Chem 101L Final Exam
In a conductivity titration, 10.15 mL of 4.95 x 10-3 M Ba(OH)₂ is titrated by 12.11 mL of a H₂SO₄ solution. Given the end point volume of titrant, what is the concentration of the H₂SO₄ solution?
4.15x10⁻³ M
Mg²⁺+2HCl→MgCl₂+H₂ Both the initial HCl solution and the final solution were transparent and colorless, but bubbles were seen during the reaction. Based on the equation and the observations of the reactions, the spectator ions in the ionic equation would be?
Cl⁻
Experiment 5, you were asked to fill the pipet bulb with water before adding the gases. Why did you need to do this?
The water will capture and dilute the water/yeast/acid displaced by the gas bubbles you will be generating this is because oxygen gas has very low solubility in water and therefore can be collected as gas
Room temp in laboratory is
25°C or °298 K
volume should be measured to how many sig figs?
3
Mass should be measured to how many sig figs?
4
For the reaction of 1.25 g sodium bicarbonate with 150 mL of 0.5 M citric acid, the value of ∆T/mol(°C/mol) would be? (molar mass of sodium bicarbonate is 84.007)
605°C/mol
When viewed through a spectroscope, calcium ions produce three distinct lines in the visible region of the spectrum, at spectroscope positions of 7.30, 7.85, and 8.45. Assuming the linear fit for the calibration of the spectroscope has a positive slope, the most likely red line would be at a position of:
8.45
Should you return unused reagents to stock bottles?
No, use the minimum amount of reagents to avoid waste disposal issues
Why is enthalpy change considered a state function? How does it relate to Hess's law?
The amount of energy depends only on the states of reactants and the state products but not on the path taken, enthalpy change in chemical reactions are the same, regardless whether the reactions occur in one or several steps the total energy change in a chemical reaction is the sum of energy changes in its many steps leading to the overall reaction
True or false: Yeast was the catalyst of the decomposition of hydrogen peroxide into O₂ gas and water?
True
exothermic reaction
When a reaction that releases more energy from the reaction to the surroundings in the form of heat causing the temperature of the solution to increase
solution
a homogeneous mixture of two or more substances
conductivity
a measurement of the ability of a substance to conduct electrical current
if all errors and uncertainties in an experiment are truly random, the value of the mean is a
a more accurate value than any single measurement and more measurements will produce a more accurate mean
intensive property
a property of matter that does not change as the amount of matter changes
catalytic reaction
a reaction sped up by a catalyst
systematic error can effect
accuracy
Measuring the same quantity by different methods give confidence to
accuracy (true value)
What's the purpose of a calibration curve?
also known as a standard curve a general method for determining the concentration of a substance in an unknown sample by comparing the unknown to a set of standard samples of known concentration
Every measurement made in the laboratory is really
an approximation based on the quality of the measuring device and the skill of the user
What should bottoms of clothing cover?
ankles
What causes absorption?
as the photon of lights are absorbed by electrons, the electrons move into higher energy levels the absorption spectrum is the exact inverse of the emission spectrum
experimental error
associated with every measurement, but this type of error can be minimized by carefully following proper lab techniques
Calorimeters are not perfect insulators. What does that mean? How do we, account for imperfect insulation?
energy is always conservedc If we have a perfectly insulating container, then no energy can flow in or out of that container. the Calorimeter is not a perfect insulator because it still does transfer heat to its surroundings. q does not = 0
If the reacting system releases heat energy to its surroundings, the temperature of the surroundings increases and the reaction is...
exothermic
a reaction that releases energy is
exothermic
the accuracy and precision of an experiment result depends on the level of...
experimental error or uncertainty
Where should glass be disposed of?
glass box, a dust pan and brush for broken glass is available to clean this up
solution
homogeneous mixture of two or more substances
Why does ice float in water?
ice is not as dense as water.
