Download Chem 160 Lab Final Questions with 100% complete solutions. NEW UPDATE!! and more Exams Nursing in PDF only on Docsity!
Chem 160 Lab Final Questions with
100% complete solutions. NEW
UPDATE!!
Describe the proper procedure for using a pipet - CORRECT ANSWER -place
the end of the pipet in the solution. squeeze the air out of the suction and place it on the pipet. After suctioning the solution, remove the suction and replace with thumb. release thumb until water line is level with the water mark. distribute to container Given the following values, calculate the mean and standard deviation
23.4, 24.1, 26.7, 28.2 - CORRECT ANSWER -Mean - 25.
SD - 2.
A student measures the density of a liquid to be 0.9992 g/ml. If the true value of the
liquid is 0.9874 g.ml, what should the student report as his %error? - CORRECT
ANSWER -1.181%
Precision - CORRECT ANSWER -standard deviation
how close together all your answers are
accuracy - CORRECT ANSWER -% error
how close your answers are to the target Cathy the chemist measures the mass of an empty 50-mL beaker to be 24.278 g. She then pipets 25.00 mL of an unknown liquid into the beaker and measures the mass of the beaker + liquid to be 48.829 g. Based on this data, what should Cathy report the
density of the liquid to be? - CORRECT ANSWER -0.9820 g/ml
a sample of an unknown metal is placed in a graduated cylinder containing water. The mass of the sample is 9.752g and its water displacement is 3.25 mL. What is its
density? - CORRECT ANSWER -3.001 g/ml
In general, is it better to measure a large or small sample? Thoroughly explain your
answer. - CORRECT ANSWER -a larger group is better because it allows easier,
and thus more accurate and precise measurements. It also avoids small mistakes and inconsistencies from having large consequences Graduated cylinders often have "TC" written on them. TC stands for "to contain." Pipets generally have "TD" written on them which stands for "to deliver." What happens to the
precision when a TC graduated cylinder is used to transfer a given volume of liquid? -
CORRECT ANSWER -The precision over multiple trials that only rely on the
graduated cylinder should stay relatively similar, however, they may get worse considering graduated cylinders are harder to measure correctly. Graduated cylinders often have "TC" written on them. TC stands for "to contain." Pipets generally have "TD" written on them which stands for "to deliver." What happens to the accuracy when a TC graduated cylinder is used to transfer a given volume of liquid? -
CORRECT ANSWER -The accuracy gets worse since it's not designed to have
the correct amount come out perfectly each time. Thus it leaves some of the liquid behind, and can't be measured nearly as accurately Simon the speedy chemist was in a hurry and some of the liquid splashed out of the graduated cylinder as he was adding his zinc washers. How would this affect his
reported density? Explain your answer - CORRECT ANSWER -His measurement
would be more dense than it should be, because there would be less water displacement Assume you have access to a balance that can accurately weigh a sample to the nearest ±0.0001 g. If you used this balance to obtain the mass of the zinc washers in Part 1, would your results be more accurate than if you used a balance that was only
accurate to the nearest ±0.001 g? Why or why not? - CORRECT ANSWER -Yes,
because overall your numbers would average out closer to the expected value Ronny the reckless chemist was performing this lab and accidentally broke a beaker
and cut himself. What should he do? - CORRECT ANSWER -He should alert his
teacher to the incident and seek immediate medical attention if necessary. When Gabby the general chemist looked at her salicylic acid crystals, they were tinged with blue. a. What are the crystals contaminated with?
