Chemistry II Lab Final
What observations might you make that suggest that a chemical reaction has occurred?
A color change or formation of a solid or a gas is indicative of a chemical reaction. Also, evolution or absorption of heat.
What is a precipitate?
A precipitate is a insoluble substance that separates from a homogeneous chemical reaction as a solid.
What are the five fundamental components of a spectrophotometer?
A spectrophotometer must possess a light source, a monochromator, a sample cell, a detector, and a meter.
Show by mathematical how one can determine the molar mass of a volatile liquid by measurement of the pressure, volume, temperature, and mass of the liquid.
Combining the ideal-gas law and the definition of moles, we have PV= nRT and n= m/M n= PV/RT= m/M or M=mRT/(PV)
Why do you equalize the water levels in the bottle and the beaker?
Equalizing the water levels equalizes the pressures and ensures that the total pressure in the bottle is atmospheric and does not contain a contribution form the pressure due to the height of the water column.
Why is hydroxylamine hydrochloride used in this experiment?
Hydroxylamine is used to reduce Fe3+ to Fe2+
Why don't you open your folded filter paper so that two pieces touch each side of the funnel?
If the filter paper were opened in this way, the solution and precipitate would simply pass through, and no filtration would result.
Can you eliminate indeterminate errors from your experiment?
Indeterminate errors are just that, indeterminant. They cannot be as ascertained or eliminated, but rather occur in a random fashion and so are accounted for with statistics.
Define metathesis reactions. Give an example.
Metathesis reactions are atom (or group) transfer reactions. For example, AgNO3 + NaCl --> AgCl + NaNO3 is a metathesis reaction in which chloride transfer occurs.
Calculate the freezing point of a solution containing 1.25 g of benzene in 100 g of chloroform.
Mol of benzene is 1.25 g/(78 g/mol) = 0.016 mol. Consequently, a solution of 1.25 g of benzene in 100 g of CHCl3 is 0.016 mol/ 0.100 kg or 0.16 molal. The freezing point lowering is thus ^T = Kfpm, or point is -63.5C - 0.75C = -64.3C
Complete and balance the following equation and then write the balanced net ionic equation NH3+HCl=
NH3+H+ = NH4+ + H+ = N4Cl+
How could you distinguish between NH4Cl and AgCl?
NH4Cl is soluble in water and AgCl is not.
Complete and balance the following equation and then write the balanced net ionic equation Na2CO3 + HNO3 =
Na2CO3 + 2HNO3 = 2NaNO3 + CO2 + H2O; (CO3)2 + 2H+ = CO2 + H2O
What is the purpose of preparing a calibration curve?
Not all substances precisely obey the Beer-Lambert law over all concentrations ranges. A calibration curve will provide the relationship between concentrations and absorbance under conditions that are similar to or identical with those used for the analysis.
Write the ideal-gas equation and give the units for each term when R = 0.0821 L-atm/K-mol.
PV = nRT; when the units of R are L-atm/ mol-K, P is in atm, V in L and T in K.
What is supercooling? How can it be minimized?
Supercooling involves the lowering of the temperature of a substance below its normal freezing point without the solidification of the substance. Supercooling can be minimized by cooling slowly with rapid stirring.
State the Beer-Lambert law, and define all terms in it.
The Beer-Lambert law is A= abc, where A is the absorbance, a is the absorptivity or extinction coefficient, b is the solution path length, and c is the molar concentration of absorbing species.
Titration of a oxalate sample gave the following percentages: 15.53%, 15.55%, and 15.56%. Calculate the average and standard deviation.
The average is 15.55 and the deviation is 0.02.
For a given substance, the amount of light absorbed depends on what four factors?
The concentration, the cell path length, wavelength, and solvent.
Suggest reasons why real gases might deviate form the ideal-gas law on the molecular level.
The ideal-gas law assumes that there are no forces of attraction between the individual gaseous molecules. Whenever this isn't so, real molecules will not obey the ideal-gas law. This would be expected to occur at very high pressures and at very low temperatures where molecules are so close to one another that they necessarily interact. The ideal-gas law also assumes that the gas particles have no volume. At high pressures their volume may become appreciable relative to the volume of the container.
Why is the KMnO4 solution filtered, and why should it not be stored in a rubber-stoppered bottle?
The permanganate is filtered in order to remove insoluble MnO2. Because rubber reduces MnO4- to MnO2, the solution should not contact rubber or other organic material.
