Calibration of a glass electrode gave a reading of 139.3 mV with 0.05 m potassium dihydrogen citrate buffer standard (pH = 3.766) and a reading of -79.0 mV with 0.08 m HEPES, 0.08 m NaHEPES, 0.08 m NaCl buffer standard (pH = 7.454), both measured at 30°C.
(a) What is the observed slope (mV/pH unit) of the calibration curve?
(b) What is the pH of an unknown that gives a reading of 21.7 mV with this electrode at 30°C?
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