Chem 211 Lab Final
What is the value of R in this lab?
62.36 L*torr/mol*K
What happens in Lab 7: Gas Collection?
Add Na2CO3 to a flask. Add HCl to the flask. CO2 will go through the tube and into the submerged graduated cylinder. Repeat with less HCl each trial. Use the volume of CO2 collected to calculate the moles of CO2.
What happens in Lab 6: Formula Weight of a Copper Compound?
Add an unknown copper compound and water to a beaker. Add Mg turnings. The solution should turn blue/green. Add HCl. The solution will bubble and clear, revealing the copper. Filter the solution using the gravity filtration method. Dry the copper and filter paper. Determine the formula weight of the copper compound and identify it.
What happens in Lab 4: Limiting Excess Reagents?
Add benzaldehyde, acetone, and NaOH into an erlenmeyer flask. Filter the solution using the vacuum filtration method. Dry the residue and filter paper. Determine the actual and percent yield of the product, dibenzalacetone.
What happens in Lab 8: Dumas Method?
Add cyclohexane to an Erlenmeyer flask. Cover the opening of the flask with a piece of foil that has a hole in it. Submerge the beaker to a point below the foil. Heat the beaker until the cyclohexane is gone. Mass the beaker, once cooled. Repeat for 3 trials. Calculate the molecular weight of the vapor.
What happens in Lab 3: Hydration of Epsom Salt?
Add magnesium hydrate salt to a crucible and heat it using a Bunsen burner for 5 minutes, then mass residue once cool. Stop heating/cooling cycle when the residue reaches a constant mass. Repeat for another trial. Determine the ratio of H20 to MgSO4 and the mass % of MgSO4 * XH20.
What happens in Lab 10: Synthesis of Alum?
Add pieces of foil to a beaker. Add KOH. Once the foil is dissolved, add H2SO4. Heat precipitate until clear. Cool and wait for crystals to form. Filter alum crystals and ethanol using the vacuum filtration method. Dry alum and mass. Determine the % yield of alum.
Extraction
Adding a solvent to a mixture that dissolves only one of the components
How to find heat capacity of calorimeter
Step 1: -mCΔT hot water = mcΔT cold water Step 2: q hot water - q cold water Step 2: C cup = q (difference of water)/ΔT water
The equation for the trendline follows the formula y = mx + b, where b is the y-intercept. The y-intercept should be set to zero. Briefly explain why.
The y-intercept is set to zero because the graph needs to reflect the 1:1 ratio between Na2CO3 and CO2. If you don't add Na2CO3, then you will not produce any CO2.
Using the mass of sodium carbonate you used in Trial 1, calculate the volume of 1.20M HCl (mL) needed for the reaction to reach completion.
0.337 g Na2CO3 = 3.18E-3 mol Na2CO3 x 2 mol HCl/1 mol Na2CO3 x 1 L/1.20 mol HCl = 5.3E-3 L = 5.3 mL
A piece of an aluminum can was cut into small pieces. Then 0.444g of the pieces from the can was used to prepare potassium alum according to the procedure used in lab. How many grams of Alum should you expect if you had a 83.8% yield?
0.444 g aluminum * 1 mol aluminum/26.982 g aluminum * 1 mol alum/1 mol aluminum * 474.42 g alum/1 mol alum = 7.81 g alum x/7.81 *100 = 83.8 x = 6.54 g Alum
If 0.839 g of KHP were titrated, and 34.58 mL NaOH solution was necessary to reach the equivalence point, what is the concentration of the NaOH solution?
