Acetic acid (which is commonly known as vinegar) has a ph of 4.8. what is the hydronium ion (h3o+) concentration of the acetic acid?

Formula
pH = - log [H+] 
H+ is the H+ concentration of the acidic content of a solution. The form of the formula you want is

[H+] = 10^-pH

Calculation
[H+] = 10^-4.8
Put it into your calculator like this.
10
^ or y^x or x^y whatever your calculator has.
+/-
4.8
=
You get
[H+] = 1.58 * 10^-5 <<<< answer
This is the usual way [H+] is expressed. If you need it in decimal form, the -5 tells you that you have four zeros to the right of the decimal point. In that case

[H+] = 0.0000158 

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