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The pH equation is as follows: pH = -log10[H3O+]. Ensure you know what all terms in the equation represent. Look at which term is used for concentration. In chemistry, square brackets usually indicate "concentration of". So the equation of pH would be read as "pH equals the negative logarithm of the concentration of hydronium ions". Read over your chemistry question. Identify the concentration of the acid or base. Write down the entire equation on paper with the known values represented in the equation. Always include units to avoid confusion. For example, if the concentration is 1.05 x 10-5 M, write the pH equation as: pH = -log10(1.05 x 10-5 M) When solving the pH equation, you must use a scientific calculator. First, hit the "negative" button. It is usually written as "+/-". Now key in the "log" button. Your screen should display "-log". Now hit an open bracket and enter in your concentration. Don't forget to add exponents when necessary. Follow with a closed bracket. At this point, you should see "-log(1.05x10^-5). Hit solve. Your pH should be approximately 4.98.
Remember the pH equation. Identify the actual concentration. Solve the equation.