0.839g KHP x 1 mol KHP/204.218 g KHP x 1 mol NaOH/1 mol KHP x 1000 mL = 4.1084E-3 mol of NaOH 4.1084E-3 mol of NaOH/34.58 mL NaOH x 1000mL/1 L = 0.119 mol/L
Using the data from Trial 2 calculate the percentage of water vapor in the cylinder.
water pressure/atm pressure x 100 18.64/765.47 x 100 = 2.435%
Formula to determine % Cu
(Mass of Cu recovered/Mass of unknown Cu compound) x 100%
Formula weight of unknown (g/mol)
(Mass of unknown Cu compound/Moles of unknown Cu compound)
Formula for percent relative standard deviation
(standard deviation/average concentration) x 100%
Accuracy
The closeness of the value to the correct value
What would the effect have been on the calculated formula weight ([9] above) of your unknown as compared to the true formula weight if the solution had been blue green when you filtered your product?
Too high
Formula for percent yield?
(Actual Yield/Theoretical Yield) x 100%
Formula % error
(MW exp - MW actual)/MW actual x 100%
Formula to find heat capacity of nails
-q (metal) = q (water) + q (calorimeter)
Calculate the volume of CO2 (mL) would you expect to collect in trial 3 based on the amount of Na2CO3 you used and your recorded temperature and atmospheric pressure?
0.171 g Na2CO3 = 1.61E-3 mol Na2CO3 PV=nRT (746.83)V=(1.61E-3)(62.36)(294.2) V=0.0400 L = 4.00E-3 mL
Examples of separation techniques
1. Decantation 2. Filtration 3. Extraction 4. Sublimation 5. Evaporation
Significant Figure Rules
1. all non-zeros are significant 2. zeros in the middle are ALWAYS significant 3. zeros at the start ar NEVER significant 4. zeros at the end are not significant unless a decimal point is present
If the yield of the reaction in question 2 is known to be only 67.89% using stoichiometric amounts, how much (g) of each of the three reagents would you need to obtain 10.000g of boron trichloride?
4.376 g B2O3 13.37 g Cl2 1.132 g C
Calculate the mass percent of water contained in a sample of MgSO4.7H2O?
7H2O = 7(18.016) = 126.112 g/mol MgSO4.7H2O = 24.305 + 32.06 + 4(16.00) + 126.112 = 246.48 g/mol 126.112 / 246.48 * 100% = 51.165% of water in MgSO4.7H2O
A sample of raw mining ore contains a hydrated salt called copper sulfate tetrahydrate, CuSO4.4H2O along with other impurities. If a 10.854g of the ore loses 2.994g of water when heated strongly, what is the mass percentage of the salt CuSO4.4H2O in the raw ore sample? Assume the only source of water is from the hydrated salt. KEY: The ore contains the hydrated salt with other unknown components. Mass % = (g CuSO4.4H2O/g ore) * 100%, Report correct significant figures.
9.621 g CuSO4.4H2O / 10.854 g ore *100% = 88.64% CuSO4.4H2O
What happens in Lab 9: Calorimetry?
Add 50.0 mL of room temperature water into a calorimeter and measure its temperature. Heat 100.0 mL of water and measure its temperature. Add the warm water to the water in the calorimeter. Record the highest temperature and determine the change in temperature. Determine the heat capacity of the calorimeter. Heat a test tube filled with nails. Add the nails to the calorimeter containing room temperature water. Record the highest temperature and determine the change in temperature. Determine the heat capacity of the nails.
Using your calculated concentration of NaOH from lab, how many grams of KHP would you need to start with in order to have an end point at 35.00 mL?
Average concentration of NaOH: 0.0863 mol/1L 35.00 mL x 1 L/1000 mL x 0.0863 mol NaOH/1 L x 1 mol KHP/1 mol NaOH x 204.218 g KHP/1 mol KHP = 0.6168 g KHP
Sublimation
Changing a solid into a gas without the intermediate liquid state
What happens in Lab 2: Separations?
Create a mixture of 50:50 sand and salt. Dissolve the salt using DI water, then filter it using gravity filtration or vacuum filtration. Dry the sand residue and filter paper. Transfer the filtrate to an evaporating dish and dry. Determine % salt and sand.
