Differences in acidity
Remember how we calculated the difference in energy released by two earthquakes? You can do the same thing with acidity:
The pH of lemon juice is 4.2. The pH of milk is 7.8. What is the difference in hydrogen ion concentration (approximately)?
(To make this problem interactive, turn on javascript!)
- I need a hint ... : The difference in pH is 7.8 − 4.2 = 3.6
- ...another hint ... : So the difference in concentration will be somewhere between
"3 zeros" and "4 zeros" (between 1000 and 10000).
- ...another hint ... : 103.6 = 3981.
- ...another hint ... : Make sure you know which one is more concentrated!
- ...another hint ... : Lemon juice is more acidic, so it should have more H+.
Therefore, lemon juice has 10 3.6 = 3981 times as many hydrogen ions as milk.
I think I have the answer: 3981 times as much.
Finally, can you figure out how to use the "unlogging" trick to calculate the concentration?
The pH of lemon juice is 4.2. How many moles of H+ are present in 5 litres of lemon juice?
(To make this problem interactive, turn on javascript!)
- I need a hint ... : Use "the log is the power" to figure out the amount
of H+ in 1 litre of lemon juice (and don't forget that it is really MINUS 4.2...)
- ...another hint ... : If you forgot the minus sign, you end up with 16,000 moles of
H+ in one litre of lemon juice, which is obviously wrong! ... Then you remember the minus sign.
- ...another hint ... : So how much of H+ in five litres?
- ...another hint ... : 10−4.2 = 0.000063 mols, so 5 litres of lemon
juice contains 5 times as much -- 0.000315 moles of H+.
I think I have the answer: 5 * 10−4.2 = 0.000315 moles of H+
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