1. Biochemistry
Test Explanations

2. Questions Related to Atoms
Carbon-14 is a radioactive isotope used to determine the age of objects composed of organic matter. Carbon-12 and carbon-13 are other isotopes of carbon. The number of what subatomic particle must differ among these 3 carbon isotopes?
Can’t be an electron, because if you change the number of electrons then you go from an atom to an ion.
Can’t be a photon, because a photon is a particle of light.
Can’t be a proton, because if you change the number of protons, then you change the element.
So, it has to be a neutron.

3. 208 is the atomic mass. 82 is the atomic number.
Atomic mass = # protons + # neutrons
82 is the atomic number.
Atomic number = # protons = # electrons
So, to determine the number of neutrons you must solve the problem: 208 = 82 + neutrons
To determine the # of neutrons you must subtract 208 – 82 = 164
Lead (Pb-208) is the heaviest stable isotope known. It has an atomic number of 82. How many neutrons does Pb-208 have?

4. An ion is simply an atom with a charge.
The only two particles in the atom that have a charge are the protons (+ charge) and electrons (- charge).
You can’t change the # protons, because then you change the element.
So, you have to change the # of electrons.
Add electrons, and you get a (-) ion
Lose electrons, and you get a (+) ion
When a small neutral molecule becomes an ion, what happens to it?

5. To determine this, you must look at the periodic table.
Carbon is in group 14. Remember to subtract 10 (since we skip the 10 groups of elements in the middle, so that leaves 4.
Carbon has 4 valence electrons, thus it can bond up to 4 times.
Or, you can look at the atomic number of carbon, which is 6.
The first energy level can hold 2 electrons and the rest can hold 8.
So, 6-2 (1st level) = 4 electrons in the outermost energy level
The octet rule says that you must have 8 to have a full energy level, so 8-4 = 4.
Carbon can form up to 4 bonds.
What is the maximum number of covalent bonds that can form between a single carbon atom and one or more hydrogen atoms?

6. Bonding Breaking which type of bond would require the most energy?
Covalent bonds have a bond energy of kCal/mol
Hydrogen bonds have a bond energy of 1-3 kCal/mol
Intermolecular forces are forces of attraction or repulsion which act between neighboring particles and tend to be weak forces.
Electrostatic bonds (salt bridges in proteins) have a bond energy of kCal/mol

7. In glycerol (C3H8O3), there is a hydroxyl group (OH) bound to each of the 3 carbon atoms. The bond between the oxygen atom of a hydroxyl group and a carbon atom in glycerol is best classified as what type of bond?
Oxygen is a nonmetal
Carbon is a nonmetal
The bond that forms between 2 nonmetals is a covalent bond.
An ionic bond forms between metals and nonmetals

8. Balancing Chemical Equations
In aerobic respiration, glucose (C6H12O6) combines with oxygen (O2) to yield carbon dioxide (CO2) and water (H2O). What is the balanced chemical equation for this reaction?
Step 1: write the chemical equation.
C6H12O6 + O2  CO2 + H2O
Step 2: add the total number of carbon atoms on each side of the equation.
Step 3: add the total number of hydrogen atoms on each side of the equation.
Step 4: add the total number of oxygen atoms on each side of the equation.

9. C6H12O6  CO2 + H2O
C = 6 and 1 = not the same, so not balanced
C6H12O6 + 6 O2  6 H2O
C = 6 and 0 = not the same, so not balanced
C6H12O6 + O2  6 CO H2O
C = 6 and 6 (1 X 6)
H = 12 and 12 (2 X 6)
O = 8 (6 + 2) and 18 (6 + 6X2) = not the same, so not balanced
C6H12O6 + 6 O2  6 CO2 + 6 H2O
C = 6 and 6
H = 12 and 12 (6X2)
O = 18 (6 + 6X2) and 18 (6X2 + 6)
All are the same, so the equation is balanced

10. Organic Molecules
Which of the following chemical formulas represents an organic molecule?
Organic molecules contain carbon
H2O can’t be right – no carbon
AgNO3 can’t be right – no carbon
C12H22O11 is right – it contains carbon
CuSO4 ∙ H2O isn’t right – no carbon

11. The molecules may exist as isomers
This is true. Isomers have the same chemical formula but different structures.
The backbone is linear, cyclic, or branched.
This is true. Because of the carbon backbone of most organic molecules and carbon’s ability to form 4 bonds.
Ionic bonds are present between the atoms.
This is false. Covalent bonds form between the atoms, which are nonmetals (C,H,N,O,P,S)
Functional groups, such as alcohols, are present.
This is true. We learned several functional groups for the organic macromolecules.
Which is the LEAST likely trait of an organic molecule?

12. Why are organic molecules so diverse?
Organic means that it contains carbon
As we know, carbon has 4 valence electrons, allowing it form up to 4 bonds (octet rule says 8 electrons for energy level to be full and atom stable).
So, the 4 bonds create carbon skeletons that vary greatly in arrangement.

13. pH
As part of an experiment, a student adds 10 mL of 5% hydrochloric acid solution to 100 mL of a non-buffered, colorless solution of sugar. What is the most likely result?
The sugar solution is neutral, so an acid was added to the neutral solution, making the solution more acidic.
Acids have a large # of H+, so the concentration of hydrogen ions (H+) increased.
The pH decreased, since acids are pH 0-5.

14. Which of the following solutions has the greatest concentration of hydroxide ions (OH-)?
Bases have a lot of OH-, so you want the item that has a pH closest to 14.

15. Each pH value is 10X stronger/weaker than the pH value before or after it.
Going from 2 – 4 = 2 pH values, so 10 X 10 = 100X change.
Since the pH value is going more towards a base (pH 14 rather than pH 0), it is increasing in OH- and decreasing in H+.
So the hydrogen ion concentration is decreasing by a factor of 100.
When the pH in the stomach increases from 2 to 4, how does the hydrogen ion concentration change?