HESI A2 Exam: Acids and Bases
• Coffee is fairly acidic. Given that, what would you guess the pH level of coffee to be?
• Buffers are made when you mix a weak acid with __________
• If a substance has a pH of 3 and turns litmus paper red, what do we know about this substance?
• What is the pH of a substance with .0013 M (description of how to do this below)?
Intro
The pH scale goes from 0 to 14. Anything between 0 and 7 is acidic, and anything above 7 is basic (alkaline). Exactly 7 is considered neutral. 2.5 would be considered very acidic. On average, the pH level of coffee is somewhere around 4.5.
For litmus paper, red is acidic and blue is basic.
Acids have a sour taste, have a pH less than 7, and produce hydrogen ions when dissolved in water.
Bases have a bitter taste, their pH is greater than 7, and they release hydroxide ions when dissolved in water. Bases also accept hydrogen ions from acids.
Remember that pH is related to the balance of hydrogen and hydroxide ions. A pH below 7 represents an excess of H+ ions. Also, a pH above 7 represents an excess of OH- ions.
Weak acids and weak bases are only partially ionized in water. Weak acids and weak bases make excellent buffers, or substances that don't easily change their pH even when acids or bases are added to them.
Buffers are made when you mix a weak acid with its weak base conjugate. Acids donate hydrogen ions and bases accept this hydrogen ion, so the acid that donates a hydrogen ion and the base that accepts that ion are known as conjugates. The acid donates hydrogen ions, but instead of lowering the pH, these ions get neutralized by the hydroxide ions in the weak base.
Math Problems
What is the pH of a substance with .00235 M?
Need a calculator for these types of problems. The formula is just -log(M). Therefore, plug in .00235 for M.
-log(.00235) = 2.63
if pH = 5.2, what does pOH equal?
pH + pOH = 14
5.2 + pOH = 14
pOH = 8.8
Given that hydrogen has a molecular mass of about 1, and oxygen has a molecular mass of a little more than 16, what is the mass (in grams) of 9.2 mol of water?
When we combine the molecular mass of hydrogen and oxygen, we get approximately 18 grams. 18(9.2) = 165.6 grams.
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