Question

A chemist must prepare 800.0mL of sodium hydroxide solution with a pH of 12.10 at 25°C. She will do this in three steps: Fill a 800.0mL 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. Round your answer to 2 significant digits.

Answer 1

Answer:

0.42 g

Explanation:

We have:

pH = 12.10 (25 °C)

V = 800.0 mL = 0.800 L    

To find the mass of sodium hydroxide (NaOH) we can use the pH:

$14 = pH + pOH$

$pOH = 14 - pH = 14 - 12.10 = 1.90$

$pOH = -log ([OH^{-}])$

$[OH]^{-} = 10^{-pOH} = 10^{-1.90} = 0.013 M$

Now, we can find the number of moles (η) of OH:

$\eta = ([OH]^{-})*V = 0.013 mol/L * 0.800 L = 1.04 \cdot 10^{-2} moles$

Since we have 1 mol of OH in 1 mol of NaOH, the number of moles of NaOH is equal to 1.04x10⁻² moles.

Finally, with the number of moles we can find the mass of NaOH:

$m = \eta * M$

Where M is the molar mass of NaOH = 39.9 g/mol

$m = 1.04 \cdot 10^{-2} moles * 39.9 g/mol = 0.42 g$

Therefore, the mass of sodium hydroxide that the chemist must weigh out in the second step is 0.42 g.

I hope it helps you!