Pure water is brought to thermal equilibrium at

CHEMISTRY · Multi-part · A Level · Chemistry · Acids and bases · Acids and bases

Problem

40^\circ\text{C}

, where the ion-product constant is

K_w = 2.92 \times 10^{-14}

. A dissolved gas slightly acidifies the water so that its measured pH is

6.79

. (a) Calculate the concentration of hydroxide ions,

[OH^-]

, in the water at

40^\circ\text{C}

, reporting your result in scientific notation with proper units. (b) Using the given value of

K_w

, determine the pH of thermally neutral water at

40^\circ\text{C}

(i.e., water in which

[H_3O^+] = [OH^-]

). (c) Compare the measured pH in the sample to the pH of neutral water at

40^\circ\text{C}

and classify the sample as acidic or basic. Justify your conclusion quantitatively.