A solution at 25 degrees Celsius with a pH of 7 contains

a) more H3O + ions than OH - ions

b) fewer H3O + ions than OH - ions

c) an equal number or H3O + ions and OH - ions

d) no H3O + ions or OH - ions

The answer is c. You probably know that a pH of 7 is usually what we call "neutral," meaning no excess of either H + or OH-. The 25 degrees Celsius is important because the dissociation constant of water is actually dependent upon temperature. The neutral pH is 7 only when the temperature is 25 degrees Celsius, which you should assume that it is unless otherwise stated.