Chem 101L Final

When should you NOT wear gloves in the lab 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 clean glassware

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

b, d

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

Remove affected clothing and rinse skin with water for 15 minutes

You are taking an evening lab 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 tend to become hypoglycemic, then you are allowed to eat at the bench to prevent health issues.

d

Define accuracy

How close a measurement is to the true value

Define precision

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

Match the following images to the following descriptions:

a. All 5 darts in middle - both accurate and precise

b. All 5 darts in upper left corner - precise, but not accurate

c. 5 spread out near bullseye - accurate, but not precise

d. Darts spread out everywhere - neither accurate nor precise

Systematic/random error arises from a flaw in the equipment or in the experimental design. This type of error can/cannot be detected and corrected.

systematic, can

examples of systematic errors

a. cloth of measuring tape is stretched out

b. Electronic scale reads 0.5 g too high for each mass measurement (improperly calibrated)

c. Improper calibration of spectrometer

d. Bluetooth disconnection / inaccuracy of thermometer

Systematic/random error arises from uncontrolled, and often times uncontrollable, variables in an experiment which has equal chance of being positive and negative. This type of error can/cannot be detected and corrected. Provide several examples.

random, cannot

examples of random errors

a. Uncertainty when reading a scale

b. Reading volume of a flask from different angles each time

c. fluctuations in light or temperature

Why does ice float in water?

a. Ice is less dense than water

b. As temperature increases, density decreases

You measure NaOH by difference using an analytical balance. The initial mass of the NaOH bottle is 5.042g. After removing the NaOH, the bottle mass is 4.812g. What is the mass of the NaOH you are using? (use correct significant figures)

a. 5.043 - 4.812 = 0.2300g

Find the density of the substances: determine whether the solids will sink or float when put into liquid CO2. Remember that 1mL = 1cm3.

a. Density = mass / volume

Consider the data table to the right. Calculate the mean and standard deviation of both data sets. Calculate the CV of both data sets. Which data set is more accurate? More precise? Explain your reasoning.

a. CV - coefficient of variation

b. Standard deviation = square root of mean

c. CV% = (standard deviation / mean) x 100

d. More accurate data - close to known / real value

e. More precise data - close to each other / lower CV value

How does temperature affect the density of a solution?

a. As temperature increases, density decreases because particles move faster and farther apart causing increase in volume and decrease in density

b. As temperature decreases, density increases because particles slow down and move closer together causing decrease in volume and increase in density

Does calibration using standards help to improve the accuracy or the precision of the experiment?

accuracy

How many μL are in a mL?

1000 μL = 1 mL

How many mL are in a L?

1000 mL = 1 L

For the graph on the right, _____ is the dependent variable and _____ is the independent variable.

a. Temperature, pressure

b. Pressure, inverse temperature

c. Pressure, temperature

d. Inverse temperature, pressure

b

On a graph, what is the independent variable and what is the dependent variable?

a. Independent variable - x-axis

b. Dependent variable - y-axis

On a graph, how can you tell if the relationship between the independent and dependent variables is direct or indirect?

a. Direct - data points are on the regression line; R2 value is close to 1

b. Indirect - data points are not on the regression line

Solid KOH is being dissolved in water: Identify the system and the surroundings. If this reaction is exothermic, what happens to the heat of each species?

a. System: KOH(s) and KOH(aq)

b. Surroundings: water

c. The heat is released from the system to the surroundings; each species' heat would decrease

A reaction is being run in a calorimeter. Find ∆T given the following values: Ti = 24.6 ℃; Tf = 29.2 ℃. Is this reaction endothermic or exothermic?

a. ∆T = Tf - Ti

b. ∆T = 4.6 ℃

c. Reaction is endothermic

Do exothermic reactions have positive or negative ∆T and ∆H?

negative

Do endothermic reactions have positive or negative ∆T and ∆H?

positive

Balance the following chemical equation: __ NaHCO3 + __ H2SO4 → __ Na2SO4 + __ CO2 + __ H2O