Where do solid wastes get disposed of?
in plastic waste buckets
What is a decomposition reaction?
occurs when one reactant breaks down into two or more products AB→A+B example, hydrogen peroxide to water and oxygen
PPE
personal protective equipment
Random error can effect
precision
gas-forming reaction
reaction that yields the production of an insoluble gas as a product typically produce H2, CO2, SO2, H2S and NH3
Coefficent of variation
relative standard deviation offers a measure of precision as a percentage of the value measured CV=100x (s/x) standard deviation/mean x100
Precision is
reproducibility repetition of a measurement taken several times
Someone got solution misted into their eyes, how should they deal with it?
rinse at eye wash for at least 15 minutes, and then make sure to go to student health
the smaller the standard deviation, the less ________ and the more _______ the results
scatter precise
Shirts in lab must at least cover?
shoulders back abdominal area Do not wear sleeveless shirts or tank tops
dissolution
simple process that involves the breaking of ionic bonds or intermolecular interactions and the subsequent formation of new bonds or interactions with solute and solvent
in aqueous solution of NaCl, is water the solvent or solute?
solvent
which is the large quantity, solvent or solute?
solvent
a measure of the scatter of measured values about a mean deviations from the mean
standard deviation
Hess's Law
states that energy changes are state functions the amount of energy depends only on the states of the reactants and the state products, but not on the intermediate steps
standards
substances with a known true value for comparison
t or f air occupies more volume when heated
t this is why hot air balloons float heated air is less dense than surrounding air
C in beers law
the concentration of the analyte in solution units of molarity (moles/liters)
l beer's law
the path length of the light through the solution contained in the cuvet. in this experiment the path length is fixed by the size of a cuvet at 1.000cm. The variable has units of cm
callibration
the process by which the reading obtained by an instrument is assessed for its accuracy and precision
transmittance
the ratio of absorbance l/l₀ if multiplied by 100, % transmittance
A- Absorbance in Beer's Law
the spectrometer uses the intensity of light passing through the sample(l) relative to the intensity of light passing through the blank (l₀) to determine the absorbance value for each sample has no units
Which item used during experiment 8 required calibration?
the spectroscope
solute
the substance present in a solution in a smaller amount
solvent
the substance present in the larger amount in a solution
When a solid is dissolved in a liquid, what oes it do the volume and mass?
there may be no apparent volume change the increased mass causes the density of the solution to be greater than that of the pure liquid
most useful substance for for studying a wide variety of chemical properties and reactions
water
When a species gets reduced it...
when a species in a reaction gains electrons
Standard deviation formula
x is a measured value n-1 degrees of freedom
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 callibration curve? a. concentration b. slope of the absorbance vs concentration plot c. length on the spectrophotometer d. transmittance
λmax is the wavelength at which the most light is absorbed. So if I was working with an incorrect λmax the slope of the absorbance vs concentration plot would be messed up after doing the wavelength vs absorbance plot
In calorimetry, the enthalpy change of the reaction, qrxn, is defined to be:
-(qsoln + qcal) the heat gained or lost by the reaction in units of joules.
How is the density of a liquid determined?
1.weighing an empty container 2. the container is weighed with a known volume or liquid inside 3. mass and volume 4.
variables in Beer's Law?
A=εlc A- absorbance ε- molar absorptivity (molar extinction coefficient) l- the path length c- the concentration of the analyte
In the reaction NaOH(s) + HCl (aq) → NaCl(aq) + H₂O(l), the spectator ion(s) would be:
Cl⁻
Does calibration using standards help to improve the accuracy or the precision of the experiment?
