## Chemistry (7th Edition)

- The rule for significant figures for the pH scale works like this: The number of significant figures of the $[H_3O^+]$ in M, has to be the same numbers $after$ the "." in pH scale. For example: $[H_3O^+] = 3.56 \times 10^{-9}M$, there are 3 significant figures. - The number that appears in the calculator; for the pH, is: pH = 8.448550.... And , we have to have 3 significant figures after the ".", so: pH = 8.449 a) $pH = -log[H_3O^+]$ $pH = -log( 2 \times 10^{- 5})$ $pH = 4.699 = 4.70$ b) $pOH = -log[OH^-]$ $pOH = -log( 4 \times 10^{- 3})$ $pOH = 2.398$ $pH + pOH = 14$ $pH + 2.398 = 14$ $pH = 11.602 = 11.6$ c) $pH = -log[H_3O^+]$ $pH = -log( 3.56 \times 10^{- 9})$ $pH = 8.449$ d) $pH = -log[H_3O^+]$ $pH = -log( 1 \times 10^{- 3})$ $pH = 3$ e) $pOH = -log[OH^-]$ $pOH = -log( 12)$ $pOH = -1.079$ $pH + pOH = 14$ $pH + -1.079 = 14$ $pH = 15.079 = 15.08$