A chemist must prepare 0.9 L of sodium hydroxide solution with a pH of 13.40 at 25°C. He will do this in three steps: Fill a 0.9 L volumetric flask about halfway with distilled water. Weigh out a small amount of solid sodium hydroxide and add it to the flask. Fill the flask to the mark with distilled water. Calculate the mass of sodium hydroxide that the chemist must weigh out in the second step.

9.04 g

Explanation:

Given that:

pH of NaOH = 13.40

pOH = 14 - pH

pOH = 14 - 13.40

pOH = 0.6

Now, from there we can find the concentration of NaOH = [OH⁻]

=

=

= 0.2512 M

Given that; volume = 0.9 L

∴ number of moles of NaOH = volume × concentration of NaOH

= 0.9 × 0.2512

= 0.2261 moles

mass of NaOH = number of moles of NaOH × molar mass of  NaOH

mass of NaOH = 0.2261  × 40

mass of NaOH = 9.04 g

"record the time to complete a chemical reaction" would most likely be part of a lab procedure.