It provides more accurate results
How might a smudge on your blank cuvet affect doing the Beer's Law experiment. How would you expect this to have your absorbance readings on samples you ran after the blank?
b= y-intercept it represents the factors unrelated to the properties of an analyte it represents background signal: current in spectrometer, scratches/smudges on cuvet this could affect the measured absorbance but are not corrected for in the measurement of the blank it can interfere with light path
density
intensive property kilograms per cubic meter gram per cubic cm =g/mL =mass/volume kg/m³
What is the purpose of using a blank in Beer's law experiment?
it is a reference blank we use it as a measure of background absorbance in the cuvette and solution (l₀) the absorbance of each standard solution is then ploted against its known concentration
As the concentration of absorbing molecules increases...
less light passes through the sample (l decreases) and absorbance increases.
the sum of the measured values divided by the number of measurements
mean average
ε in Beer's Law
molar extinction coefficient molar absorptivity how strongly the analyte absorbs light at a particular wavelength. the molar extinction coefficient is unique for each different analyte at each different wavelength. Expressed in units M¹cm⁻¹
In a calorimetry experiment, 50.0 mL each of two dilute aqueous solutions are combined and produce an extrapolated ΔT = 22.4˚C. If the density of each solution is assumed to be equal to that of water (at 20˚C, d = 0.9982 g/mL), and the specific heat of the solution is also equal to that of water (4.186 J/g˚C), the value of qrxn would be:
-9.36kJ
What are 3 of the actions that could produce a significant source of error in the results for Experiment 5?
1. Losing H₂ gas because it is much less dense than air 2. Collecting gases while the reaction is still hot, resulting in mostly H₂O vapor in the bulb. 3. Leaving moisture on the spark gun tip before firing
Steps in order if you find broken glassware in a lab drawer, or glass breaks during the lab
1. TA 2. use a pair of leather gloves found in the lab 3. Clean up large pieces and dispose of in glass container 4. Use dustpan and brush found in lab to sweep smaller stuff
Steps to light and work bunsen burner?
1. make sure all loose clothing and hair have been properly restrained 2. prep the work space to be sure no flammable materials are in the vicinity 3. inspect the tubing to the gas valve to be sure no openings that would allow gas leaks 4. inspect the burner to be sure that the air intake and fuel valves move properly and can be adjusted 5. sparker in hand, turn on gas and light flame as soon as possible to mimnimize unburned gas 6. adjust the fuel valve to set the height of the flamea dn adjust the air intake to adjust the temp (color) of flame 7. monitor the flame and all nearby materials closely until experiment is complete 8. shut off the burner and gas valve
When calculating the displacement volume of the thermometer, the volume reading on the buret is 28.47mL w/o the thermometer, and 27.82 w/the thermometer in place. How many sig figs should be recorded for the displacement volume of the thermometer?
2
The volume marking at the sealed bottom of a buret segment is 48.60 mL, and has markings for every 0.1 mL. When water is added, the bottom of the meniscus sits about halfway between the line for 28.3 mL and the next mark up. What volume of water is contained in the buret segment?
20.35mL
Given the Rydberg equation: ΔE = RH (1/(nf2)- 1/(ni2)) In the Balmer series for hydrogen, what is the initial energy level (ni) for a photon with a wavelength of 656.3 nm? REMEMBER: h = 6.626 x 10-34 J⋅s; c = 2.998 x 108 m/s; RH = 2.18 x 10-18 J
3
When dealing with hazardous materials generated in the lab, the two most important guidelines are?
-collect ALL hazardous material for proper disposal - organize the experiment to generate the least amount of hazardous material possible
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 4.812g. What is the mass of the NaOH you are using?
0.230g
The molecular equation for the reaction in Experiment 4 is: Ba(OH)₂ + H₂SO₄ →BaSO₄ + 2 H₂O Based on the conductivity properties of these reactants and products that you observed in the lab, which is the correct net ionic equation?
Ba²⁺(aq) + 2OH⁻(aq) + 2H⁺(aq) + SO₄²⁻(aq)→BaSO₄(s) + 2 H₂O(l)
During a conductivity titration, 15.0 mL of Ba(OH)2 is placed in a beaker with 35 mL of H2O. At the start of the titration, the initial conductivity reading is nearly double that of a previous trial and does not decrease to zero at the minimum as expected as sulfuric acid is added, even though the minimum occurs at the expected volume. One possible explanation for these observations would be:
Instead of adding distilled water to the barium hydroxide, the experimenter added tap water.
