When the fish-tank water has a ph of 8.0, the hydronium ion concentration is 1.0 × 10-8 mole per liter. what is the hydronium ion concentration when the water has a ph of 7.0?

You must know and use the formula for pH.

pH = - log [H3O+], where [H3O+] is the molar concentration of hydronium ion.

So, when pH is 8.0 => 8.0 = - log [H3O+] and you can use antilogarithm (the inverse function of logarithm) to find [H3O+], in this way:

[H3O+] = 10^-8 = 1 * 10 ^-8 M

When, pH = 7.0 =>

7.0 = - log [H3O+] => [H3O+] = 1 * 10^ -7 M

1*10^-7 mole / liter

If you open the bottle of perfume then nothing but the smell will come out of it and yes it is a type of gas smell because it’s say it in english so i can understand better and you