What is the pH of a 0.001 molar solution of HNO3? (A) One, (B) two, (C) three, (D) 11, or (E) 13.
pH is a measure of how acidic or basic a solution is. It is measured on a scale from zero to 14 with a pH of seven being neutral, neither acidic nor basic. Anything below seven, an acid and anything above seven, a base. The lower below seven we go, the higher is the acidity. And the higher above seven we go, the higher is the basicity. The pH values of some common solutions are shown on the diagram. We can see that black coffee has a pH of about five. Our stomach acid, a very acidic pH of about one. Drain cleaner is highly basic, with a pH of about 14. And our blood has a pH almost neutral of about 7.4, which is slightly basic.
But how is pH calculated? The formula for pH is the negative log of the concentration of H3O+ ions in solution, in other words, the concentration of hydronium ions in solution. There is an inverse relationship between pH and the concentration of hydronium ions. When pH is low or acidic, the concentration of hydronium ions is high. And when pH is high or basic, the concentration of hydronium ions is low. A solution of HNO3 in water is acidic. This is nitric acid. Let’s use the concentration of this acid given to us to determine the pH of this nitric acid solution.
The concentration is 0.001 molar, which is a concentration in moles per liter. When the unit is moles per liter or molar, we can refer to the term concentration rather as the molarity of the solution. When HNO3 molecules interact with water, the HNO3 molecules undergo complete ionization, which means all the HNO3 molecules break apart into ions, react with water to form hydronium ions and nitrate ions, NO3–.
This complete ionization occurs because HNO3 is a strong acid. The hydronium and nitrate ions are both dissolved in aqueous solution, although this has not been shown in the equation here. The equation is already balanced. For every one molecule of HNO3 that ionizes, one hydronium ion is formed. So we know that the concentration of hydronium ions is also 0.001 molar. Putting these into the pH equation, we get a value of 3.0. And pH is unitless. Finally, the pH of a 0.001 molar solution of HNO3 is three.