Answer
Solution A: $[H_3O^+] = 1 \times 10^{- 4}M$
Solution B: $[H_3O^+] = 1 \times 10^{-6}M$
Work Step by Step
1. Use the following equation to calculate the hydronium ion concentration on solution "A":
$[H_3O^+] = 10^{-pH}$
$[H_3O^+] = 10^{- 4.0}$
$[H_3O^+] = 1 \times 10^{- 4}M$
** There is only 1 digit after the decimal point on $4.0$, therefore, the number of significant figures of the $[H_3O^+]$ must be equal to 1.
2. Repeat the same process for solution "B":
$[H_3O^+] = 10^{-pH}$
$[H_3O^+] = 10^{- 6.0}$
$[H_3O^+] = 1 \times 10^{-6}M$
** There is only 1 digit after the decimal point on $6.0$, therefore, the number of significant figures of the $[H_3O^+]$ must be equal to 1.