Formula for density
D = mass/volume
A 250 mL Erlenmeyer flask has a measured volume of 287mL. The experiment is performed with a this flask but the volume of 250 mL is mistakenly used in the molar mass calculation. What effect does this error have on the calculated molecular weight?
If a lower meausurement of volume is used in the molar mass calculation, the result would be a higher molecular weight because they are inversely proportional to each other.
If the barometer reading is recorded to be higher than the actual value. What effect does this error have on the reported value of the molar mass of the compound?
If the barometer reports a higher pressure, the molar mass of the compound would be lower because they are inversely proportional to each other.
If the water bath only reached a temperature of 94.9C but your still used the value of 100C in the calculation what effect would this error have on the reported value for the molar mass of the compound?
If the temperature of the water bath used in the calculation was a higher value, the reported value for the molar mass of the compound would be higher because they are directly proportional to each other.
When performing this experiment if your mistakenly added 6 mL of volatile liquid instead of 4 mL at the start. What effect would this have on the calculated molecular wieght the compound?
If you added 6 mL instead of 4 mL, the calculated molecular weight would be unchanged because the amount of liquid cyclohexane added would be completely evaporated and the gas cyclohexane would cling to the inner walls of the flask. The measurement depends on the volume of the flask, not the amount of liquid cyclohexane added.
Formula for % of salt/sand?
Mass of salt or sand (recovered) / mass of mixture (initial) x 100%
Briefly Explain why NaOH was not included in the limiting reagent calculation for the dibenzalacetone synthesis.
NaOH was not included because it functions as a catalyst for the reaction and is not consumed.
If the unknown copper salt you worked with was copper (I) instead of a copper (II) would the calculated formula weight ([9] above) of the unknown salt change?
No
Formula: Ideal Gas Law
PV=nRT
Decantation
Pouring a liquid off a solid
Evaporation
Removing a liquid from a mixture by turning it into a gas
Multiplying or dividing sig figs
Round to least number of sig figs
Adding or subtracting sig figs
Round to the least number of decimal places
How would the following error affect % salt and % sand? Error: The residue on the filter paper was not washed during the filtration.
Salt: too low Sand: too high
How would the following error affect % salt and % sand? Error: The beaker holding the mixture was not rinsed after pouring into the filter.
Salt: too low Sand: too low
Filtration
Separation of a solid from a liquid by pouring the mixture through a semipermeable barrier which allows only the liquid to pass through
Assume after recording the mass of KHP on the weigh paper it was not completely transferred into the Flask. Your following titration was completed correctly. Would your experimental value for concentration of NaOH be too high, too low, or unchanged as compared to the true value?
The experimental value would be too low because less g of KHP would also mean less moles of KHP and NaOH when converted. A small amount of moles of NaOH would lower the molarity.
Assume an air bubble had been in the buret tip when you took your first reading and it was replaced by NaOH during the course of the titration. Would your experimental value for the concentration of NaOH be too high, too low, or unchanged as compared to the true value?
The experimental value would be too low because the measured volume of the NaOH solution would include air and the NaOH solution, lowering the molarity.
Assume the buret had been wet with water because it was not adequately rinsed with base. Would your experimental value for concentration of NaOH be too high, too low, or unchanged as compared to the true value?
The experimental value would be too low because the measured volume of the NaOH solution would include water and the NaOH solution, lowering the molarity.
What would the effect have been on the calculated formula weight ([9] above) of your unknown as compared to the true formula weight if the solution had not quit bubbling when you filtered your product?
The formula weight would have been too low because the recovered copper metal would have included excess solid magneisum that had not converted to aqueous Mg2+.
In our experiment we assumed a perfect system, (no heat lost to the room). We know in reality that some heat will escape to the room. Because we did not account for this loss do you think the true Specific heat capacity of nails (J/g°C) is larger or smaller than our calculated values?
