Video Transcript
Orange juice has a pOH of 9.8. Is it acidic, basic, or
neutral?
Acidic means having the properties
of, or containing, an acid. An acid is a substance that does or
can lose protons in a reaction, where protons are hydrogen ions represented by the
symbol H+. More specifically, this type of
acid is called a Brønsted–Lowry acid. An acid can lose protons, but a
base does or can gain protons. Since acids lose protons, or
hydrogen ions, and bases gain them, the presence of an acid or base will change the
hydrogen ion concentration. Therefore, the hydrogen ion
concentration, where concentration is represented by square brackets, dictates
whether a substance is acidic or basic.
But instead of using the hydrogen
ion concentration, we usually use a related measure called the pH. The pH is calculated as the
negative log base 10 of the hydrogen ion concentration. Most species will have a pH value
between zero and 14. Species with a pH between zero and
seven will be acidic, and species with a pH between seven and 14 will be basic. Species with the pH of seven are
neutral species. An example of this would be pure
water. But the question doesn’t ask us
about the pH. It asks us about the pOH. This is a related measure with a
similar equation to that of the pH.
To work out the pOH, you calculate
the negative log base 10 of the hydroxide concentration. The pOH also dictates whether a
substance is acidic or basic. It also has a scale from zero to
14, and species with a pOH of seven are neutral just like with the pH. Species with a pOH from seven to 14
are acidic, and species with a pOH from zero to seven are basic, so it’s opposite to
the pH. The question tells us that orange
juice has a pOH of 9.8, so we can tell the orange juice is acidic.
We can calculate the pH from the
pOH using the relationship that pH plus the pOH equals 14. In this example, the pOH is
9.8. To make the pH the subject, we need
to subtract 9.8 from both sides of the equation. 9.8 minus 9.8 is zero, so these
terms cancel, leaving us with the pH equals 14 minus 9.8. So the pH of orange juice is
4.2. If we find 4.2 on the pH scale and
9.8 on the pOH scale, we can see that both values show us that orange juice is
acidic. So the answer to the question “Is
orange juice acidic, basic, or neutral?” is acidic.
