Chem 160 written lab practical review
in the lab each student should
Be on alert, watching for his own safety and the safety of others
complete balanced equation for the precipitation reaction in aqueous solution of barium nitrate with sodium carbonate
Ba(NO3)2 + Na2CO3 —> BaCO3 (s) + 2NaNO3
(experiment 9)Consider the combination of a proton and hydroxide ion to make water (or the reverse, the dissociation of water into ions). H+ (aq) + OH- (aq) → H2O (l) ∆H = -55,800 Joules What is ∆H in Joules when 0.117 mol of water is formed
-6530 joules
in experiment 8, In this procedure, Tina the titrating student did not completely dissolve her antacid tablet prior to beginning the titration and still had some large chunks of tablet in her solution at the end of her titration. How would this affect her results (i.e. would the calculated mass of CaCO3 in the antacid be high, low or unchanged)?
-low - Explain reasoning: Undissolved antacid leads to fewer moles of H+ reacted which leads to an increased amount of OH- reacted. This in turn leads to a lower moles of antacid calculated.
In experiment 8, In this procedure, Gary the general chemist added an extra 10 mL of water to his Erlenmeyer flask after the addition of the nitric acid to the antacid tablet. How would this affect his results (i.e. would the calculated mass of CaCO3 in the antacid be high, low or unchanged)?
-unchanged -moles of acid is not changed
in experiment 8, If 25.00 mL of 0.5348 M HNO3 is added to an antacid tablet and the excess HNO3 requires 38.34 mL of 0.1027 M NaOH to reach the equivalence point, how many moles of HNO3 react with the antacid?
.0094 (limiting) convert 25 ml of .5348 M HNO3 to moles of HNO3 convert 38.34 ml of .1027 M NaOH to moles HNO3 subtract .01337 mol HNO3- .00398 mol HNO3 = .00939 mol of HNO3 reacted with antacid
What is absorbance when transmittance is 54.3%?
.265 A=-logT (t is between 0-1)
(experiment 9) A 25.64 g sample of solid was heated in a test tube to 100.00°C in boiling water and carefully added to a coffee-cup calorimeter containing 50.00 g of water. The water temperature increased from 25.10°C to 28.49°C. What is the specific heat of the solid? (You may assume all the heat was transferred to the )
.3867
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 (in mL)?
0.251
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?
0.982
In experiment 2, if you recover less than the original mass of your sample mixture, choose two things that could have happened to the "missing" amount?
1. did not precipitate out of solution completely 2. residual precipitate was not completely removed from the beaker to the filter
In experiment 2, how do you isolate this first solid? How do you isolate this first solid? If some of the solid remains stuck in the beaker, how can you get it out without changing the amount you isolate?
1. you isolate the first solid by gravity filtration 2. To remove solid from the beaker, you can wash it with cold or lukewarm water because this compound is only soluble in hot water
In experiment 2, once both compounds in the mixture are dissolved, what must be done to precipitate out one of them as a solid and which compound precipitates?
1. you most cool the solution in an ice/water bath to precipitate one of the compounds 2. From the ice bath, the Salicylic acid precipitates
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?
1.2 (with precision 2)
In experiment 4, A substance is known to have a molar extinction (ε) coefficient of 18,300 mL·cm-1mg-1 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. Report your answer after multiplying it by 10^5.
1.79
In experiment 5, 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).
10.8 mm (convert 14.7 cm to mm and then divide by 13.6)
Geoffrey the General Chemist used 2.498 g of a mixture of CuSO4 5 H2O and salicylic acid in this experiment. He recovered 1.324 g of salicylic acid and 1.247 g of CuSO4 · 5 H2O. What is his percent recovery?
102.9%
a student measured 14.7 cm for d. calculate the mercury equivalent of this water level difference
14.7/13.6 = 1.08 mmHg
Given the following values, calculate the mean. 23.4 24.1 26.7 28.2
25.6
(experiment 9) In a calibration of a calorimeter, 50.3 g of water at 60.5oC is added to 48.7 g of water at 19.7 Celsius to give a mixture at 39.1 Celsius. What is the heat capacity of the calorimeter used in this experiment?