b. How could this have been avoided? - CORRECT ANSWER -a. Copper (II)
Sulfate b. Heating it longer to dissolve the entire solution Freddy the freshmen chemist calculated his percent recover for the salicylic acid crystals and realized he obtained only a small fraction of the salicylic acid that was in the original sample even though he was very careful to avoid boiling the original mixture solution and he transferred copious amounts of white crystals from the beaker to the
filter paper. Give possible explanations for Freddy's poor recovery - CORRECT
ANSWER -His filter paper may not have been set up right, allowing the crystals to
slip through or he also could have dropped some
Phillip the freshman chemistry student was stirring his salt during heating. If some of the hydrated salt adhered to the stirring rod and was removed prior to weighing the anhydrous sample, would his calculated number of water molecules be higher or lower
than it should be? Explain your reasoning. - CORRECT ANSWER -He would
have a higher number of water molecules since there would be a reduced weight making it seem like more water was removed. Shelly the speedy chemist did not heat her crucible prior to the addition of the hydrated salt. Would her calculated number of water molecules be higher or lower than it should
be? Explain your reasoning. - CORRECT ANSWER -It would be higher since
there are may have been water left behind in the crucible adding to her calculations Is it possible to heat the solid too long for this experiment? Why or why not? -
CORRECT ANSWER -While it may be hard to do, you could eventually heat it
too long that it would begin to decompose
What is the mass % of water in CuSO4·5H2O? - CORRECT ANSWER -36.07%
A 7.218 g sample of hydrated CoCl2 is heated. After heating, the sample weighs 3. g. a. How much water was lost in the process? b. How much anhydrous salt (CoCl2) is left behind?
c. What is the formula of the hydrate? - CORRECT ANSWER -a. 3.279 g or
0.1821 mol b. 3.939 g or 0.03034 mol c. CoCl2 6H2O Examine the absorption spectrum of complexed aspartame in Figure 2. Based on this figure, what color do you think the solution of complexed aspartame appears to the
eye? (622 nm ) - CORRECT ANSWER -blue, but absorbs orange
What is the purpose of the positive and negative controls in Part A of the procedure? -
CORRECT ANSWER -The purpose is to establish a baseline for how the
sweetener will act if it has aspartame compared to if it does not.
What is absorbance when transmittance is 0%? - CORRECT ANSWER -100%
What is absorbance when transmittance is 100%? - CORRECT ANSWER -0%
What is absorbance when transmittance is 54.3%? - CORRECT ANSWER -
UNKNOWN FIND THIS ANSWER
A substance is known to have a molar extinction (ε) coefficient of 18,300 mL·cm-1·mg- at its wavelength of maximum absorption. The absorption of a solution containing this substance in a 1-cm sample cell was found to be 0.327 at this same wavelength.
Determine the concentration of the solution. - CORRECT ANSWER -IDK HOW
TO SOLVE FOR C CAUSE IDK B with A =ebc a= 0.327, e = 18,300, Why are positive and negative controls important in science? (I.e. what would happen if
these controls were not used?] - CORRECT ANSWER -They are important for
establishing a baseline so that observations can be made When Felicity the freshman chemist performed this lab, she didn't transfer the entire amount of the sweetener from the packet into the 50.0 mL graduated cylinder (i.e. some got left behind and was never weighed or transferred to the graduated cylinder). How would have this affected her calculation of the mass% aspartame (i.e. would it be
higher, lower, or unchanged)? Explain your answer. - CORRECT ANSWER -Her
mass% would be lower since she was working with a smaller amount Cale the chemist also performed this lab and when he made his unknown, he did not mix the solution well and the solution in the bottom of the flask appeared a darker blue than the solution at the top of the flask. How would this affect his calculation if he used the lighter-colored part of solution to determine the mass% aspartame (i.e. would it be
higher, lower, or unchanged)? Thoroughly explain your answer. - CORRECT
ANSWER -It would have been a lower mass% since the amount wasn't mixed. The
