Number of hydrogen ions present in 10 millionth part of 1.33 cm³ of pure water at 25° C is

10 million =10⁻⁷

∴ 10 millionth part of 1.33 cm³ = 1.33 × 10⁻⁷cm³ = 1.33 × 10⁻⁷ mL

For pure water, [H+]=10⁻⁷ mol/L

⇒ 1L water contains [H+]=10⁻⁷ moles

or 1mL water contains [H+] = $\frac{{10}^{-7}}{1000}$​=10⁻¹⁰ moles

or 10 millionth part of 1.33 cm³ water contains

[H+] = 1.33 × 10⁻⁷ × 10⁻¹⁰ moles = 1.33 × 10⁻¹⁷ moles

∴ Number of H+ ions =1.33×10⁻¹⁷ × N_A​

= 1.33 × 10⁻⁷ × 6.022 ×10²³

= 8.009 × 10⁶ = 8.01 million