Distilled water is found to have a of 6.55 when measured at
CHEMISTRY · Single correct · A Level · Chemistry · Acids and bases · Acids and bases
Problem
Distilled water is found to have a pH of 6.55 when measured at 50∘C. Which statement best explains why the sample is nonetheless considered neutral?
(a) As temperature rises, the pOH increases while the pH stays roughly constant, so any pH below 7 represents neutrality at 50∘C.
(b) Readings from glass pH electrodes are systematically low at elevated temperature, so values slightly below 7 are attributed to instrument error and the water is judged neutral.
(c) Because Kw increases with temperature, the neutral pH drops; at 50∘C the neutral value is about 6.55, so the sample is neutral even though its pH is below 7.
(d) Autoionization of water actually diminishes at higher temperature, driving the pH of all solutions lower; therefore any solution with pH<7 at 50∘C is considered neutral.