Experiment #19

The Method (s)

Determine the wt% sulfate in a fertilizer

PLAN 1

1)      Wear PPE – gloves and goggles.

2)      Wash beaker with distilled water to remove impurities, dry & weigh.

3)      Weigh crystals of fertilizer in the pre-weighed beaker.

4)      Weigh a filter paper.

5)      Dissolve some fertilizer crystals in some water.

6)      Add BaCl₂ solution to precipitate the sulfate as BaSO_{4 (s)}

7)      Use a filter funnel and the weighed filter paper to filter out the precipitate.

8)      Wash all precipitate from the beaker into the funnel with a wash bottle & distilled water

9)      Place the filter paper on a tray and place in an oven to dry.

10)   Weigh the dry precipitate and filter paper and subtract the original mass of filter paper to find the mass of precipitate

11)   Calculate the moles of precipitate and sulfate, the mass of sulfate and the wt% sulfate in the fertilizer via wt%_sulfate = m_sulfate/m_fertilzser x 100

12)   Repeat the experiment for reliability

PLAN 2

bold = adjustments to the plan. The changes made to minimise error are numbered. You need to explain the error that each is designed to overcome as part of Q2 of your discussion.

1)      Wear PPE – gloves and goggles.

2)      Wash beaker with distilled water to remove impurities, dry & weigh.

3)      Weigh three pre dried⁽¹⁾ filter papers on a watch glass⁽²⁾.

4)      Weigh 10g of fertilizer crystals into a clean dry beaker.

5)      Make a 250ml standard solution* of this fertilizer in nitric acid⁽³⁾.

6)      Use a volumetric pipette to transfer 10ml of the solution to a clean test tube.

7)      Add 0.25 mol/L BaCl_{2 (aq)} – until no more ppt formation is apparent. Let the precipitate settle 24hr. Add more BaCl_{2 (aq)} and repeat until no more ppt forms⁽⁴⁾.

8)      Use a filter funnel and the three⁽⁵⁾ pre-weighed filter papers to filter out the precipitate.

9)      Wash all precipitate from the beaker into the funnel with a wash bottle & distilled water

10)   Wash the ppt with water ⁽⁶⁾.

11)   Dry the filter papers and precipitate on the watch glass in an oven to constant mass⁽⁷⁾.

12)   Weigh the dry precipitate and filter paper and subtract the original mass of filter paper to find the mass of precipitate.

13)   Calculate the moles of precipitate and sulfate, the mass of sulfate and the wt% sulfate in the fertilizer via wt%_sulfate = m_sulfate/m_fertilzser x 100.

14)   Repeat the experiment for reliability.

* Ensure the correct steps are followed to ensure accuracy of measurement and concentration – see titration and standard solution experiment.

The equipment used

The precipitates in different stages of settling* (most to least settled) 

*sedimenting

RESULTS    

All tests had 10.00 g of fertilizer dissolved into a 250.0 mL standard solution. 10.00 mL of this standard was reacted with excess aqueous barium chloride to produce a barium sulfate precipitate which was weighed.

GROUP	Mass ppt (g)
A	0.25
A	0.23
A	0.26
A	0.11
A	0.16
B	0.26
B	0.39
B	0.08
B	0.24
C	0.43
C	0.62
C	0.65
C	0.51

CHEMICALS

A = Iron (II) Sulfate - hmmm

B = Magnesium Sulfate - hmmm

C = Potassium Sulfate - hooray!  

 

 

Summary Feedback

 

NOTE - AAS can ONLY be used for analysing metal ion conc. It cannot be used to analye sulphate ion conc

 

A) After 19 experiments you should know that you can’t skimp on safety. The danger of each chemical is distinct. Choose 1 Ba compound e.g. BaCl₂ and HNO₃ and specify the dangers (and precautions) for each. Don’t forget about disposal issues. ‘Take care’ is not sufficient instead say ‘wear gloves to avoid contact with skin’ ie say exactly what ‘taking care’ means

 

B) Make sure you look up a proper chemical definition of each term – use textbook? – many definitions given were too specific and thus not thorough. You need a valid, general definition for each term.

C) Detail, detail, detail – don’t just say the precipitate might pass through the filter paper – this is unusual – say why e.g. "The very fine nature of the BaSO₄ ppt means it can pass through the filter paper…this would lead to an underestimate of sulfate content". NB the MOST important issues to discuss are: adding acid, adding excess BaCl₂, filtering, washing, drying – be detailed for each.

