CHEM 101 LAB FINAL UNC

When should you wear gloves in the laboratory setting? (Select all that apply.)

a. When weighing out reagents at the balances.

b. When entering data into your ELN on your laptop.

c. When handling glassware.

d. When writing notes on scratch paper using a pen and paper.

a, c, d

You make a solution of 0.1 M NaOH and accidentally spill the solution, covering your gloved hand and onto your arm. Your wrist was exposed, and the solution came into direct contact with your skin. What should you do?

Alert your TA, take off your gloves, and wash your hand for at least 15 minutes

You are taking an evening laboratory session and want to bring a snack to help you make it through the evening. When should you be allowed to have food at the bench top in the lab?

a. If the experimental work is over and cleaned up.

b. At the beginning because it is dinner time and being a student is exhausting.

c. Only if the food remains in the students backpack while at the bench top.

d. Never.

e. Never, unless you have a tendency to become hypoglycemic, then you are allowed to eat at the bench to prevent

health issues.

Never

Accuracy

how close a measurement is to the true value

Precision

a measure of how close a series of measurements are to one another

_________ error arises from a flaw in the equipment or in the experimental design. This type of error ________ be detected and corrected. Provide several examples.

Systematic; can

_________ error arises from uncontrolled, and oftentimes uncontrollable, variables in an experiment which has equal chance of being positive and negative. This type of error _______ be detected or corrected for. Provide several examples.

Random; cannot

Number of significant figures for beaker

0 places after decimal

Number of significant figures for graduated cylinder

1 place after decimal

Number of significant figures for Erlenmeyer flasks

0 places after decimal

Number of significant figures for Volumetric flask

0 places after decimal

Number of significant figures for micropipette

0 places after decimal

Number of significant figures for glass thermometer

1 place after decimal

Given the mass of a beaker, the mass of the beaker with a sample, and volume, how would you calculate the density of the sample?

Find the mass of the sample by subtracting the mass of the empty beaker from the mass of the beaker with the sample. Once you have the sample's mass, you can use the formula for density, which is mass divided by volume. Divide the mass of the sample by its volume to get the density. Make sure the units for mass and volume are compatible, as density is usually expressed in grams per milliliter (g/mL) or grams per cubic centimeter (g/cm³).

Percent error formula

|measured value-true value| / true value x 100

Looking through your spectroscope, you see several discrete, clearly defined lines. What is a possible source for this spectrum?

a. Fluorescent lamp

b. Incandescent lightbulb

c. Candle

d. Sunlight

fluorescent lamp

After observing light from a fluorescent bulb as a series of colored bands through a spectroscope, a table is compiled with two columns. The left column corresponds to the independent variable—the literature wavelength of each colored band and the right column is the dependent variable—the position on the scale of the spectroscope at which each colored band appears. The data are plotted as a scatter plot and fitted to a line with the following equation: y = 0.0084x - 1.237. What is the wavelength that corresponds to a line which appears at 6.57 on the scale of the spectroscope? Plug 6.57 into y and solve for x.

929.4 nm

An electron moves from the ground state (n=1) to an excited state (n=5), is this emission or absorbance, and is the change in energy positive or negative?

absorbance; positive

An electron moves from the ground state (n=1) to an excited state (n=5). Describe the relative stability of the electron after the transition.

The electron is less stable because it is in an excited state. Electrons in excited states are higher in energy and will naturally tend to return to a lower, more stable energy level (the ground state) by emitting energy, often in the form of light.

Describe the behavior of a photon:

A photon behaves as both a particle and a wave, carrying energy and momentum, traveling at the speed of light, and interacting with matter through absorption or emission, depending on its frequency and wavelength.

You see a flame that has a yellow color, and an emission line on the spectroscope at 6.03 mm. You have a calibration curve of

yy = 117.0xx − 133.4 where y is the wavelength in nanometers and x is the position on the spectroscope in millimeters. What

is the energy of this electron as it relaxes and emits a photon?

The Rydberg constant (RH = 2.18 x 10^-18 J) is a value useful for calculating the change in energy involved in electronic transitions between an excited state and a state where nf = 2 in hydrogen atoms. Can the Rydberg constant be applied to determine changes in energy observed in mercury emission lines using the same relationship? Why or why not?

No, because the energy levels of mercury atoms do not follow the same simple relationship that hydrogen's do.

A solution containing an unknown metal ion is sprayed into an open flame, giving rise to an orange color by eye. Upon

researching metal ions which burn with this color, you find several candidate ions which also burn orange and are unsure how

to identify the unknown. What test(s) would you perform to identify the identity of the unknown?

Use a spectrometer to find the wavelengths corresponding to peaks in % transmittance vs wavelengths; compare that known metals to find the identity

Balance the following chemical equation: __NaHCO3 + __H2SO4 __Na2SO4 + __CO2 + __H2O

2, 1, 1, 1, 2

What's the purpose of a calibration curve?

to relate the measured response of a system, such as absorbance in spectroscopy, to the known concentration of a substance. BWcan use the curve to determine the concentration of an unknown sample.

