HW #2 Flashcards
What is ph?
The mathematical definition of pH is the negative of the log of the hydrogen ion concentration. It is a measure of the acidity if the solution. The lower the pH the more acidic the solution. Because of the log term a pH change of one unit means a tenfold change in hydrogen ion concentration.
What are acids and bases?
Acids are compounds that release hydrogen ions (protons) when dissolved in aqueous solution. In other words they are proton donors. Bases are compounds that are proton acceptors.
Why do we want to know the pH?
It is important to know the oH because many biological reactions require a very tight range of pH values. For example an enzyme that is active at pH 7.0 may be completely inactive at pH 8.0. Solutions used in science often must have their pH controlled in order to have an experiment function correctly. While logical variations in pH may occur in certain subcellular organelles a cell must maintain a pH near neutrality in order to stay alive.
Why do oil and water mixed together separate into layers?
Oil molecules are amphipathic having both polar (hydrophilic) heads and non polar (hydrophobic) tail portions. When oil and water separate into layers the polar head groups of the oil molecules are in contact with the aqueous environment and the non polar tails are sequestered from the water. Van der waals interactions between nonpolar molecules provide the energetic basis for this spontaneous molecular arrangement.
How do buffers work?
Buffers work based on the nature of weak acids and their conjugate bases that compose the buffer. If a source of extra hydrogen ion is added to a buffer solution it reacts with the conjugate base to form the weak acid. If a source of hydroxide ion is added to the buffer it reacts with the weak acid to form water and the conjugate base. In this way either added H+ or OH - is used up by adding it to a buffer. This keeps the pH much more stable than if the same acid or base had been added to an unbuffered system.
How do we choose a buffer?
We choose a buffer primarily by knowing the pH that we wish to maintain. For example if we are performing an experiment and we want the solution to stay at pH 7.5 we look for a buffer that has a pKa of 7.5 because buffers are most effective when the pH is close to the buffer pKa.
How do we make buffers in the lab?
The most efficient way to make a buffer in the lab is to add either the weak acid form or the weak base form of the buffer compound to a container add water and then measure the pH with a pH meter. The pH will be either too low or too high. We then add strong acid or strong base until the pH is the desired buffer pH. Then we bring the solution up to the final volume so that the concentration is correct.
