Knowing your stuff? Running quiz

Kicking off a new thread of a running quiz of little ditties to get the old grey matter working (or not).

Anyone to contribute with quizzicals, answers, discussion….

You have 2 fish tanks, each filled 1000 litres of pure water.
Tank A…. you add Hydrochloric Acid so that you have 0.37g of HCl to get a pH of 5.0
Tank B …..you add 60g of Acetic Acid to get a pH of 3.9.

Q1. To which tank have you added the stronger acid?
a) Tank A, or
b) Tank B, or
c) They are both the same.


Q2. In the above Tank A. If 0.37 gram of HCl gives a pH of 5. How much EXTRA would be needed to give a pH of 4.0 (assuming no change in temp or pressure, and that the water does not obtain any other chemical).

ian

1: a


2:
far too nasty for a sunday morning hangover and a formula I promptly forgot 23 years ago after first year exams.
Guess: 3.33g (or 3.3g?)

c

and question 2 is a trick question

murph wrote:

1: a


2:
far too nasty for a sunday morning hangover and a formula I promptly forgot 23 years ago after first year exams.
Guess: 3.33g (or 3.3g?)

100% right Murph. {I'd have even accepted 3.7g for the Q2 for a good 90%, but 3.3g is 100%. }

@ Dar the only trick in Q2 was that I'd asked how much EXTRA....for which if anyone misread the question they would have got 3.7g (but I'd have said OK even in that case).

It so happens that at that pH and quantity of a strong acid used, the effect of the buffering capacity of water towards pH would be negligible (and would have probably only made a difference of 0.0000001gram anyway...I didn't calculate that, but it is that order or magnitude)

If, however, Q2 was asking about Acetic Acid (Tank 2) then that would have been a much more difficult question and not so straight forward.

Do the answers need an explanation? (as there it hoped to be a discussion around this thread...ie I hope it goes off-topic a wee bit ).

ian

I must have been on auto-pilot this morning!

Maybe you should have put in the suplemental question on pH4 with acetic acid!
(Although you did not specify the starting temp/pressure and I have NOT recovered sufficiently to do the maths. The bottle of Riscal in front of me might help in a while.)

Although I have vauge recollections of a 5 or 10molar solution being pH4.2...

The acetic acid question would require a temperature noted, and the pKa would need to be given at that temperature.
For the HCl, at the dilution used, the temperature effect would not have too much of an influence if we are only doing a comparative question (as was given).

OK, I'll give a question and solve it (with reasoning) for Acetic Acid in Tank B.
If tank B is at standard temperature and pressure, and the pH for 60g acetic acids in the 1000 litres is measured at 3.9.
Then how much EXTRA Acetic Acid is required to drop the pH by one unit to pH 2.9?

(we assume that the solution is sufficiently, but not overly, dilute such that we can use concentrations to do our pH calculations).

The answer is we need 6000g in 1000 litres to get to a pH of 2.9, so we need 6000-60 (=5940) grams extra (ie almost a 100 fold increase in the amount of added acetic acid).

The same trend (ie 100 times more) would be approximately true if we go from pH 4.9 to 3.9.

In general, at the pHs of ~5.5 and under, a ten-fold increase in acetic acid will produce a pH drop of ~0.5 (in pure water of course at standard temp and pressure).

Now, if we have a pH close to pH then things are little different......why?
The effect of waters acid dissociation starts to be an increasing effect on the pH.
If have added acetic acid (0.6 micrograms in 1000 litres) to get a pH of 6.95 then a 10 fold increase in acetic acid will drop the pH by only 0.1 unit to pH 6.85 (I'm approximating....and this almost impossible to do in reality because of carbon dioxide, water movement etc etc).

Also, in these I am making an assumption (and it is a valid assumption) that the 'Activity' of the acid in solution is equal to the 'concentration'.

pH = -log {Hydrogen ion Activity}, and is only approximately equal to -log[Hydrogen Ion Concentration].
But for most solutions the approximation works well for doing work on the bulk measurable pH. At interfaces (eg filter surfaces), or extreme concentrations, this may not be the case.

Hence, why asking about acetic acid would be an unfair question.

The main point of these questions were to emphasise the logarithmic nature of pH and that small changes in pH (which is only one of the measures of acidity) can actually be a massive change in the amount of acid in solution.

ian

I thought Murph might set a question.

So, another weekend conundrum...

You do a Nitrite test and a Nitrate Test.

You get exactly the same result for both tests.....say 5ppm (ignoring the actual impact of the actual example value on your fishes health....AND assume that the both test its have the same sensitivity).

What might explain getting exactly the same result?

igmillichip wrote:

I thought Murph might set a question.

So, another weekend conundrum...

You do a Nitrite test and a Nitrate Test.

You get exactly the same result for both tests.....say 5ppm (ignoring the actual impact of the actual example value on your fishes health....AND assume that the both test its have the same sensitivity).

What might explain getting exactly the same result?

For most test kits, the above result would probably mean that there is zero nitrates and 5ppm nitrites if the test is done correctly with in-date reagents (even though the test says 5ppm nitrates).

Most nitrate test kits on the market are nitrite test kits......they test for nitrites in the test water.

So what happens is that the nitrate test kit first converts the nitrates to nitrites (often by zinc or other suitable reducing agent), and the colour is a measure of the TOTAL nitrites within that test tube.
ie these types of nitrate test kit gives a result that is the nitrite plus nitrate in the tank water.

So if you actually had 5ppm nitrites and 5ppm nitrates, then the nitrate test is more than likely give you a nitrate reading of 10ppm nitrates.

Now, it is also possible that the nitrates are very high and the during the test too much of one reagent (the reducing agent...eg zinc) is added......this would push the reduction of nitrates much further than to producing nitrites.....you'd actually get nitrogen gas given out....and you'd end up with an unrealistic result (ie an unknown).

Now....not all test kits are the same, but if the test kit is of the above type then this is a possible scenario.

The bottom line is...
even though many nitrate test kits will also measure the nitrites, that should NOT be reason to consider a nitrate test useless and inaccurate: if the level of nitrite in a tank is so high as to interfere with the realistic sensitivity of a nitrate test kit, then you have more to worry about than the nitrate levels.

accurate or precise results from a test kit can only be obtained if the quantities of reagent are strictly followed....ie don't add an extra drop for good-luck.

thoroughly clean all test tubes immediately after any test.....residual reagents (not the aquarium) will either give false highs or false lows.
In the above example, it may be that some residual reducing agent (eg zinc) is left in the test tube from a previous test....and that would have reduced the amount of nitrate to measure too much.

Similarly, tests that require a count of drops may also suffer from residual reagents left in a test tube.

ian