40 Days of Chem - 9.3.3 = Part 3

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

Experiment #10

Results

CHEMICAL	Type	UI colour	UI pH	pH
ammonia	cleaning	blue	9	9.5
drain cleaner	cleaning	purple	10	11
bicarb soda	cleaning	grey-green	8	8.2
stain remover	cleaning	pale green/yellow	6-8	8.1
oven cleaner	cleaning	purple	10	10.5
deep heat	cosmetic	orange	5	6.5
Listerine	cosmetic	red	3	4.9
mascara	cosmetic	yellow	6	7.6
deodorant	cosmetic	orange	5	6.4
perfume	cosmetic	green	7	6.8
sunscreen	cosmetic	orange	4	5.7
tea	food	red	3	5.7
cola	food	red	3	4.2
lemonade	food	red	3	3.5
milk	food	yellow	6	7.2
orange fizz	food	red	3	4.3
kiwi	food	red/orange	3	3.3
OJ	food	red	3	4.1
coffee	food	yellow brown	6	6.3

Worksheet Feedback

MC questions – Read the question carefully and answer the questions that is asked

Q11 – you actually need to write a sentence or two to explain the difference. Eg for d) saying 10⁴ difference means absolutely nothing. However, saying ‘pH 5 is an acidic solution with a concentration of hydronium ions 10⁴ times greater than a basic pH 9 solution” explains the difference AND more importantly demonstrates your understanding of Chemistry as you have brought in the connection between pH and acidity and linked the numerical pH value to its physical meaning. Sometimes explaining the difference can be as simple as defining each term. Eg for g) ‘A pH meter is an electronic device for measuring pH while an indicator solution is a chemical mixture that changes colour at specific pH’.

Q13) -  We are now in yr 12 Chemistry and the BrØnsted Lowry model is OUR model of acids and bases. Thus when asked to show ionisation of an acid in water you must show the acid donating protons to the water. In addition, only STRONG acids have the complete reaction arrow ®. Weak acids (being incompletely ionised) are in equilibrium with their conjugate base and thus need the ↔ arrow when ionising.

Q14) - Don’t fall for the trick in b) and e). If you are given grams per mL – you need to convert that into moles per mL and then moles per litre before you can calculate pH (or pOH). In Part e many used the moles of OH in the pOH equation rather then the CONCENTRATION of OH. Also remember that the # decimal places in a pH answer must be the SF of the question. For a) there are 2 sig figs (in scientific notion EVERY figure before the x10 is significant) do the pH answer needs 2 decimal places.

Q15) - As above but in reverse – the # decimal places in a pH needs to be the # Sig Fig in the final answer. So for a) there are 2 decimal places thus the answer should be to 2 sig fig. DON’T FORGET UNITS. pH is one of the only times you can leave off units (as there are no units!).

Q17) - We have finished 9.3.1 so we should have mastered these questions by now. IF the solution is yellow then that means each indicator is yellow (or clear). Thus you have to figure out the smallest possible pH range. In addition if an indicator goes from yellow to blue at pH 6.2 – 7.6 that means it is yellow BELOW 6.2, green between 6.2 and 7.6 and blue ABOVE 7.6.

Eg Indicator X changes from orange to blue at pH 7.4 to 8.3.  Indicator Y changes from green to orange at 4.4 – 6.4. Indicator Z is changes from blue – clear at 6.0 – 7.1. If a mixture of all three turns a solution orange, is the sample acidic, basic or neutral. Think then highlight below for the answer.

If the solution is orange, solution X must be orange so the pH < 7.4. This means it could still be acidic basic or neutral.

Indictor Y also has to be orange so that means the pH must be > 6.4. So now we know the possible pH range is 6.4-7.4.

However, indicator Z has to be clear else the sample would be ‘blueish’ colour. This that tells the pH > 7.1. Thus, the possible pH range is 7.1 – 7.4 and the sample is slightly basic.

Summary

A)     You must use the pH data to determine if each substance is acidic basic or neutral and this must be a column in the table. Do not include your predicted results, just the actual results.

B)      Do not say ‘universal’ indicator when you mean ‘all’ indicators. Call them ‘chemical’ or ‘liquid’ pH indicators – or ‘pH indicator solutions’.

C)      There are three main ways to test for pH: liquid indicators, paper indicator strips and pH probes. Each is useful in different situations eg one is very good for precise numerical pH readings, one is good for testing non-aqueous soil, one is good If you just need to test the A/B/ N nature of small sample, some are good for testing pH ranges of household liquids, one is good for automated pH reading, some are good for coloured solutions, one is good if destructive testing is permissible, some are good if it needs to be non-desructive testing, some are good if cost is a factor.

D)     You need to start using our notes for your summaries and when answering questions – ie take definitions etc from our notes. Do not take definitions from the Internet. Always include an equation to explain a relationship where possible.

E)      Don’t forget to mention the general A/B/N nature of cleaning products AND food.

F)      Remember you have to say what advantages the pH probe had (over indicator solutions) in this experiment (testing household chemicals) – eg effectiveness in coloured solutions…