CHEM 112 Lab Practical Review

What is the equation for relative standard deviation?

%RSD = standard deviation/average x 100%

What current should not be exceeded during the experiment?

0.6 amps

What are the two key components that all chromatographic separations include?

1. A mobile phase consisting of a solvent and an eluted sample 2. A stationary phase, the media through which the mobile phase passes

What are the four steps of a basic chromatography experiment?

1. A sample is spotted on the paper 2. The paper is then placed in a solvent bath 3. The solvent climbs the paper due to capillary action 4. The components of the spotted mixture are eluted once the solvent front reaches the sample > The resulting separation is due to the different mobilities of the components of the sample mixture.

What color would be absorbed/observed with each of the following wavelengths? 1. 620-750 nm 2. 590 nm - 620 nm 3. 570 - 590 nm 4. 495 - 570 nm 5. 450 - 495 nm 6. 380 - 450 nm

1. A: Red O: Green 2. A: Orange O: Blue 3. A: Yellow O: Violet 4. A: Green O: Red 5. A: Blue O: Orange 6. A: Violet O: Yellow

What will the following changes do to a reaction rate? 1. Increase the temperature 2. Decrease the surface area of a solid reactant 3. Add a catalyst 4. Decrease the concentration of a reactant

1. Increase 2. Decrease 3. Increase 4. Decrease

What is the color of bromothymol blue at the following pH ranges? 1. <6 2. =7 3. >8

1. Yellow 2. Green 3. Blue

What are the steps used in carrying out a measurement using a pH probe?

1. remove probe from storage solution 2. rinse probe with deionized or distilled water and blot dry 3. Place probe in solution to be measured 4. Observe readout on meter of sample solution pH 5. Rinse probe, blot dry, then place in storage solution

What is a faraday?

96485 Coulombs/mol electron

How is the total number of electrons calculated?

= mols metal*Avogadro's number*(number of electrons transferred)

What effect does increasing the concentration of a dissolved solute have on each of the following colligative properties? > boiling point > freezing point > vapor pressure

> elevation > depression > salt can be added to roads to lower the freezing point of water and ensure that the roads don't ice over. > depression

What is the equation for Beer's Law? What does it do?

A = ebc. > Beer's Law directly relates the absorbance of a sample to the concentration, c, of the sample and the path length, b, of the spectrophotometer. > As b is constant throughout an experiment, a plot of absorbance versus concentration for a series of samples with known concentrations yields a straight line.

Why is a blank measured before a sample when using the spectrophotometer?

A blank is measured to remove the signal caused by the cuvette walls and the solvent

What is the retention factor? How is it calculated?

A measure of the distance traveled by a given molecule and the affinity of the component for the stationary phase. > Rf = distance traveled by the compound/ distance traveled by the solvent > The distance traveled by the solvent is measured as the distance between the solvent front and the initial position of the spotted sample. > The distance traveled by a particular compound is measured as the distance between the position of the spot after elution and the initial position of the sample.

What is a colligative property? What are some examples?

A physical property of solutions that depends on the number of dissolved solute particles in solution, but not the identity of the solute. Examples: freezing point depression, boiling point elevation, vapor pressure lowering, osmotic pressure

What is an isosbestic point?

A point of an absorption spectra where two species have the same molar absorptivity

What does the term chromatography refer to?

A wide array of analytical techniques used for the separation of ions and molecules in both liquid and gaseous phases.

What are the derived units of a coulomb?

A*s

What is adsorption?

Adsorption is a surface phenomenon in which molecules adhere to the surface of a material. > Charcoal is commonly used in industries to adsorb pollutants from water sources and is a conventional standard.

Which type of research does the government encourage?

Applied research

Where are the tin chloride solutions poured after the electroplating experiment?

Back into the original vial

What is an important thing to remember while operating a centrifuge?

Balance the sample with a counterweight of the same mass directly across from it

Which type of research is carried out for the purpose of gaining knowledge rather than to fulfill a specific application?

