A solution has [H⁺] = 2.0 × 10⁻³ M, a value with two significant figures. Reported to matching precision, its pH is:

Core Concept

On logarithmic scales, the digits after the decimal point carry the significant figures of the original concentration. Since 2.0 × 10⁻³ has two significant figures, pH = -log(2.0 × 10⁻³) = 2.70 gets exactly two decimal places. The digit before the decimal only locates the power of ten.