A sodium hydroxide solution has pH = 12. 100 mL of this solution has how many grams of NaOH ? [rounded up to two decimal places]Correct answer is '0.04'. Can you explain this answer?

Question

Answer

Given:

pH of sodium hydroxide solution = 12
Volume of solution = 100 mL

To find:

Mass of NaOH in the solution

Solution:

pH and pOH are related to each other by the equation:

pH + pOH = 14

pOH = 14 - pH

pOH = 14 - 12 = 2

The concentration of hydroxide ions (OH-) can be found from the pOH value using the equation:

pOH = -log[OH-]

[OH-] = antilog(-pOH)

[OH-] = antilog(-2)

[OH-] = 1x10^-2 mol/L

The concentration of sodium hydroxide (NaOH) can be assumed to be the same as the concentration of OH- ions in the solution.

Concentration of NaOH = 1x10^-2 mol/L

The number of moles of NaOH in 100 mL of solution can be calculated using the formula:

moles = concentration x volume (in L)

moles = 1x10^-2 x 100/1000

moles = 1x10^-3 mol

The mass of NaOH can be calculated using its molar mass:

mass = moles x molar mass

mass = 1x10^-3 x 40

mass = 0.04 g

Therefore, the mass of NaOH in 100 mL of the solution is 0.04 g (rounded up to two decimal places).

Answer

PH = 12 then pOH = 2
concentration of OH minus ions = .01M [-log(OH)=2]

since NaOH is a strong electrolyte it dissociates fully so the concentration of NaOH is also 0.01

No of moles of NaOH =0.1×0.01 = 0.001 [molarity×volume(L)= no of moles]

molar mass of NaOH = 40g/mol

mass of NaOH in 0.001 mol = 40×0.001 = 0.04g

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