What is the pH of a solution comprising 100⋅mL volumes EACH of HCl(aq) at 0.20⋅mol⋅L−1 and NaOH(aq) at 0.18⋅mol⋅L−1?

1 Answer
anor277
Sep 25, 2017

pH≡2

Explanation:

We interrogate the chemical reaction....

HCl(aq)+NaOH(aq)→NaCl(aq)+H2O(l).

Moles of HCl(aq)=100×10−3⋅L×0.2⋅mol⋅L−1=2.0×10−2⋅mol.

Moles of NaOH(aq)=100×10−3⋅L×0.18⋅mol⋅L−1=1.8×10−2⋅mol.

There is thus an excess of hydronium ions, to the tune of 2.0×10−2⋅mol−1.8×10−2⋅mol=2.0×10−3⋅mol

But this is dissolved in a solution whose volume is now 200×10−3⋅L

And so [HCl]=2.0×10−3⋅mol200×10−3⋅L=0.01⋅mol⋅L−1

But pH≡−log10{[H3O+]}=−log10(0.01)=2.0

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