2, 0, 0, 2, 2

In the laboratory should you add acid to water, or water to acid?

add acid to water

Which is the larger quantity: solvent or solute?

solvent

In aqueous solution of NaCl, is water the solvent or solute?

solvent

React NaCL(aq) with AgNO3(aq) to produce NaNO3(aq) and AgCl(s). What are the spectator ions? What is the net ionic equation?

a. Spectator ions: Na, N, O

b. Net ionic equation: Ag+ + Cl- → AgCl(s)

What is a precipitate?

a. The insoluble product of a precipitation reaction

Why do we use 2 cups in a coffee cup calorimeter setup?

to trap in heat

Why do you need a stir bar in the calorimeter?

a. To evenly mix the solution and to evenly distribute the heat

T/F: To calculate change in temperature, you use Tinitial - Tfinal

a. False; Tfinal - Tinitial

Does making a bond release or require energy?

release energy

Why do bond forming reactions release energy?

a. It goes from a high energy state to a low energy state

Would you expect a bond forming reaction to be endothermic or exothermic?

exothermic

Balance the following equation: __ H3PO4 + __ CaCl2 → __Ca3(PO4)2 + __ HCl

2, 3, 0, 6

What is the full balance equation for the neutralization of barium hydroxide with sulfuric acid?

Ba(OH)2(aq) + H2SO4(aq) → Ba(SO4)(s) + 2H2O(l)

Explain the difference between a stoichiometric reaction and a catalytic reaction, give one example for each that was not part of your experiments (be specific with type of redox, combustion, or acid-base reaction).

a. Stoichiometric reaction - reaction with products and reactants

ex.) Figuring out how many grams of propane is produced from a specific amount of propane burned using a combustion reaction

b. Catalytic reaction - reaction that uses a catalyst to activate

ex.) Hydrolysis of sucrose using an acid-base catalyst (H+ ion)

c. Redox reaction - an oxidation reaction paired with a reduction reaction

d. Combustion reaction - a chemical reaction that occurs when a substance reacts with oxygen, releasing energy in the form of heat and light

e. Acid-based reaction - a reaction where an acid reacts with a base to neutralize and produce water and salt

What is the oxidation state of the metal in this redox reaction (before and after)? 2KMnO4 + 3H2O2 → 2MnO2 + 2H2O + 3O2 + 2KOH

a. before: 7

b. after: 4

What is a decomposition reaction? Name one that you have learned in 101L.

a. When something is broken down into smaller molecules

b. Ex.) Decomposition of aqueous hydrogen peroxide into water and oxygen

For the following reaction, which species is reduced? HCl(aq) + Zn(s) → H2(g) + ZnCl2(aq)

Hydrogen

The following reaction is an example of what type of reaction? 2H2 + O2 → 2H2O

combination or synthesis

Balance the following combustion reaction: __ C3H8 + __ O2 → __ CO2 + __ H2O

0, 5, 3, 4

Balance the following reaction: __ CO2 + __ H2O → __ C6H12O6 + __ O2

6, 6, 0, 6

Why is a catalyst needed in the decomposition of hydrogen peroxide into water and oxygen?

a. Decomposition occurs naturally but really slowly so a catalyst is needed to speed it up

T/F: Yeast was a catalyst of the decomposition of hydrogen peroxide into oxygen gas and water.

true

What experimental components hindered the launch of the pipet rockets in Experiment 5?

a. Contaminated systems, contaminated gas, ratio of gases, order of adding gases, purity of gases

In experiment 5, you were asked to fill up the pipet bulb with water before adding the gases. Why did you need to do this?

a. To make sure we were only collecting the gases produced (not the air) and so that we could measure the amount of gas produced and used

If you performed a conductometric titration of silver nitrate, using lithium chloride, under what conditions might you see the following results? Explain your answer.

a. Graph with straight regression line

b. Graph with negative to positive regression line

c. Graph with positive regression line

d. Graph with position to negative regression line

b; Explanation: negative because the volume of the solution continuously decreases until it reaches a minimum conductivity, then begins to increase

