Lab Exam Review

Question 1

Define precision.

Answer 1

Precision refers to the reproducibility of a result or measurement.

Question 2

Define accuracy.

Answer 2

Accuracy refers to how close a result or measurement is to the actual value.

Question 3

Using the technique of weighing by difference, how is the weight defined?

Answer 3

The weight of the material is the difference between the two weights of the vial before and after you transfer the material from the vial.

Question 4

Why do organic solids often have lower melting points than ionic solids?

Answer 4

The polar/polar interactions of organic solids are weaker than ionic bonds.

Question 5

What is the stationary phase of cation-exchange chromatography?

Answer 5

The stationary phase consists of a large quantity of acid groups attached to a polymeric resin that is slurried with water and applied to a column.

Question 6

What is the mobile phase of cation-exchange chromatography?

Answer 6

The mobile phase contains the inorganic salt dissolved in a suitable solvent, which is applied to a column. As the mobile phase passes through the column, exchange between the H+ ions on the polymeric ion-exchange resin of the stationary phase and the cations of the salt in the mobile phase occur.

Question 7

What is the Lewis definition of an acid?

Answer 7

An acid is an electron pair acceptor (a proton in aqueous medium).

Question 8

What is the Lewis definition of a base?

Answer 8

A base is an electron donor (proton acceptor in aqueous medium).

Question 9

What is responsible for the oxidation of vitamin C?

Answer 9

Iodine.

Question 10

What direction does current flow in a battery?

Answer 10

Current flows from the anode (reduction) to the cathode (oxidation).

Question 11

How is concentration related to absorbance?

Answer 11

A solution having a high concentration of a light-absorbing molecule will absorb more light than a dilute solution of the same molecule. In short, the greater the concentration, the greater the absorbance. This fact is summarized in the Beers-Lambert Law.

Question 12

Define transmittance.

Answer 12

Transmittance is used to quantify the amount of light a solution will absorb. It is the ratio of radiant power transmitted by a solution to the initial radiant power of the light source.

Question 13

What is a calibration curve and how is it used to apply the Beer-Lambert Law?

Answer 13

A calibration curve consists of a plot of absorption values for varying concentrations of standard solutions. Once the calibration curve has been plotted, the absorbance of the unknown solution is measured and its concentration determined based on the curve provided by the solutions of known concentration. This is an applicated of the linearity of the Beer-Lambert Law.

Question 14

What type of error does poor precision result in?

Answer 14

Random error.

Question 15

What type of error does poor accuracy result in?

Answer 15

Systematic error.

Question 16

What product is formed when a Group 1 metal is reacted with a halogen? Why do all of the halogens react in the same manner?

Answer 16

An ionic compound is formed when a Group 1 metal reacts with a halogen. Halogen atoms are highly electronegative and reactive. Because they have seven valence electrons (and atoms strive for eight valence electrons [with exceptions]) halogens want to gain an electron. Group 1 metals have one valence electron and want to lose it. Therefore, these two groups react with each other easily; the halogen accepts an electron, and the metal donates an electron, forming an ionic compound.

Question 17

In performing the technique of vacuum filtration, what should you do to the filter paper once it has been placed into the Buchner funnel?

Answer 17

The filter paper should be washed with solution. The liquid phase should be decanted into the funnel before the precipitate is added.

Question 18

Briefly describe the octet rule.

Answer 18

The octet rule states that atoms “want” to have eight electrons in their valence shell (the outermost shell). Some atoms already have eight electrons in their valence shell (such as the noble gases), so they are relatively inert. Other atoms, having fewer than eight valence electrons, will react with other atoms in order to fill their valence shell. The octet rule applies primarily to the main group elements, though there are exceptions (such as hydrogen).

Question 19

Define valence shell expansion. Give an example of a compound that illustrates this exception.

Answer 19

Some heavier elements past the third row of the periodic table can use their d orbitals to fill more than eight electrons in their valence shell. An example of this is sulfur hexafluoride. Each of the six fluorine atoms adheres to the octet rule, but the central sulfur atom has twelve valence electrons. Those four atoms that exceed the octet rule are placed in the sulfur atom’s 3d orbitals.

Question 20

What does it mean when a salt is 100% ionized?

Answer 20

Ionic solids are held in a lattice shape governed by coulombic attractive forces. When ionic solids (salts) are dissolved in water, the elements that make up the solid become individual cations and anions. An ion that completely dissolves (breaks up into cations and anions) in an aqueous solution is 100% ionized. An example of this can be seen with the salt NaCl. When this solid is dissolved in water, it will split into Na+ cations and Cl- anions. When the original lattice of solid NaCl has dissolved completely into Na+ cations and Cl- anions, the salt is said to be 100% ionized.

Question 21

In a titration what does the term “equivalence point” mean?

Answer 21

The equivalence point refers to the point at which the molar concentrations of the titrant and the unknown substance are equivalent.

Question 22

What is the Bronsted-Lowry definition of an acid?

Answer 22

A Bronsted-Lowry acid is any molecule that can donate a proton (H+ )ion to another molecule.

Question 23

What is the Bronsted-Lowry definition of a base?

Answer 23

A Bronsted-Lowry base is any molecule that can accept a proton (H+) from another molecule.