larger amount of aspartame would not have have been measured since it was at the bottom Which calibration curve would give the better R2 value (absorbance vs. concentration or %transmittance vs. concentration)? Use the Beer-Lambert Law and the mathematical relationship between absorbance and transmittance to help you fully explain why this
would be expected. - CORRECT ANSWER -The straight line is a better option
since an R2 value is how well the data fits into a straight line What is the periodic trend in ionic size for the cations of the alkaline earth elements
(Mg2+, Ca2+, Sr2+, Ba2+) used in this experiment? Explain. - CORRECT
ANSWER -The ionic size increases as you go down the table
Name the following ions: NH4+, SO42-, CO32-, C2O42-, CrO42- - CORRECT
ANSWER -Ammonium, sulfate, carbonate, oxalate, chromate
Write the complete balanced equation for the precipitation reaction in aqueous solution
of barium nitrate with sodium carbonate. - CORRECT ANSWER -Ba(NO3)2 (aq)
- Na2CO3 (aq) → BaCO3 (s) + 2NaNO3 (aq)
determined molar mass? Explain your answer. - CORRECT ANSWER -It would
have a higher molar mass since the calculated mass is a higher amount If water contaminated the sample after it was weighed out, what effect (i.e. higher, lower, or none) would this have on the determined molar mass? Explain your answer. -
CORRECT ANSWER -It would have no effect since water is necessary to get a
molarity Ellen Meyer wrote down the molarity of NaOH solution that was reported on the bottle and used this value in her calculations. However, unbeknownst to Ellen the molarity of the NaOH solution was actually less that what was reported on the bottle. Would the molar mass of the unknown acid determined in this experiment be greater or less than
the actual molar mass? Explain your answer. - CORRECT ANSWER -The molar
mass would be greater since she used a larger molarity Tilly the titrating fiend consistently obtained a dark pink endpoint for all three of her titrations. a. How would this affect the accuracy of her molar mass? Explain your answer. b. How would this affect the precision of her molar mass? Explain your answer. -
CORRECT ANSWER -a. the accuracy would be low since she was far from the
desired color each time b. the precision would be consistent since she got the same color everytime Gerry the general chemist was performing this laboratory and accidently spilled some
acid on her arm. What should she do? - CORRECT ANSWER -Alert her
professor and wash the area as soon as possible What label (units) will attach to the molar mass of magnesium to be determined in this
experiment? - CORRECT ANSWER -g/mol
A student measured 14.7 cm for "d" (see the illustration in the procedure). Calculate the
mercury-equivalent of this water-level difference (in mm of Hg). - CORRECT
ANSWER -10.8 mm Hg
For purposes of discussion, use the following data for this question: 0.0404 g of Mg, the pressure of dry hydrogen is 711.4 mm Hg, the temperature of hydrogen is 24.2°C, and the volume of H2 is 42.5 mL. a. Calculate pressure in atm, volume in liters, and temperature in Kelvin b. Use PV=nRT to calculate moles of hydrogen. c. Moles of magnesium inferred from (b) above d. Calculate the atomic weight of magnesium (g/mole)
e. Calculate the % error for the atomic weight (known is 24.305 g/mol) - CORRECT
ANSWER -a. 0.936 atm, 0.0425 L, 297.2 K
b. 0.00163 mol H c. 0.00163 mol Mg d. 24.785 g/mol Mg e. 1.97% When Fiona the first-year chemistry student performed this lab, her magnesium metal escaped the copper turnings and floated to the top of her gas buret and never completely reacted with the hydrochloric acid. Would her calculated atomic weight of magnesium be higher or lower than it should be? Explain your reasoning. -
CORRECT ANSWER -It will have a smaller atomic weight since there isn't as
much hydrogen gas When Geraldine the general chemistry student performed this lab, she noticed that she had small bubbles lining the interior of her buret right after filling her buret with distilled water. She decided to continue with her experiment anyway. a. How would this affect her calculated atomic weight of magnesium (i.e. would it be higher, lower, or unchanged)? Explain your reasoning. b. What could Geraldine do to prevent these bubbles from skewing her laboratory