What causes absorption?

when a substance absorbs energy from light, causing electrons to move to higher energy levels within the atoms or molecules of the substance. This happens when the energy of the incoming light matches the energy difference between the substance's electron energy levels.

Define the variables in Beer's law

A: absorbance- the intensity of light that passes through the cuvet.

E: Molar Extincion Coefficient- how well the mixture absorbs a certain wavelength of light.

L: path length- for this experiment it is 1.000 cm.

C: concentration- in this case concentration of the mixture in mol/L.

What is the purpose of blank in Beer's law experiment?

To calibrate the spectrometer by setting the absorbance of the blank cuvette

What volume of a 3.0 x 10-3 M solution is needed to prepare 50.0 mL of 5.0 x 10-5 M; how many moles of solute are in this

solution?

2.49 x 10ˆ-6 moles

Use the trend line A = 17453C + 0.0745 to calculate the concentration of a solution that has an absorption of 1.0, where A

and C are absorption and concentration, respectively.

5.3 x 10ˆ-5 M

After measuring the absorbance vs Concentration (and working with the same spectrophotometer), you realized you had

worked with a wrong λmax. Which of the following will get affected in the regression line for the calibration curve?

a. concentration

b. Slope of the absorbance vs Concentration plot

c. Length of the spectrophotometer

d. Transmittance

Slope of the absorbance vs Concentration plot

Why do we only record absorbance at a wavelength that gives the greatest absorbance value for Allura Red (λmax)?

Molecules all have a specific wavelength at which they absorb most. This allows us to make easier comparisons between molecules of the same kind at different concentrations and ensures that we are comparing actual absorbance and not background absorbance. Also, it would be impossible to compare at different multiples wavelengths.

A solution absorbs 50% of a specific wavelength of light that passes through it in a spectrophotometer. What is the %

transmittance and absorbance at this wavelength for this solution?

50%; Absorbance= -log (0.5)=0.301 absorbance

After you finish collecting your absorbance readings on the spectrophotometer, you notice a dark smudge on your cuvette

you used to blank while doing a Beer's Law experiment. How would you expect this to have affected your absorbance

readings on samples you ran after the blank?

Your absorbance readings for all the others would be lower because the absorbance reading from your blank cuvette would be higher than if it did not have a dark smudge.

What are the characteristics of an ideal gas?

The gas particles have negligible volume, the gas particles are equally sized and do not have intermolecular forces with other particles, and the gas particles move randomly in agreement with Newton's Laws of Motion

What is the difference between an ideal gas and a real gas?

An ideal gas is a theoretical gas composed of many randomly moving particles that are not subject to interparticle interactions and a real gas occupies space and the molecules have interactions

Which noble gas is the most ideal and which is the least? Why?

Helium is the most because it has a full electron shell and exists as a single atom. Radon is the least ideal because it has the largest atomic radius and the weakest intermolecular forces among them.

What is the name of the law that relates temperature a pressure?

Gay-Lussac's law, Amontons' law, or the pressure law

What is the definition absolute zero? What does this imply about the pressure at absolute temperature?

Absolute zero is the theoretical temperature when there is no motion and no heat. There is no pressure at absolute zero

What is the difference between electron geometry and molecular geometry?

Electron geometry refers to the arrangement of all electron groups (both bonding pairs and lone pairs) around a central atom, while molecular geometry only considers the arrangement of atoms (bonding pairs) and ignores lone pairs

True or False. As molecules escape from the liquid to the vapor phase, the average kinetic energy of the solution is lower and the temperature

of the solution is higher than the original solution.

false

If molecule A has an enthalpy of vaporization, ΔHvap, that is larger than molecule B, what does this imply about the intermolecular forces of

molecule A relative to molecule B?

a. Higher

b. Lower

c. Equal

d. Not enough information to determine

Higher

which would result in a larger ΔT as the organic liquid evaporated. Assume that the compounds in question are similar to one another: A small ΔHvap or a large ΔHvap?

small

which would result in a larger ΔT as the organic liquid evaporated. Assume that the compounds in question are similar to one another: Weaker intermolecular forces/stronger intermolecular forces?

Weaker intermolecular forces

which would result in a larger ΔT as the organic liquid evaporated. Assume that the compounds in question are similar to one another: Less structural branching/more structural branching?

Less structural branching

which would result in a larger ΔT as the organic liquid evaporated. Assume that the compounds in question are similar to one another: Incapable of hydrogen bonding/Capable of hydrogen bonding?

Incapable of hydrogen bonding

Liquids with ___________ intermolecular forces have a ___________ enthalpy of vaporization, and the molecules escape into the vapor

phase ___________.

a. Stronger, larger, with difficulty

b. Stronger, smaller, easily

c. Weaker, larger, easily

d. Weaker, smaller, with difficulty

Stronger, larger, with difficulty

For several molecules with the same chemical formula, how does the degree of branching influence the relative intermolecular forces?

Why?

a greater degree of branching weakens the intermolecular forces. highly branched molecules typically have lower boiling points and evaporate more easily than their less-branched counterparts.

Miscibility

the ability to mix without separating into two phases

immiscibility

inability to mix or become homogeneous.

spectator ions

Ions that do not take part in a chemical reaction and are found in solution both before and after the reaction