Basic research

How can the molar mass of a solute be determined?

By measuring the degree to which a known mass of the solute depresses the freezing point of a known mass of solvent. > The molality of a solution can be calculated by dividing the change in freezing point by the molal freezing point constant for the solvent. > The number of moles of the solute can then be determined according to moles solute = kg solvent x m. > Finally, the molar mass of the solute can be calculated provided that the mass of the solute dissolved in the solution is known.

In electroplating, the object to be plated is which part of an electrolytic cell?

Cathode

What is the equation for the confidence limits?

Confidence limit = mean +/- (t*s/sqrt(n)) > t is a value for various levels of probability > s is the sample standard deviation > n is the number of measurements in the data set > The value for t is dependent on the number of degrees of freedom. The number of degrees of freedom is equal to the total number of replicate measurements minus one.

The quantity of charge moving through a circuit each second (amperes)

Current

After removing the chromatography paper from the developing chamber, what is the first thing to do with the chromatography paper?

Draw the solvent front line

What is the equation for the energy of a mole of photons?

E = E of single photon * 6.022x10^23

What is the equation for the energy of a photon?

E = h*v h = Planck's constant = 6.63x10^-34 v = frequency

How is Ecell calculated?

Ecathode - Eanode

The amount of positive or negative particles (coulombs)

Electrical charge

The work that can be done to move an electric charge from one point to another (volts)

Electrical potential

What is a voltmeter used to measure?

Electrical potential

Fluid that exits the column

Eluate

Fluid that enters the column

Eluent

When using a serological pipet, the correct volume is dispensed by...

Emptying the contents of the pipet completely into the desired container

How to mix a stock solution with water to a final volume of 250 mL?

Fill the container partially with water, add the correct amount of stock solution, then fill to the 250 mL mark with water

Which way do electrons flow?

From anode to cathode

What is a gross error?

Gross errors are usually caused by failure of equipment, or a mistake and/or carelessness on the part of the experimenter. > A leak in a closed system > A student forgets to add small volumes of titrant and misses the endpoint of a titration during the lab experiment

Which purpose is paper chromatography most often used for?

Identification

Define the end point of a titration

It is when a change that indicates equivalence is observed in the analyte solution

What characteristic property of the dye was used in the research project?

Its maximum wavelength

For the reaction A--->B, what is the equation for the equilibrium constant?

K = [B]/[A]

The velocity of a solvent as it travels through the column

Linear flow rate

Liquid or gas solvent that carries the sample through the separation column

Mobile phase

Which agencies serve as sources for federal research funding?

National Science Foundation, Department of Energy, National Institute of Health

A hollow capillary tube with inner walls that are coated with stationary phase material

Open tubular column

A hollow tube that is filled with particles coated with stationary phase material

Packed column

What are PAHs?

Polycyclic Aromatic Hydrocarbons; a family of organic compounds > Suspected human carcinogens, toxic to aquatic wildlife > Common by-products of industrial and agricultural processes > Formed by incomplete combustion of coal and gasoline, as well as organic substances such as wood and tobacco.

Compare precision to accuracy

Precision means that several measurements are close to each other. Accuracy means that individual measurements are close to an accepted value.

What is a random/indeterminate error?

Random errors result from fluctuations in variables that affect experimental results. They yield measurements that are equally likely to be above or below the average or mean value. > A student takes the mass of a standard sample 10 times on the same balance and the set of masses are different by 0.0015 grams.

Where does reduction occur? Where does oxidation occur?

Reduction occurs at the cathode. Oxidation occurs at the anode.

What is the equation for relative average deviation (relative precision)?

Relative precision = average deviation from mean/ mean times 100% > The smaller the relative precision is, the more precise a set of measurements is.

Elapsed time between sample injection and detection

Retention time

What is a voltaic cell?

Spontaneous chemical reactions produce electricity which is then supplied to an external circuit. > Ex: battery

The separation medium that is bonded to the surface of particles packed in a column

Stationary phase

What are buffered systems?

