Salad (lettuce, tomato, and onion) a pure substance or a mixture ?

I'd say it's a mixture, because you are putting different ingredients together to make something new; aka this salad.

when [oh-] is 10-5  m, and [h3o+] is 10-9  m, the poh is 5 and ph is 9. again in both cases the sum of ph and poh is 14. however, the hydrogen ion concentration is not always going to be equal to exactly 1 x 10 raised to a negative number. for example, we skipped over the value of 2.0 x 10-7.

c) should be the answer.