28.4
If a 25.0 mL sample of 1.50 M HCl solution were diluted 125-fold, what would be the volume of the diluted solution (in L)?
3.13 L
What is the mass % of water in CuSO4 5H2O?
36.08%
Geoffrey used 2.498g of the original mixture and recovered 1.247 of CuSO4 5H2O. What is the percent recovery of the Copper (II) sulfate?
49.92%
Geoffrey used 2.498g of the original mixture and recovered 1.324g of salicylic acid. What is the percent recovery from the original mixture?
53%
Beer-Lambert Equation
A=E b c, A=absorbance, e= wavelength dependent constant, c= concentration of the absorbing material, b= length of the light path
If you spill a large amount of chemical on the floor:
Alert nearby students and call the instructor for instructions about how to clean it up
net ionic equation for barium nitrate and sodium carbonate
Ba 2+ + CO3 2- —> BaCO3 (s)
In experiment 6, What is the periodic trend in ionic size for the cations of the alkaline earth elements (Mg2+, Ca2+, Sr2+, Ba2+) used in this experiment? You may wish to consult section 8.4 (pages 269 - 271) of your textbook to help do this.
Ba2+ > Sr2+ > Ca2+ > Mg2+ increase down the column
If you spill solid chemicals on a balance:
Brush off any spills
If you are pregnant or have a medical condition:
Check with your physician prior to working in lab; notify instructor
In experiment 2, Would it be better to rinse the crystals after filtration with cold solvents or warm solvents?
Cold because it dissolves easier in warm solvent. (adding warm solvents would then dissolve the mixture again after you just filtrated it)
An unused/leftover chemical should be:
Disposed of in the designated waste container
consider the relative solubilities of the group 2 cations in the various precipitating reagents. one the basis of the trends you observed, list the 4 alkaline earth cations in the order in which they should appear in the periodic table. Start with the one whose oxalate salt is the most soluble
Mg2+, Ca2+, Sr2+, Ba2+
After dispensing a chemical from a container:
Replace the stopper immediately
If you feel ill in lab:
Tell the instructor
Which compound remains dissolved in solution after the first compound is isolated as a solid? What are the next two steps that you must take in order to precipitate this second compound out of solution?
The compound that remains in solution is copper(II) sulfate pentahydrate. To precipitate it, you must first evaporate most of the solution water off then add ethanol.
you are permitted to enter the lab if
The instructor is present
After you have isolated both compounds, separately, from your mixture, why do you wait a week before weighing them?
There is still residual liquid on the solids that must evaporate - the extra week provides the time for this to happen.
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?
Tyndall effect
precision
a measure of how close a series of measurements are to one another
correlation coefficient
a statistical measure of the extent to which two factors vary together, and thus of how well either factor predicts the other; R^2
for purposes of discussion, use the following data for this question: 0.0404 g of Mg, pressure of dry hydrogen is 711.4 mmHg, temp of hydrogen is 24.2 C, volume of H2 is 42.5 mL a) calculate pressure in atm, volume in liters, and temp in K b) use PV=nRT to calculate moles of hydrogen c) moles of mg inferred from above d) calculate atomic weight of mg e) calculate % error
a) 0.936 atm; 0.0425 L; 297.4 K b) 0.001631 mol c) 1.631 x 10^-3 mol d) 24.77 g/mol e) 1.9%
In experiment 3, a 7.218 g sample of hydrated CoCl2 is heated. After heating, the sample weighs 3.939. How much water was lost in the process? (grams and moles) How much anhydrous salt (CoCl2) is left behind? What is the formula of the hydrate?
a. 3.279g or .1820 mole of water was lost b. 3.939g of CoCl2 or 0.0303 moles was left behind c. CoCl2 6H2O
Light emitted by sodium vapor lamps (like those on highway 30 outside BHS) has a wavelength of 5.89 x 102 nm. a) Calculate the frequency of this radiation in Hz. b) Determine the energy in joules (J) of one photon of this c) Determine the energy in joules (J) of one mole of photons which have this wavelength. d) In what region of the light spectrum (IR, UV, visible, x-ray etc.) is this radiation found?
