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…