UPSC Combined Geo-Scientist, Mains · 2016 · Mains Paper III

Q1(a). Temperature and neutrality

Question

An aqueous solution has pH 6.76 at the printed temperature 400° C, with K_w=2.92×10⁻¹⁴. Classify it and justify.

Approximately neutral at the precision of the data; strictly, pH 6.76 is very slightly acidic relative to neutral pH 6.7673.

At neutrality [H⁺]=[OH⁻]=√(K_w). Hence pHₙ=-1/2log (2.92×10⁻¹⁴)=6.7673.
Compare the measured pH with this temperature-dependent value, not automatically with 7.
The printed temperature appears erroneous: the supplied K_w is characteristic of water near 40° C. The classification here uses the explicitly supplied K_w.

The full model answer

As written for the answer booklet: the diagram carries the working, the lines stay short. Read here only; it is not offered as a download.

CoversTemperatureneutralityAcid–base equilibrium