a. 5.09 x 10^14 Hz b. 3.37 x 10^-19 J c.2.03 x 10^5 J d. Visible '
- log T
absorbance
store your books and bags
on the designated rack in the lab
In experiment 4, Which calibration curve gave 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
absorbance vs. concentration
In experiment 5, 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)? b. What could Geraldine do to prevent these bubbles from skewing her laboratory results?
a. lower b. add more water to get rid of bubbles before inserting the rubber stopper
(experiment 5) 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)
a. pressure= .936 atm, V= .0425 L, T= 297.4 K b. 0.00163 moles H2 c. 0.00163 moles Mg d. 24.8g/mol e. 2%
in experiment 9: Sometimes the heat capacity of the coffee cup calorimeter is ignored during calculations. a. what type of error would this introduce into calculations? b. If the heat capacity of the calorimeter was ignored, how would this affect the calculated heat of neutralization (i.e. would it be greater or lower in magnitude, or unchanged)? c. If the heat capacity of the calorimeter was ignored, how would this affect the calculated heat of solution (i.e. would it be greater or lower in magnitude, or unchanged)?
a. systematic b. lower c. slightly higher or unchanged
In experiment 7, Tilly the titrating fiend consistently obtained a dark pink endpoint for all three of her a. how would this affect the accuracy of her molar mass? b. how would this affect the precision of her molar mass?
a. underestimated molar mass b. high precision
In experiment 2, when Gaby 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?
a: CuSO4 b: add H2SO4
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 accuracy and precision when a TC graduated cylinder is used to transfer a given volume of liquid? (I.e. does it change the accuracy, precision, or both? Why?)
accuracy
what does the valve on the bottom of the bunsen burner do
adjusts amount of natural gas going into the burner
what does the bunsen burner barrel do?
adjusts the amount of air going into the burner
the following should be reported to the instructor
all accidents no matter how minor.
In experiment 2, Freddy the Freshman Chemist calculated his percent recovery for the salicylic acid crystals and realized that 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 the majority of crystals from the beaker to the filter paper. Give a possible explanation for Freddy's poor recovery.
allow more time to crystallize
(NH4)+
ammonium ion
If you do not understand a direction or part of a lab procedure, you should
ask the instructor before proceeding
When heating a liquid in a test tube, turn the mouth of the test tube _______ from yourself and others.
away
In experiment 9, Why should the heat lost by the hot water exceed the heat gained by the cold water?
because heat is lost to the calorimeter
Which flame is hotter: yellow or blue?
blue is a hotter flame then yellow
In experiment 4, 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?
blue??
How could Geoffrey's 102.9% recovery from above have been avoided?
by further drying the sample
(CrO4)2-
chromate ion
(CO3)2-
carbonate ion
In experiment 4, what is the purpose of the positive and negative controls in Part A of the procedure?
determine what sweetener has aspartame
(experiment 9) The heat transfer in a reaction is 8740 J. Write the correct value (with correct sign) for q if heat is: evolved released absorbed
evolved: -8740 released: -8740 absorbed: 8740
d= m/v
formula for density
What label (units) will attach to the molar mass of magnesium to be determined in this experiment?
g/mol
In experiment 7, 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?
greater
What type of mixture are you given in experiment 1? (Copper (II) sulfate and Salicylic Acid)
heterogeneous mixture
In experiment 7, If a solid, unknown acid were contaminated with pure, adsorbed water prior to weighing out the sample, what effect (i.e. higher, lower, or none) would this have on the determined molar mass?
higher
in experiment 5, 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?
higher
In experiment 3, 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?
higher because more water release than accounted for in the first place
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? (I.e. would his reported density be higher or lower than it should be?)
higher because of the change in water volume
In experiment 3, 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?
higher because this leads to a lower anhydrous mass subtracted from the hydrate mass.