Where is the endpoint of a reaction found on a graph?

a. The point where the two regression lines meet on a graph

If the experimenter made a mistake and put lithium chloride in the beaker and silver nitrate in the buret how would the slope of the first and second lines change?

a. The slope of the first line would be negative and the slope of the second line would be positive

b. The slope of the first line would be negative and the slope of the second line would be negative

c. The slope of the first line would be positive and the slope of the second line would be positive

d. The slope of the first line would be positive and the slope of the second line would be negative

d

When a species gets reduced it...

a. Loses electrons

b. Gains electrons

c. Loses hydrogen

d. Gains hydrogen

b

How do you find ∆Hrxn?

∆Hf products - ∆Hf reactants

Why is enthalpy change considered a state function? How does this relate to Hess's Law?

a. Considered a state function because it is independent of the specific mechanism that a reaction follows to move from reactants to products

b. Relates to Hess's Law because the ∆H for the overall reaction equals the sum of the ∆H values for the two separate reactions

What is a state function?

A function that depends only on the initial and final states of a system, not on the path in between

For an endothermic reaction, circle the correct sign of qcal, qsoln, and qrxn.

a. qcal: positive / negative

b. qsoln: positive / negative

c. qrxn: positive / negative

a. qcal: negative

b. qsoln: positive

c. qrxn: positive

Calorimeters are not perfect insulators. What does that mean? How do we, as scientists, account for imperfect insulation?

a. It is impossible to have perfect insulation in the calorimeter. Some of the enthalpy change due to the reaction is leaked from the medium to calorimeter itself.

Given the following regression lines, calculate Ti and Tf if a reactant was added at t = 3.5 min. Which equation is used to calculate Ti and Tf?

a. T = 0.10013(t) + 16.46245

b. T = -0.14998(t) + 30.00143

First equation is used to calculate Ti and second equation is used to calculate Tf

How do you find ∆H?

products minus reactants

What's the purpose of a calibration curve?

a. To relate the instrument and the property we want to measure

b. To calibrate the spectrometer so the experimental values are accurate

c. Ensures overall good quality in the preparation of standards

What causes absorption?

a. When an electron is moved to a higher energy level

b. Different concentrations have difference absorbances

Define the variables in Beer's Law.

a. Absorbance, molar extinction coefficient (molar absorptivity), path length, concentration

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

a. To calibrate

b. To determine how much light passes through something you know the value of to compare to something you don't know the value of

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?

use M1V1 = M2V2

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.

a. Plug in 1.0 for A and solve for C

After measuring the absorbance vs. concentration, 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

a

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

a. Bigger value means more accurate results

How do you find absorbance?

-log x (% transmittance / 100)

If you know the % absorbed, how to you find % transmittance?

100 minus 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?

Absorbances recorded would be less than what they actually are

Using your spectroscope, you observe a light source and see several strong, distinct lines. Is this source continuous or discrete?

a. Continuous = see all light and lines

b. Discrete = see only specific lines

Given a line with a wavelength corresponding to 500nm, calculate the energy of the emitted light,

E = hc/λ

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

a

If you are given a regression line equation, such as y = 0.0084x - 1.237 and you know the absorbance value, how do you find the wavelength?

Plug in absorbance value for y and solve for x

An electron moves from the ground state (n = 1) to an excited state (n = 5). Is this emission or absorbance? Is the change in energy positive or negative? Describe the relative stability of the electron after the transition.

a. Absorbance

b. Positive

c. More unstable

Describe the behavior of a photon.

When an electron relaxes, it releases a photon (a packet of energy)

If you are given a regression line equation, such as y = 0.0084x - 1.237 and you know the absorbance value, how do you find the energy?

Solve for wavelength and then use E = hc/λ to find energy

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 Rydberg constant is used in the formula for the Balmer series, which is only for hydrogen

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

a. Wavelength and/or absorbance test