# pH again

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)?

(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 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?

(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 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+.