Anyway I redid it from 'memory' and astonishingly the results exactly match the theory. How lucky is that!.
Formula
|
Name
|
UI colour
|
UI pH
|
Probe pH
|
pH
|
Na2CO3
|
purple
|
10
|
11.2
|
11
|
|
Na2SO4
|
green
|
7
|
6.8
|
7
|
|
NaCl
|
green
|
7
|
6.6
|
7
|
|
KCl
|
green
|
7
|
7.2
|
7
|
|
K2CO3
|
purple
|
10
|
11.1
|
11
|
|
KNO3
|
green
|
7
|
7.1
|
7
|
|
NaNO3
|
green
|
7
|
7.1
|
7
|
|
NaCH3COO
|
blue-green
|
8
|
8.3
|
8
|
|
Na3C6H5O7
|
blue-green
|
8
|
8.2
|
8
|
|
NH4Cl
|
yellow
|
6
|
5.7
|
6
|
|
(NH4)2SO4
|
yellow
|
6
|
5.6
|
6
|
|
NaHCO3
|
green
|
7
|
7.5
|
7.5
|
Generally the reports were very good. Suggestions
(A) You should use 2 equations per non-neutral salt. Some people were very thorough and explained the reason for the salts being non-neutral from 'first principles' - ie BL theory. But this Q asked you to'write equations to ACCOUNT for' the salts being non neutral. It did not ask for an explanation for each. You explain in general the reasons for salts being non-neutral in the next question. All you need to do is write equations to show why they are non-neutral. Eg
NH4Cl was acidic. This is because
NH4Cl (aq) => NH4+ (aq) + Cl- (aq) (this show the ions produced when the salt is in solution)
NH4+ (aq) + H2O (l) => NH3 (aq) + H3O+ (aq)
(this equation shows that the NH4+ it is a proton donor – ie an acid, thus NH4Cl is an acidic salt) You don’t need to write the stuff in italics.
(B) You need more detail about the indicators used in titrations. Specifically define endpoint and equivalence point and give the indicator colour ranges (you need to practise) I’ll do one in full here. The others are up to you.
“When titrating a strong acid and strong base the salt produced will be neutral (NB you may have already said why in the previous Q of the report, but if not you need to put it here like I do next). This is because the salt will consist of the very weak conjugate base of the strong acid and the very weak conjugate acid of the strong base. Thus the solution will be neutral at equivalence point. Thus and indicator with END point at pH 7 must be chosen. Thus Bromothymol Blue is used as it changes from yellow to green to blue at pH 6 – 7.6. NB the use of the terms equivalence point and endpoint. NB the specific colour changes matches to the specific pH’s
(C) The conclusion needs to address all the aims. Thus, it needs to say what equipment was chosen and that the pH’s were measured and some were A, B & N
(D) I accidentally used C and D for the same thing. So, see (C) above
(E) The citrate ion is what is produced when citric acid has donated all it protons ie C6H5O7 3-. In addition this means that sodium citrate is Na3C6H5O7.
(F) The bicarbonate ion is slightly basic. This ion is in high concentration is sea water. Hence sea water is slightly basic.
(G) For this question you need to introduce Bronsted Lowry theory, ie define what a acid and base are and what a conjugate pair is (eg a conjugate base is formed when an acid has donated its proton - ie it has one less H+ than the parent acid). You need to give examples of what the pH will be for salts made from SA-SB, SA-WB, WA-SB with reference to strength of conjugates.
(H) Your table needs to be complete & correct.
(I) You can put the table on a second page to save space (some of the explanations for the last two questions can be quite long.
(J) A GREAT way to introduce indicator choice questions is : "In neutralisation Acid + base =>salt +water. Thus the pH at the equivalence point (define) depends on the acidic/neutral/basic behaviour of the salt. The endpoint (define...) of the indicator needs to match the pH of the equivalence point (define...) and thus its colour change must correspond to the salt's pH".
Finally – have a play with these animations. Most are about acidic environment. However, depending how your skills at balancing equations and figuring out molecule polarity (make sure you choose the 'real molecules' tab) are you might find some of the non acid/base ones useful too.
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