 

D) In the conclusion, state the class average and the theoretically calculated value

 E) You must show working for your calculation of the theoretical wt% sulfate in the fertiliser. Make sure you use the results posted to the Blurg (above) for your calculations for the group wt %. Also don’t forget the fertiliser was ammonium sulfate NOT barium sulfate.

 F) This summary can be 2 pages!

 G) Reliability means repeated with consistent results. We repeated but were the results consistent (they were to an extent but were not perfect) – what were the main sources of error? Remember that assess means you need to introduce the concept first – ie DEFINE reliability & ditto when assessing validity

 H) Make sure your calculations are clearly set out. ie don’t just write "n = 3.02 / 2233.37….". Use subscripts "n_BaSO4 = 3.02/233.37…"

 

I) Nitric acid was used as it will dissolve any phosphates in the fertilizer and also any carbonates (which are converted to CO2 and bubble out of solution). Thus the only ions that will precipitate with the Barium ions will be sulfate ions. Thus we know the ppt will be BaSO4 only (ie carbonates and phosphates will NOT ppt in acidic conditions)

But NB sulfates WILL precipitate in acidic, neutral and basic conditions – see Expt #18 results!

J) Because we added nitric acid there will be nitrate ions (and because there might be phosphate, chloride, potassium, magnesium, ammonium etc ions from the fertilizer) in the solution. These ions will be in the water surrounding the wet ppt. Unless we wash them through the filter with extra water they will crystallise when we evaporate the water from the ppt and add to the mass of the ppt … UPDATE-  thank you to those that read this before posting but Don't take shortcuts. nitrate ions will not 'crystallize' upon evaporation. They will crystalise as a salt with some of the cations in the solution - eg excess barium ions, metal ions from the fertilizer etc.

K) There were 4 possible fertilizers to calculate the wt% sulfate for – don’t forget ammonium sulfate

 

9.4.1 HSC Questions

2001 Q25 - Generally well answered but make sure you include:
* NOX is produced in all combustion engines - even with  optimal fuel:air ratios - however if there is excess air (oxygen) then the production of NOX increases.
* If you are going to present a combustion equation  -make sure it is BALANCED!
* Note that complete combustion also gives optimal energy output & fuel efficiency
* Actually say that insufficient oxygen causes incomplete combustion  - this was overlooked in a few of your answers
* Finally - to answer the Q - you must say why you need to monitor the products - ie by monitoring what chemicals are produced in the exhaust the conditions of combustion can be altered to add more oxygen (If C and CO are produced) or less oxygen (if excessive NOX produced) and thus increase fuel effieicncy and decrease pollution.

2012 Q33  - Generally well answered but make sure you include:

* The correct cell chemistry and reactions - this will always lose marks
* That the dry and silver cells are non-rechargeable and thus contribute to bulk landfill
* A full ASSESSMENT ie introduction to the chemistry and chemicals of the cell, positives of collaborating,  consequences of not collaborating (-ves?) and a clear judgement at the end - even labelling it'judgement' is a good idea so the marker won't miss it.
* Actually say specifically what the chemists who are collaborating do - don't waffle - just get to the point eg. Analytical chemists might analyse the soil and rivers near the rubbish tip to determine the concentration of (Zn - using AAS? and H+ - using a probe), they would communicate this with Environmental Chemists who would investigate the impact of these concentrations. They both might need to collaborate with Research chemists to determine a way to halt the spread of pollution. The research chemist may collaborate with industrial chemists to determine new ways of producing or packaging or recycling the cell. Electro-chemists would be involved in any research to manufacture new cells which are less harmful and rechargeable...

Experiment #18

Here are some  photos of Experiment #18 - thank you to the photographer. Because of some 'interesting' results I combined images from this year and last year. Click on the images for a larger version.

The key is;

1)      OH^-_(aq)

2)      NO₃^-_(aq)

3)      Cl^-_(aq)

4)      CO₃^2- _(aq)

5)      SO₄^2- _(aq)

6)      PO₄^3- _(aq) in neutral conditions

7)      PO₄^3- _(aq) in acidic conditions

8)      PO₄^3- _(aq) in basic conditions

9)      I^- _(aq)

PHOTO RESULTS

TABLE OF RESULTS

The full table of results can be downloaded at the link here. The file has two tabs, the first showing the correct results. The second tab showing our creativity and 'interesting' results. If nothing else these results demonstrate that old saying 'One person's sunshine is another persons sewerage'

AND - Here is another preview of Kingdom Rush 2 - not long now!



