At 25 °C, how many dissociated OH− ions are there in 1285 mL of an aqueous solution whose pH is 1.23? number of OH− ions:

Question

At 25 degree C, how many dissociated OH- ions are there in1285 ml of an aqueous solution whose pH is 1.23? Find the number of OH- ions

Answer 1

To find the number of OH- ions, we first need to find the number moles of OH- ions and then use Avogadro's number.

From the pH, we can find the concentration of OH- ions:

pH = 1.23

pOH = 14 - pH = 12.77

[OH-] = 1x10^-12.77

[OH-] = 1.70x10^-13 M

Because this is such a low concentration, we now have to add in the contribution that H₂O is making to the OH^-. That will be 1x10^-14 M (Kw for water). So the final [OH-] = 1.70x10^-13 + 1x10^-14 = 1.80x10^-13 M

Since there are 1285 ml (1.285 L) this equates to 1.80x10^-13 mols/L x 1.285 L = 2.31x10^-13 mols OH^-

Number OH^- ions = 2.31x10^-13 mols x 6.02x10²³ ions / mole = 1.39x10¹¹ OH- ions