Question

An aqueous solution at 25 degrees C has a H3O+ concentration of (2.7 x 10^-7 M).

Calculate the OH− concentration.

Be sure your answer has the correct number of significant digits

Answer 1

The an aqueous solution kept at room temperature has

pH + pOH=14

pH = -log [H+]
pH = -log 2.7x10^-7
pH = 6.57

so use this equation to find the pOH of the solution.

pOH=14−6.57=7.43

Now, the pOH of the solution is given by the concentration of hydroxide anions, OH−

pOH=−log([OH−]

To find the concentration of hydroxide anions starting from this equation, rewrite it as

log([OH−])=−pOH

10log([OH−])=10−pOH

this will get you

[OH−]=10−pOH

[OH−]=10−7.4

[OH−]=3.72x10−8 M