FEEDBACK

Worksheets

Generally well done. Tips:

b) When explaining flame tests it is important to say that when the atom absorb (heat) energy from the flame the electrons jump to higher orbitals, but when (they cool &) the electrons jump back to lower orbitals they emit light with specific energy (and thus colour) corresponding to the energy DIFFERENCE between the orbitals. These energy jumps, and thus colours, are unique to each element.

d) & e) ppm and unit conversions. A number of silly mistakes and errors here. REMEMBER that 1L = 1000g. Unit conversion is a skill that takes a fair bit of practise to master but once you get it – you have it forever.

i) be careful to answer the question. The studetn didn't know what the ions were so would have had to go through the whole cation test process - you just needed to indicate whatthye would observe along the way.

k) & l)  You need to revive excess and limiting reagent precipitation questions - particularly when asked to calculate concentrations of ions remaining in solution - you need to understand that any ion that ends up in the precipitate is no longer 'in solution'.

m) Ionic equations only include reacting ions (no spectator ions) and MUST include states. You have to be very careful to use our solubility tules when determining which ions form the precipitate.

n) & q) When explaining which ion must/must be in solution you have to give reasons. Questions n (a) and q needed some extra care in this regard. E.g. for n(a) it was insufficient to say it contained lead because it produces precipitates with sulfate, chloride and hydroxide ions. You needed to also why it COULDN’T be calcium – ie because it does NOT form a precipitate with chloride ions.

r) Again - remember that 1 mL = 1g, 1 L = 1000 g

BTW – Question n (c) and Question (o) had mistakes. Apologies.
n (c) should have said “…AND a precipitate with AgNO3.”
(o) should have had the last two columns identical with ppt for both hydroxide and sulfate but not chloride.

Summary

Generally VERY good

A) Safety – need to specify which are the heavy metals, their dangers, precaution and disposal strategies. Ditto for the acidic and basic chemicals. Finally you needed to specify why there was no flame test for lead. Don’t worry about the other chemicals.

B) Flow charts – States are need for all reactions. Take care with your charges on ions. Equations must be balanced.

C) Relevant Chemistry – make sure you are thorough in your explanation of the logic behind the choice of steps – focus on the broad reasons for choosing the steps for any sequence of precipitation testing – bit just the steps we used in our flowchart BUT you should include a specific example from our flowchart to illustrate the consequences of not testing in the correct order. eg when testing for cations if we had added sulfate first…

D) Some people were missing part of the table – please ensure the table is complete.

E) Sulfate precipitates with barium ions in acidic conditions but phosphate and carbonate wont (because carbonates react to form carbon dioxide in acidic conditions and phosphates are soluble in acidic conditions) BUT ALL THREE will precipitate with barium ions in neutral and basic conditions

F) Compare is a fairly low level verb – just similarities and differences – ie “Both are about the dangers of lead but…” then just a sentence comparing the differences of each issue style, purpose, accuracy, reliability. – 5 lines max? (but don’t forget the similarity – they are both about…)

G) This is a 1 page summary and 1 page flowchart report

H) Nitrates do NOT form precipitates. When you look at the formula for nitrate I want you to see NO₃, ie NO precipitates

While marking this summary I was watching 'Giggle & Hoot' on ABC2 with my kids. This song really grabbed me. Probably the best song I have heard all year. On par with 'Life's a Happy Song' and 'Feel Inside - & Stuff Like That'.

9.3.3 HSC Questions Feedback

This set of questions stood out as something that we can work on. You all knew hat pH was and how it related to strength and concentration but the way that these concepts are expressed is crucial – you have to be very careful with terminology (eg weak & strong) and you have to clearly make the link between strength, concentration [H+] and pH.

WARNING – AVOID THE TRAP we really have to stop falling for the tricks where Q ask about increasing or decreasing pH. HIGHER [H+] = LOWER pH

2001

Q20b)  You need to correctly use the terms strong and weak, You need to link them to %I and thus the final hydrogen ion CONCENTRATION and thus to pH via the formula. ‘Even though both acids are at the same concentration (0.1 mol/L) , HCl is a strong acid with 100% ionisation and thus will have a [H+] of 0.1 mol/L. Citric acid is weak and thus has a %I < 100 &  will have [H+] of less than 0.1mol/L. Thus the pH of the hydrochloric acid is lower than citric acid as pH = -log[H+] (ie pH HCl = -log[0.1] = 1) and thus as [H+] increases, pH decreases’ Note you could figure out the citric and HCl %I form the data – this is a GREAT way to answer these questions and reinforce the concept of strng and weak. But it is also something to watch for because according to the data in this question %I for citric is 25% which means that for THIS QUESTION saying the %I of citric is 8% is technically wrong.