Systems that are capable of resisting changes in pH. Small amounts of added acid or base are neutralized by the buffer components. > All buffers consist of a conjugate acid-base pair. > The acidic component, a weak acid, neutralized any added base and the basic component, a weak base, neutralizes any added acid > The two buffer components exist in equilibrium which will always be reestablished.

Describe Fajans method

The Fajans titration uses an adsorption indicator to detect the endpoint of a titration in which a precipitate is formed. > The endpoint is detected when an anionic dye binds to the positive surface of the precipitate

What is the Q-test? How does it work?

The Q-test is a simple and widely accepted method to analyze questionable data points. This test can be performed on the most deviant result in a series of measurements. > The absolute value of the difference between the questionable result, xq, and its nearest neighbor, xn, is divided by the spread, w, of the entire set of data to give a Q value. > If the calculated experiment value of Q is larger than the table value of Qcrit, the questionable result may be rejected and not included in subsequent calculations.

Which electrode gains mass?

The cathode

What is the equation for average deviation?

The deviations added together divided by the amount of deviations

What is deviation?

The difference between the measurement and the mean value.

What condition occurs when a chemical reaction is at equilibrium?

The forward and reverse reactions are happening at equal rates

What is the eutectic temperature?

The lowest temperature at which the solvent is still a liquid.

Why should a pencil be used to mark chromatography paper?

The mobile phase can dissolve the ink and move it up the paper

What occurs at the mid-point/half-equivalence point in the titration of a monoprotic weak acid and a strong base?

The moles of conjugate base and weak acid present in the solution are equal

Define the equivalence point of a precipitation titration?

The point at which the precipitate no longer forms. > Chemical indicators are used to trace the equivalence point

What is electroplating?

The process in which metal ions are coated onto an electrode through the use of an applied electrical current.

What is a standard/calibration curve used for?

The standard curve allows a researcher to determine the concentration of a sample of unknown concentration by recording the unknown samples absorbance and comparing the measured absorbance to the standard curve. > The slope of the curve represents e, the molar absorptivity.

Why must the cuvette be placed in the same orientation each time that it is used?

The transmittance of the cuvette must be measured in the same place each time

Why do electrolyte solutions conduct electricity?

There is a presence of free-flowing ions

What is a systematic/determinate error?

These errors result from instrumental error or methodological errors by an experimentalist's misunderstanding of proper measurement procedures. > Using a pH probe that drifts away from calibrated settings > Using the wrong acid-base indicator > Only reading even-numbered volumes from the buret while adding the titrant > A thermometer used in an experiment reads 5 degrees lower than the real temperature

The suggested procedure for spotting a sample on a paper for paper chromatography involves transferring a droplet of concentrated solution from a capillary onto the plate, allowing it to dry, then repeating the process over the existing spot. Why is it advised to spot multiple times?

To facilitate visualization by having a higher compound concentration

What is an electrolytic cell?

Use electrical energy to cause an otherwise nonspontaneous chemical reaction to occur.

Which electrode do anions flow toward in the salt bridge?

anode

How is the charge of an electron calculated?

charge = total charge (a*s)/number of electrons

What is the equation for the change in a reactant, A, over time?

d[A]/dt = -k1[A]

Why is molality used rather than molarity in calculating the freezing point depression? How is molality calculated?

delta(Tf) = Kf*m > The the temperature of a solution changes, its volume will also change, which will affect its molarity but not its molality. > Molality = moles solute/kg solvent

What is the equation for frequency?

frequency = speed of light/wavelength > Speed of light = 2.998x10^8

What is the Henderson-Hasselbalch equation?

pH = pKa + log[base]/[acid] > The greatest degree of buffering occurs where the desired pH is equal to the pKa, or the point where the concentration of the acidic component equals the concentration of the basic component. > The pH range in which a buffering pair is most effective is typically defined as being within 1 pH unit of the pKa of the acidic component.

What is the equation for the rate of a reaction with respect to k and concentrations of reactants?

rate = k[A][B]