The heat capacity of the nails is larger because the combined energy absorbed by the environment, cup and water would mean a greater amount of energy lost by the nails. An increase in q for the nails means an increase in its heat capacity.
If a student's coffee cups are discarded after determining the heat capacity and used new coffee cups for the specific heat capacity of the nails experiment. How would this error impact the results?
The new coffee cup would not have the same heat capacity of the original cup. Since the energy absorbed by the cup and water is lost by the nails, the calculated heat capacity of the nails would be higher or lower depending on the difference of heat capacity between the old and new cups.
If you had not dried the solid, would the calculated percent yield have been higher, lower, or unchanged?
The percent yield would have been higher because it would have include dibenzalacetone and the unevaporated water.
Pricision
The range of error, tolerance, or uncertainty around the measurement
How would your calculated ratio of water molecules present in Epsom salt be affected if you heated your samples too strongly and decomposition started to take place?
The ratio of water molecules would be too high because SO3 would be included in the amount of water lost.
How would the calculated ratio of water molecules present in Epsom salt be affected if the procedure was changed so the analysis was performed using 2.500g of hydrated salt instead of 1.500g? Assume you still heated the sample to a constant mass.
The ratio would remain unchanged because the amount of water lost through heating is proportional to the sample of hydrated salt.
If 100.0mL water of at 50.0C is added to 100.0 mL of water at 20.0C. What temperature would the mixture of water reach?
The temperature would equalize at 35 C. -T + 50 = T -20 T = 35
What happens in Lab 5: Titrations?
Wash and fill a buret with NaOH. Add KHP and DI water to an Erlenmeyer flask. Add phenolphthalein indicator to the flask. Add NaOH from the buret to the flask until it turns light pink. Determine the concentration of NaOH. Determine the standard deviation and percent relative standard deviation.
A chemistry professor has a cup of coffee containing 50.0mL of room temperature coffee at 25.0C. The professor also has a new pot of hot coffee at temperature of 98.0C. What volume of hot coffee will the professor need to add to his cold coffee to reach the ideal drinking temperature of 82.0C? Assume that no heat is lost to the coffee cup or to the environment. Also assume that coffee has the same density (1.0g/mL) and heat capacity (4.184 J/g*C) as water.
[(-50.0)*(4.184)*(82.0-25.0)]/[(4.184)*(82.0-98.0)] = 178 mL
Boron trichloride is prepared from the following reaction. 2B2O3 + 6Cl2 + 3C --> 4BCl3 + 3CO2. If 3.456g of B2O3 is mixed with 9.9216g of Cl2 and 2.459g of C, then... a) what is the theoretical yield of BCl3? b) How much of each of the excess reagents would remain once the reaction is complete?
a) 10.931 g b) 0.209 g of B2O3 9.0813 g of C
How would the following error affect the calculated percent yield of alum? a) The Aluminum foil did not completely react. b) Alum sample was left in the beaker and not transferred to the filter paper. c) Alum Sample was not dried completely in the oven.
a) There would be an decrease in percent yield of alum because of the lower mass of alum synthesized. b) There would be a decrease in percent yield of alum because of the lower mass of alum synthesized. c) There would be an increase in percent yield of alum because the mass of alum synthesized would include the alum and water.
A researcher attempting to determine the hydration number of hydrated salt first washed their crucible. They did not dry the crucible very well as they intended to heat it anyway. They massed this damp crucible and added the Epsom salt and massed a second time. a) would the researcher have the correct initial mass of the Epsom salt? b) Would the researcher have the correct value for molar ratio water to magnesium sulfate?
a) Yes, the mass of the epsom salt would be correct because you zero the scale before mesuring the mass of the epsom salt. b) No, the molar ratio would be too high because the mass of the water in the crucible would be added to the mass of the mixture. When the mixture is heated, more moles of water will be lost than MgSO4.
Formula to find Q of water
q = mcΔT
Formula for standard deviation
sq. root of (sum of (x-x average)^2/number of trials-1)