2002

Q22b &c) for b) you needed to know that sulfuric acid was diprotic, thus can produce a higher [H+] then HCl when at the same concentration, so sulfuric acids pH will be lower. For c) ditto Q20b) 2001e above, without the calculations.

2003

Q8) We have been over the diprotic nature of sulfuric acid SO many times…

2004

Q7) WARNING – AVOID THE TRAP we really have to stop falling for the tricks where Q ask about increasing or decreasing pH. HIGHER [H+] = LOWER pH

Q24a) This was dilution so use the dilution equation. In addition the #SF in the ‘normal’ numbers in the data is used to calculate the # decimal places in a pH answer (& the #decimal places in pH data = the #SF you need when calculating the answer)

Q24b)  If you are given two chemicals and asked about their uses you should give a different use for each. Also Citric acid is NOT ascorbic acid (vitamin C). Citrus fruits have both these acids in them but it does not mean that they are the same thing. Just like, Pepsi has water and sugar and caffeine in it but they are different chemicals.

Q24c)  You have to give the pH equation. You have to say that higher [H+] = lower pH then you have to say why the [H+] – ie hydrogen ion CONCENTRATION - is different in each and thus why the pH is different in each. Many fell into the trap of talking about the ‘amount’ or moles of H+ in solution – THAT DOES NOT MATTER. pH depends on hydrogen ion CONCENTRATION. Also – NEVER talk about things being ‘more’ or ‘most’ or ‘less’ acidic. This means nothing in Chemistry and reflects badly on you as a Chemistry student. If you mean ‘stronger’ say ‘stronger’. If you mean ‘more concentrated’ say ‘more concentrated’. Finally you need to ay that HCl was strong a t100% ionised and that citric and acetic were weak but had different [H+] as citric is 8% ionised and acetic 1% ionised(giving citric a higher hydrogen ion concentration and a lower pH than acetic).

2005

Q8)  We have been over the diprotic nature of sulfuric acid SO many times…

2007

Q10)  “...must be added…” I’ve thrown this trick at you a few times. It is cruel but you need to be aware and looking for it in ALL dilution questions.  

Q21a&b)  We have been over the diprotic nature of sulfuric acid SO many times…

Q21c)  The same inexplicable numerical error showed up in approximately half of the papers. Again – work together to discuss solutions to the questions but go and work independently. IF you are just copying answers without thinking: a) it is not helping  b) you are not learning, c) you are not showing the person you are copying from respect (because if you were thinking you might have found their mistake and helped them) & d) it means that I can’t fully diagnose problems you are having.

2008

Q14)  This was basically identical to 2006 Q17b and 2012 Q28 but we did well in those and poorly in this. The main error was using the moles of hydrogen ions rather than the concentration in the pH calculation. A silly, avoidable, mistake.

2010

Q21) You need to distinguish yourself from the crowd so if you are given data USE IT! Just like 2001 Q20b – you have the data to calculate the %I so do so. Basically if you ever get a compare the pH question and you can calculate %I then do it. (remember in our first assessment the rule that if you were provided with a stimulus you had to use it in your answer – this is the same – if provided with numbers and you can do a calculation to support your answer then do the calculation)

2012

Q11)  WARNING – AVOID THE TRAP we really have to stop falling for the tricks where Q ask about increasing or decreasing pH. HIGHER [H+] = LOWER pH. You shoud know the different %ionisation of hydrochloric, acetic an citric acids.

Q18) 1) Weak acids are in EQUILIBRIUM with their conjugate base, water and hydronium. Changing any of these chemicals will shift the equilibrium. Eg CH₃COOH _(aq) + H₂O_(l)  ↔ CH₃COO^-_(aq)  + H₃O⁺_(aq)  Diluting is adding water, thus by LCP the reaction will shift to remove water, thus reducing the acid conc and increasing the conc of conjugate and hydronium – thus raising the %I.

2)  WARNING – AVOID THE TRAP we really have to stop falling for the tricks where Q ask about increasing or decreasing pH. HIGHER [H+] = LOWER pH

9.3.2 HSC Questions Feedback

Generally good results but disappointing that half the class did not read the instructions to leave 2002-Q21, 2006-Q22 or 2009-Q18 AND in addition, did not do the extra 2010 and 2011 questions that were sent with the instructions.