When are you permitted to enter or work in a lab without TA permission
NEVER
Examples of situations where you should fill out an incident report
No matter how small the injury, you should report it to the TA and be sure to wash the area thoroughly to be sure you have washed away any potential contamination -touching a hot beaker, and getting a small red mark -feeling faint and dizzy in lab because you skipped lunch - scratch on ankle due to broken glass at end of semester check-out - etc.
Where do personal items go?
Not at the lab bench personal items are placed in designated cabinets at the front of the classroom
My lab partner spilled acid on his wrist and watch, what should I do?
Remove the watch and immediately rinse his wrist for at least 15 minutes to be sure all hazardous material has been washed away Let the TA inspect his wrist to see if it is okay
Best resource for understanding the nature of the chemical hazards of material you work with in lab?
SDS safety data sheets
Because of toxicity and flammability hazards, never dispose of solvents, toxic substances, or reactive chemicals... where?
The drain Chemicals must be disposed of in the liquid waste disposal located underneath the ventilation hood at the front of the room
When should you wear gloves in the laboratory setting?
They must be worn at all times when handling glassware and chemicals
What is the most important consideration to preserve safety when it is necessary to feed glass tubing, thermometers, or other apparatus through a rubber stopper?
Using the correct hand position so that if something goes wrong you can avoid coming into contact with broken or sharp ends
What should you do when there is imminent danger to your personal safety, because several bottles of reagents in the fume hood caught fire and are in danger of explosion?
You should NOT stay to fight the fire Close the blast shield on the hood, pull the fire alarm and quickly evacuate the building
combustion reaction
a major class of chemical reactions commonly referred to as burning a reaction between any combustible material and an oxidizer to form an oxidized product exothermic reaction releases heat the presence of oxygen as a reactant and carbon dioxide, water, and heat as products
What is a state function?
a property whose value does not depend on the path taken to reach that specific value
What is a precipitate?
a substance to be deposited in solid form from a solution
Do what you oughta, add
acid to wata always add acid to water NEVER add water to acids
3.5g of white solid is added to a calorimeter containing 30 mL of 0.5 M HCl at 22°C During the reaction, bubbles appear in a steady stream, and the solution remains transparent. Based on observations the reaction would be classified as?
acid-base and gas-forming
acid-base reaction
acids dissolve in water to produce H+ ions bases dissolve in water to produce OH-
in the lab should you add acid to water, or water to acid?
add acid to water
random error
arises from limitations in the ability to make physical measurements equal chance of being positive or negative and is always present example: uncertainty when reading a scale, and measurement between two marks must be estimated to the best of the ability of the measurer but can never be perfect this affects the precision, or reproducability of a measurement
solubility
can be measured as the concentration of solute in a saturated solution at a specified temperature
systemic error
can sometimes be corrected example: a balance that is calibrated incorrectly and always reads 2.3g heavier than the material being weighed represent a limitation in the accuracy of a measurement
Why is a catalyst needed in the decomposition of hydrogen peroxide into water and oxygen?
catalase found in yeast
What kind of shoes should be worn in a lab setting?
close toed... covering entire foot (ballet flats are also unacceptable)
How should we transport chemicals?
closed containers
a reaction that absorbs energy is
endothermic
if the system absorbs heat energy from its surroundings, the temperature of the surroundings decreases, and the reaction is...
endothermic
The temp at the start of a reaction was 22.8°C and final temp was 32.9°C. Based on these observations, the reaction between sodium bicarbonate with HCl would be?
exothermic, because energy is transferred from the reaction to the surroundings
When should you not wear gloves in the lab setting?
gloves must be removed before leaving the lab gloves should be removed when touching computer, or other personal items. gloves should be removed when leaving lab
When handling chemicals, what should we always use
hood ventilation never keep an open beaker of solvent outside a hood
Where can incident report forms be found?
inside first aid kits by the main hood
redox reaction
involve electron transfers if one species gains electrons, another species must lose electrons
Why do we use two cups in a coffee cup calorimeter setup?
it helps insulate the insulater.
