To find the concentration of hydronium ions \([H_3O^+]\) from the given pH, we can use the following equation:
\[ [H_3O^+] = 10^{-\text{pH}} \]
Given that the pH is 4.91, we can substitute this value into the equation:
\[ [H_3O^+] = 10^{-4.91} \]
Now, we calculate the value:
\[ [H_3O^+] \approx 1.23 \times 10^{-5} \]
Rounding this to 2 significant figures, we get:
\[ [H_3O^+] \approx 1.2 \times 10^{-5} , \text{M} \]
Thus, the concentration of hydronium ions \([H_3O^+]\) in the solution is approximately \(1.2 \times 10^{-5} , \text{M}\).