How many significant figures are there in the measurement 64,000 mg? a. 2 b. 3 c. 4 d. 5 (possibly a but idk)

I'm pretty sure that significant figures is just the amount of numbers there is. So, in this case I think the answer would be D. 5

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.