Does breaking a bond release or require energy?
it takes energy to break a bond think of ATP becoming ADP. When it breaks from that third phosphorous it loses energy
How does temperature affect the density of a solution?
liquids and solids also expand and contract as temp changes, which changes the density
When a species gets oxidized it...
loses electrons or provides electrons
calorimetry
measures changes in temperature from the energy produced by a reaction
endothermic reaction
more energy is required to break the bonds, therefore, the reaction proceeds when energy is transferred from the surroundings to the reaction causing the solution temperature to decrease
What are dangers of chemicals going down drain?
municipal sewage treatment plants are not equipped to remove these materials from sewage a spark or an open flame near sink can cause an explosion in sink or further down the drain
When reading the label on a reagent container, what are the three most important pieces of information?
name concentration hazard warnings
Should you ever be allowed to eat inside the lab?
no
best way to put out a small contained fire?
turn off heat source immediately use a watch glass to cover the beaker and minimize the oxygen around the flame
Based on your observations in Experiment 5, an exothermic reaction may be identified via observation in the lab when it:
when a reaction is exothermic it releases energy into the surroundings away from the system like when P leaves ATP and becomes ADP. It uses energy to break the bond and releases it. we see this in experiment 5 with the combustion reaction of H2O2 into H2O and O2 gas.
When should you NEVER use a sealed container or secondary containment in lab?
when heating a solution over a hot plate, burner, or heating mantle
Does making a bond release or require energy?
releases Think of two objects banging together and the impact it takes
Any accidents in the lab, no matter how minor should be?
reported to the TA
precision
reproducability repetition of a measurement taken several times
A beaker of solid reagent drops onto the bench and cracks, What is the correct disposal?
should be added to solid waste container and the broken glass should be in broken glass box.
What happens if you spill 0.1 M NaOH on skin and clothes?
skin contact can produce burns, so, we must (like most chemicals) rinse immediately at the water faucets at the sink where the skin was exposed to corrosive materials. If you are exposed to alot of it there is a shower at the fron of the room
experimental error can be classified as either?
systemic OR random
how do you calculate temperature change?
tfinal- tinitial
To carry out Experiment 6 quantitatively, one major adjustment was made to the procedure followed in Experiment 2. That adjustment was:
the calorimeter was calibrated to account for heat loss from the system
accuracy
the closeness of a measured value to a standard or known value measuring the same quantity by different methods gives confidence in the nearness to the "true" value
Callibration
the process by which the reading obtained by an instrument is assessed for its accuracy and precision
stoichiometric reaction
the quantities of a the reactants and products are such that all of the reactants are consumed and none remain after completion of the chemical reaction useful for measuring chemical reactions such as those that occur in corrosion processes
precipitation
the reverse of dissolution the products of a reaction exceed their solubility and form a new phase that could be a gas, liquid, or solid
If a student uses a thermometer that is improperly calibrated and always reads a value of 1.5 degrees above the true temp. what would be effected on the graph?
the slope of the regression line would be correct, but the intercept would read a value that was greater that the true value
enthalpy
the total internal energy, in a given system
dependent variable
the variable being tested and measured
independent variable
the variable that is changed or controlled in a scientific experiment to test the effects on the dependent variable
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?
this reaction is exothermic the system is KOH, the surroundings is the water, container, etc.
What should you do with loose/long hair/clothing?
tie back securely constrained under a lab coat
Why do you need a stir bar in the calorimeter?
to stir the liquid and distribute heat in the vessel