Calculation of pH
______________ of water results in there always being some hydronium ions and some hydroxide ions present in every aqueous solution, whether it is acidic, basic, or neutral.
In any aqueous solution, [H3O+][OH−] =______________ at 25°C. This constant is known as the water ionization constant, Kw. For pure water the concentration of hydronium and hydroxide are ______________ and have a value of 1×10‑7 M. A solution is ______________ if it contains equal concentrations of hydronium and hydroxide ions; acidic if it contains a ______________ concentration of hydronium ions than hydroxide ions; and basic if it contains a lesser concentration of hydronium ions than ______________ ions.
For numbers that may span a very wide range of orders of magnitude (such as concentration of H+) it is helpful to compare them on a logarithmic scale. A popular scale is based on the p-function, where “p” means to take the base-10 log of the quantity of interest. pH is therefore an efficient way to express the concentration of H3O+ in solution, where pH = ______________. If the pH is known, [H3O+] can be calculated by taking the inverse log of the negative pH: [H3O+] = 10–pH. A pH < 7 means the solution is ______________, a pH > 7 means the solution is ______________, and a pH = 7 means the solution is neutral. Since the autoionization constant Kw is ______________ dependent, these correlations between pH values and the acidic/neutral/basic adjectives will be different at temperatures other than 25 °C.
One can also apply the logarithm to both sides of the equation [H3O+][OH−] = 1.0×10−14, giving the expression: pH + pOH = ______. Similar to pH, pOH = -log[OH–]. These relationships can be used to go from a measured pH value to the amount of either H+ or OH– ions in a solution or to predict the pH from a concentration value. pH can be measured using a range of indicators or more precisely using a pH meter.
Classify the following solutions as acidic, basic, or neutral at 25°C.
pH = 13.00
[H3O+] = 1.0 × 10−4 M
pOH = 8.00
[OH−] = 1.0 × 10−7 M
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