Water

Question 1

Define the term hydrogen bond

Answer 1

Water is a polar molecule, due to its electronegative oxygen atom. This allows it to form hydrogen bonds between any other electronegative atom (usually N or O) and an electropositive H atom.

Question 2

Describe the interactions between water molecules and solutes that define solubility

Answer 2

Molecules that form H bonds are water-soluble. In water, the solute-solute and water-water H-bonding is replaced with water-solute H-bonding, which. is more favorable. This also works for charged molecules
Uncharged molecules are not soluble. They rearrange themselves in water so as to minimize the disruption of the surrounding water molecules, forming “cages”.

Question 3

Define the term amphipathic and give an example of an amphipathic molecule.

Answer 3

Amphipathic molecules contain both hydrophobic and hydrophilic parts. For example, phospholipids have hydrophilic phosphate heads and hydrophobic fatty acid tails. In water, they minimize disruption by forming bilayers or micelles.

Question 4

Define the term pH and derive an expression that relates the degree of proton dissociation of a weak acid to its ionization constant and the pH.

Answer 4

pH is a measure of proton concentration. pH= -log[H+]

pH= pKa + log [A-]/[HA]

Question 5

Describe what is meant by the term “buffer” and describe how a buffer works.

Answer 5

Buffers are solutions of weak acids. In solution, acids dissociate to a conjugate acid-base pair. Weak acids do not dissociate very much. In solution, 2 reactions are occurring at once.

H2O < -> H+ + OH-

HA < -> H+ + A-

Both these equilibriums have to maintain Ka and Kw.
If OH- is added, it combines with free H+ to form H2O.
If H+ is added, it combines with the free OH- to form H2O.
These ions are replaced by the dissociation of the acid and the water, so the pH is kept constant.

Question 6

Define Ka

Answer 6

Ka, the acid ionization constant, is the equilibrium constant for chemical reactions involving weak acids in an aqueous solution. The numerical value of Ka is used to predict the extent of acid dissociation

Question 7

Define pKa

Answer 7

The pKa value is one method used to indicate the strength of an acid. pKa is the negative log of the acid dissociation constant or Ka value. A lower pKa value indicates a stronger acid. That is, the lower value indicates the acid more fully dissociates in water

Question 8

What is the Henderson Hasslebatch equation?

Answer 8

pH = pKa + log10([A-] / [HA])
{pH}	=acidity of a buffer solution
{pKa}=negative logarithm of Ka
{Ka}	=acid disassociation constant
[HA]	=concentration of an acid
[A^-]	=concentration of conjugate base

Question 9

What is the relationship between pKa and Ka?

Answer 9

pka= -log10ka
Ka is the acid dissociation constant and represents the strength of the acid. pKa is the -log of Ka, having a smaller comparable value for analysis. They have an inverse relationship. Larger the Ka, the smaller the pKa, and the stronger the acid

Question 10

Define weak and strong acid?

Answer 10

A weak acid is an acid that partially dissociates into its ions in an aqueous solution or water. In contrast, a strong acid fully dissociates into its ions in water