results? - CORRECT ANSWER -a. it would be lower since hydrogen gas is
escaping b. she could restart, or quickly plug the buret to stop escaping gas Frank the freshman chemist forgot to account for the vapor pressure of water in his buret when doing his calculations. How would this affect his calculated atomic weight of magnesium (i.e. would it be higher, lower, or unchanged)? Explain your reasoning. -
CORRECT ANSWER -His atomic weight would be higher since there would be a
larger amount of hydrogen gas How are the excess solutions treated prior to disposal (i.e. what happens to the solutions collected in the hood prior to them being dumped down the drain with lots of
water?)? - CORRECT ANSWER -It is neutralized with baking soda
Write balanced chemical equations for the reactions between HNO3 and... (a) magnesium hydroxide
(b) calcium carbonate (Note: a gas is produced in this reaction.) - CORRECT
ANSWER -a. 2HNO3 (aq) + Mg(OH)2 (aq) → Mg(NO3)2 (aq) + 2H2O (l)
b. 2HNO3 (aq) + CaCO3 (s) → Ca(NO3)2 (aq) + H2O(l) + CO2 (g) It is found that 41.27 mL of 0.2017 M NaOH is required to neutralize 25.00 ml of HNO solution in a titration experiment. Calculate the molarity of the HNO3 solution. -
CORRECT ANSWER -0.33296 mol/L HNO
The amount of acid to be titrated has to be delivered by a pipet so that its volume is accurately known, so why can additional deionized water be added without affecting the
By using the general equation for dilution of a fixed amount of solute, (M1)·(V1) = (M2)·(V2), where M represents concentration and V represents volume, how much of a solution of 0.0949 M sodium sulfate would be needed to prepare 10.0 mL of 0.00238 M
sodium sulfate? - CORRECT ANSWER -V1= .251 mL
If a 25.0 mL sample of 1.50 M HCl solution were diluted 125-fold, what would be the
volume and concentration of the diluted solution? - CORRECT ANSWER -0.
M
Zia took 10.0 mL of water and added it to 10.0 mL of barium solution and found the sulfate concentration of the diluted sample was 0.0030M. What was the concentration of
the original water sample? - CORRECT ANSWER -M1=.
When a large, colorless, crystalline or glassy solid is examined for light transmission, one notes a slight bit of reflection of light at the interface with air but generally high transmission ability for light. This is called transparency. Why, when the same solid is ground to a powder, but no change occurs in its composition or crystal structure, does it
becomes white and opaque to transmission of light? - CORRECT ANSWER -It
creates a colloid and causes the tyndall effect As you may have noticed, barium sulfate tends to settle at the bottom of the solution. In addition, once barium sulfate forms in solution, the particles begin to coalesce (clump together). Millie the general chemistry student made all of her standard solutions at the beginning of lab and let them sit before she measured their turbidity. What kind of error would this settling and clumping of barium sulfate introduce (i.e. systematic or random)?
Explain your answer. - CORRECT ANSWER -It would be a systemic error since
she didn't follow directions and let her product settle for all of her solutions Josephine the general chemistry student was careful to make and stir all of her standard solutions immediately before measuring their turbidity for her calibration curve. However, she let her tap water solutions sit for 10 minutes prior to measuring their turbidity. If Josephine measured the top portion of the solution, would her experimental sulfate ion concentration be higher or lower than it should be? Explain your answer. -
CORRECT ANSWER -There will be a lower concentration since the barium
sulfate would have all settled at the bottom of her solutions Fillmore the first-year student accidentally dropped a 4 L container of 0.10 M barium
nitrate on the floor creating a large spill. What should he do? - CORRECT
ANSWER -He should alert his professor for assistance
VSEPR theory utilizes an intuitively simple idea to predict the shapes of molecules and ions (or the shape about a given atom in a complex structure). State the idea behind
this theory. - CORRECT ANSWER -The idea is that the electrons will want to get
as far away from each other as possible and will therefore follow basic geometry to do so