In experiment 2, Dissolving both compounds in the mixture is the first step in the experiment. What solvent works best for this?
hot water (can dissolve both compounds)
accuracy
how close a measurement is to the true value
What is absorbance when transmittance is 0%?
infinity
In experiment 8, 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 results of the titration?
it does not change the moles of the acid originally present
What is wrong with Geoffrey's result of 102.9%?
it is above 100%
Is a larger or smaller sample better?
larger is better
What is the correct flame appearance when heating something
visible outer and inner "cone" of blue flame
In experiment 5, 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)?
lower
In experiment 3, Clarissa the chemist did not heat her salt sample long enough to drive off all of the hydrated water. Would her calculated number of water molecules be higher or lower than it should be?
lower because a greater perceived amount of anhydrous mass
In experiment 10, 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?
lower because the sulfate has a chance to sediment to the bottom
In experiment 4, 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?
lower than the true amount
10. 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?
more accurate because there are sig figs
In experiment 5, 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?)?
neutralize with base
A cracked test tube or chipped glassware should:
never be used
In experiment 7, 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?
no effect
In lab you are allowed to eat and drink only:
none of the above
You are excused from wearing goggles in lab:
none of the above
you can change the procedure of the experiment if
none of the above
for your safety, before leaving the lab
wash your hands and other skin
(C2O4)2-
oxalate ion
measured value - accepted value/accepted value x 100
percent error
standard deviation / average of trials *100
percent relative standard deviation
does a graduated cylinder or pipet give you more accurate and precise results?
pipet gives you more accurate and precise results
In experiment 8, The lab states that it is very important to thoroughly rinse the buret with the titrant before beginning the experiment. Tyler the titrating student did not rinse his buret properly and residual water (not tritrant) was left in the buret prior to filling the buret with base. How would each of the following be affected? Precision, accuracy and mass or CaCO3?
precision: worse accuracy: worse mass of CaCO3 in antacid: lower
you should prepare for the lab by
reading the assigned experiment before coming to the lab.
if you accidentally mix the wrong chemicals, you should
report it to your instructor
Why is salicylic acid more soluble in ethanol than CuSO4 5 H2O?
salicylic acid is an organic compound and more soluble in ethanol
((trial-average)^2 + (trial-average)^2 + (trial-average)^2)^1/2 / # of trials - 1
standard deviation
(SO4)2-
sulfate ion
When filtering the copper(II) sulfate crystals (e. the 2nd filtration), why do we filter the crystals with ethanol instead of water? [What would happen if you used water in that filtration step?]
water dissolves CuSO4 so it will re-dissolve crystals
If no specific instructions for disposing of waste chemicals are given, you should dispose of all liquids and solutions into:
the sink with lots of water
In experiment 8, you will know that you have completely titrated the excess nitric acid in the solution when
the solution has a persistant faint pink color
In experiment 7, You will know that you have reached the equivalence point in the titration when ________________.
the solution has a persistent faint pink color
What is the hottest portion of the flame?
the top of the inner "cone" of the blue flame
Why are pipet volumes reported with an additional significant figure?
they are more accurate to that value
Frances the Freshman recorded the temperature of her final solution (Part 2) before it stabilized. If her temperature recorded was higher than the actual temperature (due to lack of stirring), how would this affect her calculated value for the heat of neutralization (i.e. would her heat of neutralization be greater or lower in magnitude, i.e. bigger absolute value, than it should be)?
this would increase her heat of neutralization because increased heat of reaction
In general, why do chemists use weigh boats for measurements?
to avoid putting chemicals directly on the balance weighing pan
I / I (blank)
transmittance
I/ I zero
transmittance
Before evacuating the lab in case of earthquake or other emergency, you will:
turn off gas valves and electrical equipment
In experiment 4, 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)?
unchanged
What is the Tyndall effect?
when light is scattered by a colloid
In experiment 3, Is it possible to heat the solid too long for this experiment?
yes, eventually, decomposition would occur
What is absorbance when transmittance is 100%?
zero