2001

Q5) Read the question – UNPOLLUTED rainwater

Q9) It is expected that you know that LCP is about position of equilibrium (yield) NOT about rates of reaction. Also you should know that catalysts only affect the rate NOT the yield of a reaction.

Q10) Read the equations very carefully. IF something is not equilibrium then LCP does not apply.

2003

Q7) Read the WHOLE question carefully ‘reacts with’ is very different to ‘acts as’

2004

Q9) Be careful – this was a limiting reagent Q and like all these Q the limiting reagent is usually written second to trick those who don’t realise it is a limiting reagent question.

Q10) BE careful  - decreasing the pressure causes the equilibrium to shift to the side with more moles of gas (to counteract the decrease in pressure)

Q18) Analyse is a high level verb. For this Q you needed to ID that metal (left side) oxides were generally basic, non-metal (right side) acidic and that there were some metals near the semimetals that are amphoteric. THEN to fully answer the Q you should have included:

• Example of acidic (eg CO₂), basic (eg CaO) and amphoteric (eg Al₂O₃)
• An example reaction showing at least one of th above
• NB that some oxides with low ox# are neutral eg CO, N₂O
• That the far right side of the PT – Noble gases do not form oxides at all

2005

Q22a) Whenever you are asked to plot experimental data be aware of outliers. If you THOUGHT about the reaction you would realise that it would have been impossible for less H2 to be formed at 0.56 g than at 0.33g. The graph should have been two straight lines with intersection corresponding to the stoichiometric amount of Zn.

Q22b) After 1g of Zn the HCl was the LIMITING reagent thus any Zn beyond 1g would not increase the amount of H₂ produced. It is important that you use the term/s limiting and/or excess in your answer. How to know that 1g was the stoichiometric mass of Zn – either from the intersection of the two straight lines on the graph OR by calculating the maximum moles of H₂ and thus the stoichiometric moles of HCl/Zn  

Q22c) In this Q the molar mass has 4SF, the molar volume is 4SF, the mass of An is 2SF and the volume of HCl is 1SF – BUT the volume of HCl is not used in the calculation so the correct # SF is…

2006

Q22) you were not asked to answer this Q but NB that the Q only asked about sulfur dioxide and it said discuss the evidence – thus you had to look at the evidence that SO₂ was increasing (acid rain damage, ice cores etc) and the difficulties (low conc, highly soluble, only measured in recent times) AND you have to use equations in your answer (ie 2+)

Q23bii) This was poorly answered - as soon as you have an equilibrium sign and are asked to explain the effect of –anything- then refer to LCP. Adding NaOCl increase the conc of OCl^-, thus via LCP the equilibrium shifts… AND please don’t make the embarrassing mistake that decreasing acidity = decreasing pH.

2007

Q22b) you should have mentioned that S combusts to form SO₂, that SO₂ is a respiratory irritant, that SO₂ causes acid rain (with equation) and the effects of acid rain on the environment (built and natural). Then you just had to say that reducing S would have a positive impact. Evaluate was a poor verb to choose for a 4 mark Q. I’m not sure how critical it was to get full marks.

2008

Q20a) the unit of Molar mass is g/mol  (ie M = m/n = grams/mol)

2009

Q2) see 2001 Q5

Q15) This is a tricky but important question. There was no excuse for leaving it blank. This is a question in your assessment task so those who didn’t do it I suggest you work to catch up.

Q18) Again you did not need to do this Q – but NB that it was very similar to 2006 Q22) but this time about N rather then S. So the same strategy applies except that NO_X also causes ground level ozone/smog so it needed to be mentioned.

2010

Q18) This linked to 9.3.4 – none of the ins in the options were in the equilibrium reaction – so at first glance it might seem that none had any effect. But NB that there was H+ in the equilibrium so anything that affects [H+] will effect this equilibrium. I wonder if any of those salts will add or remove H+ from solution.

Q26) Another excess/limiting Q – just be aware that 0.50 and 0.20 are 2 SF.

2012

Q15) I wonder what acid I would get if I reacted SO₃ with H₂O?

Exciting news: While procrastinating about this marking I discovered my first ever emerald ore in Minecraft & I got an achievement for killing a ghast with its own fireball! It is amazing that such a ridiculous-looking, pixelated, monster could be so terrifying in-game. The sounds of one approaching puts me on the edge-of-my seat. (pictures from here and here)

& there is an awesome Minecraft desktop wallpaper here