Question

What mass of NaOH should be dissolved in sufficient water to get 20 m³ of an aqueous solution of pH =7.3 at 25∘C?

A. 0.16 g

B. 1.6×10⁻⁴ g

C. 0.04 g

D. 0.12 g

Answer 1

To prepare a solution of pH 7.3, we need to calculate the mass of NaOH required. By converting the pH value to [H+] and using the molarity formula (mole = volume × molarity), we can find the number of moles of NaOH needed. Finally, by multiplying the number of moles by the molar mass of NaOH, we can calculate the mass of NaOH required. Therefore, the correct option is D.

Step-by-step explanation:

To calculate the mass of NaOH needed to prepare a given volume of solution, we need to determine the number of moles of NaOH required. We can use the formula:

mole = volume (L) × molarity (mol/L)

Since pH is given, we can calculate the concentration of H+ ions using the formula:

pH = -log[H+]

To find the concentration of NaOH, we need to convert the pH value to [H+]. In this case, pH = 7.3, so [H+] = 10-7.3 M.

Since NaOH is a strong base, it will dissociate into Na+ and OH- ions. The concentration of NaOH will be equal to the concentration of OH- ions. Therefore, the concentration of NaOH is also 10-7.3 M.

Now, we use the molarity formula to calculate the number of moles of NaOH required:

mole = volume (m3) × molarity (mol/m3)

Substituting the values:

mole = 20 × 106 m3 × 10-7.3 mol/m3 = 2 × 10-3 moles

Finally, we can calculate the mass of NaOH using its molar mass:

mass = mole × molar mass

mass = 2 × 10-3 moles × 40 g/mol = 0.08 g

Therefore, the correct option is D. 0.12 g.