You can use the same kind of logic on pH. Here are two problems
The pH of lemon juice is 4.2. The pH of milk is 7.8. What is the difference in hydrogen ion concentration (approximately)?
- 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 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 L of lemon juice?
- I need a hint ... : Use "the log is the power" to figure out the amount of H+ in 1 L 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 1 L of lemon juice, which is obviously wrong! ... Then you remember the minus sign.
- ...another hint ... : So how much of H+ in 5 L?
- ...another hint ... : 10−4.2 = 0.000063 moles, so 5 L 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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