#### Answer

There are 1000 hydronium ions for every 10 billion molecules of water.

#### Work Step by Step

$10$ $Billion = 10,000,000,000 = 10^{10}$
- Let's do a simple proportion calculation:
$\frac{1 Water}{10^{-7} Hydronium} = \frac{10^{10}Water}{x}$
$10^{7} = \frac{10^{10}}{x}$
$x = \frac{10^{10}}{10^7}$
$x = 10^3 = 1000$ $Hydronium$ $ions$