
This is very significant as to give a reading of how healthy / polluted the waterways are.

 

[liamh16]

Describe the role of a chemist employed in an industry or enterprise, and a chemical principle used by the chemist. (Choose an occupation other than teaching.)

Robert Evans is an Environmental chemistry emplyed by Orica Ltd. He works at Botany Industrial Park, Sydney. A chemical principle that Evans uses is Neutralisation. He needs to have a thorough understanding on the reaction that takes place between acids and bases, incase he needs to neutralise a chemical spill or a spill that could be potentially hazardous to the enivironment.

Describe how commercial radioisotopes are produced, and, how transuranic elements are produced.

 

[danz90]

Describe the role of a chemist employed in an industry or enterprise, and a chemical principle used by the chemist. (Choose an occupation other than teaching.)

Robert Evans is an Environmental chemistry emplyed by Orica Ltd. He works at Botany Industrial Park, Sydney. A chemical principle that Evans uses is Neutralisation. He needs to have a thorough understanding on the reaction that takes place between acids and bases, incase he needs to neutralise a chemical spill or a spill that could be potentially hazardous to the enivironment.

Describe how commercial radioisotopes are produced, and, how transuranic elements are produced.

Commercial radioisotopes are produced in nuclear reactors as well as particle accelerators.
Example: Technetium-99m is a commercial radioisotope widely used in medicine for diagnostic imaging purposes.
Molybdenum-98 is bombarded with a neutron in a nuclear reactor, to produce Molybdenum-99.
Molybdenum-99 then undergoes Beta-decay to form Technetium-99m. ( ^99m₄₃ Tc )

Transuranic elements are those which have an atomic number > 92. (ie after Uranium). These elements are usually produced in heavy ion particle accelerators. Eg. Darmstadtium-269 is produced when a Nickel-62 ion is accelearted in a particle acceleator (with the combination of electrical and magnetic fields to generatic kinetic energy) , and is bombarded into a Lead-208 ion. The nuclei of the two ions combine to form the nucleus of a Darmstadtium-269 atom.


Question: Outline a procedure you would follow to produce a standard solution of Lead ions at a concentration of 10µg/100mL , starting with a small quantity of lead nitrate crystals.

 

[Undermyskin]

It's a shit one to use because it's deliquescent.

Undermyskin, I challenge you:
If you have a pH 3 acetic acid solution and a pH 3 HCl solution, predict which one will require more alkaline solution to neutralise it in a titration.

So sorry that I hadnot seen this question until a few mins ago while surfin through the last few pages.

Supposing we have the same volume for both of them, obviously for acetic acid to have pH 3, it should be of much higher concentration than HCl due to its weaker ionization energy. While 10^-3 M is [HCl], with 1% molecules being ionized (somewhere around than, if I'm not wrong), acetic acid in this case needs to have a concentration of 0.1 M. And neutralizing depends on the concentration of the acid, not the pH, a bigger volume of the same alkaline solution is required.

Danz90's: Outline...

10 microgram/100mL= 100 microgram/1000mL=0.1 mg/L

- Weigh out exactly 1.6 g of lead nitrate
- Dissolve in a 1L of distilled water in a volumetric flask. (remember to make up to the exact calibration mark, avoid paralax, etc.)
- Using a clean 10mL pipette, transfer into another 1L volumetric flask, fill up to the mark, shake to ensure a uniform solution.
- Using a clean 10mL pipette, transfer 10 mL of this solution into another 1L volumetric flask, fill up and shake. Done!