The total amount of water \( w \) that Martina used can be expressed by the equation:
\[ w = 5 \times 8.5 \]
Here, \( 5 \) represents the number of containers, and \( 8.5 \) ounces is the amount of water in each container. Thus, the complete equation that models the total amount of water used is:
\[ w = 5 \times 8.5 = 42.5 \text{ ounces} \]
So, the total amount of water \( w \) that she used is 42.5 ounces.