# 化學題目PH值計算

A solution is prepared by adding 15.8g of HCl to enough wather to

make total volume of 400ml.What is the PH of the solution? How

much hydrogen ion is contribute by the auto-ionization of water?

Rating

Molar mass of HCl = 1 + 35.5 = 36.5 g/mol

No. of moles of HCl = 15.8/36.5 = 0.433 mol

Volume of the solution = 400 ml = 0.4 L

Molarity of HCl = 0.433/0.4 = 1.08 M

Assume that HCl is completely ionized in water.

The amount of hydrogen ion contributed by the auto-ionization of water is negligible.

Molarity of H+(aq) ions, [H+] = 1.08 M

pH of the solution = -log(1.08) = -0.033

pOH = 14 - pH = 14 - (-0.033) = 14.033

[OH-] = (10)-14.033 = 9.27 x 10-15 M

Consider the auto-ionization of water.

H2O(l) ≒ H+(aq) + OH-(aq)

The auto-ionization of water contributes equal amounts of H+ and OH- ions.

[H+] contributed by the auto-ionization of water = 9.27 x 10-15 M

2009-07-18 14:15:55 補充：

No. of moles of [H^+] contributed by the auto-ionization of water

= (9.27 x 10^-5) x (400/1000)

= 3.71 x 10^-5 mol

The answer may slightly varies due to that different significant figures are taken in the calculation steps.

• What's your concentration of HCl(l)？

2009-07-18 15:46:07 補充：

假設濃度是100%，這樣不是不合理嗎？照理說，100%的HCl應該是氯化氫氣體。