So pH and pOH together should add up to fourteen. So as it stands, you have the pOH (concentration of base). All you need to do is subtract this from fourteen, giving you 5.6 pH units. In order to get the concentration of hydrogen ions, you use this number and plug it into the following equation:
pH = -log [H3O+]
5.6 = -log [H3O+]
[H3O+]= 10 ^ -5.6
so [H3O+] = 2.51 x 10^-6 keeping with significant figures.
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Verified answer
So pH and pOH together should add up to fourteen. So as it stands, you have the pOH (concentration of base). All you need to do is subtract this from fourteen, giving you 5.6 pH units. In order to get the concentration of hydrogen ions, you use this number and plug it into the following equation:
pH = -log [H3O+]
5.6 = -log [H3O+]
[H3O+]= 10 ^ -5.6
so [H3O+] = 2.51 x 10^-6 keeping with significant figures.
Does this all make sense?
pH + pOH = 14.00
pH = 14.00 - pOH
pH = 14.00 - 8.40
pH = 5.60
[H+] = 10^(-5.60)
[H+] = 2.5 x 10^(-6)
Answer: The hydrogen ion concentration of the solution is 2.5 x 10(-6)
Here's a real answer.
pH + pOH = 14.00
pH = 14.00 - pOH = 14.00 - 8.40 = 5.60
[H3O+] = 10^-pH = 10^-5.60 = 2.5 x 10^-6
pOH = 14-pH
8.4 = 14 - pH
pH = 14 - 8.4
pH = 5.6
pH = -log [H+]
[H+] = 10^-pH
[H+] = 10^-5.6
[H+] = 2.51188643 Ã 10-6 M