How does the pressure of an ideal gas at constant volume change as the temperature increases?
The pressure of an ideal gas increases as the temperature increases at constant volume.
Consider the following equilibrium: BaSO4 = Ba2+ + SO4 2-; ^H>0 In which direction will the equilibrium shift if BaCl2 is added? why?
The reaction will shift to the left in order to decrease the [Cl-]
Consider the following equilibrium: BaSO4 = Ba2+ + SO4 2-; ^H>0 In which direction will the equilibrium shift if NaCl is added? why?
The reaction will shift to the left in order to decrease the [Cl-]
Consider the following equilibrium: BaSO4 = Ba2+ + SO4 2-; ^H>0 In which direction will the equilibrium shift if H2SO4 is added? Why?
The reaction will shift to the left in order to decrease the [SO4 2-]
Consider the following equilibrium: BaSO4 = Ba2+ + SO4 2-; ^H>0 In which direction will the equilibrium shift if heat is added? why?
The reaction will shift to the right in order to absorb the added heat.
Why does the solution decolorize on standing after the equivalence point has been reached?
The solution slowly decolorizes after the equivalence point has been reached because permanganate undergoes photochemical reduction and because it is slowly reduced by water.
How does the volume of an ideal at constant temperature change as the pressure increases?
The volume of an ideal gas decreases as the pressure increases at constant temperature.
How does the volume of an ideal gas at constant temperature and pressure change as the number of molecules increases?
The volume of an ideal gas increases as the number of molecules increases at constant temperature and pressure.
List three colligative properties and suggest a rationale for the choice of the word colligative to describe these properties.
Three colligative properties are boiling point, freezing point, and vapor pressure. They are called colligative properties because they are related to the number and energy of collisions between particles and not to what the particles are.
If 0.4586 g of sodium oxalate, Na2C2O4, requires 33.79 mL of a KMnO4 solution to reach the end point, what is the molarity of the KMnO4 solution?
0.04052 M
Calculate the value of R in L-atm / mol-K by assuming that an ideal gas occupies 22.4 L/mol at STP.
0.082 L-atm/mol-K
A solution containing 0.200 g of an unknown substance in 2.50 g of cyclohexane was found to freeze at 5.1C. What is the molar mass of the unknown substance?
1.1 x 10^2 g/mol
Balance these equations: KBrO3 --> KBr + O2 MnBr2 + AgNO3 --> Mn(NO3)2 + AgBr
2KBrO3 --> 2KBr + 3O2; MnBr2 + 2AgNO3 --> Mn(NO3)2 + 2AgBr
What volume of 0.100 M KMnO4 would be required to titrate 0.33 g of K2[Cu(C2O4)2] * 2 H2O?
7.48 mL
If 0.80 of a gas occupies 295mL at 25C and 680 mm Hg of pressure, what is the molar mass of the gas?
74 g/mol
Distinguish between volatile and nonvolatile substances.
A volatile substance has a high vapor pressure, and a nonvolatile substance has a low vapor pressure at room temperature. Obviously, volatility is a relative term and depends upon temperature and pressure. Volatility increases with increasing temperature and decreases with increasing pressure.
How are percent transmittance and absorbance related algebraically?
Absorbance A = log I0/I and percent transmittance %T = (I/I0)100, so %T/100= I/I0 or 100/% =I0/I and log(100/%T) = log (I0/I) = A. Or we may write A = 2- log %T.
Complete and balance the following equation and then write the balanced net ionic equation AgNO3+HCl=
AgCl+HNO3; Ag+ + Cl- = AgCl
What is the value of error analysis?
An error analysis allows you to judge the reliability of your data and gives an indication of the potential sources of error.
If the percent transmittance for a sample is 50 at 350 nm, what is the value of A?
Because %T = (I/I0)(100), log(I/I0) + log 100, or log(I0/I) = 2 - log %T . 2-log %T = 2 - log50 = 2 - 1.7 = 0.3.
Before a chemical equation can be written, what must you know?
Before a chemical equation can be written, you must know the reactants and products of the reaction and their formulas.
Under what conditions of temperature and pressure would you expect gases to obey the ideal-gas equation?
Gases obey the ideal-gas law at relatively high temperatures and low pressures.
What is the fundamental difference between gravimetric and volumetric analysis?
Gravimetric analyses involve a weighing as the determing measurement, whereas volumetric analyses involve a volume measurement as the determining measurement.
At present, automobile batteries are sealed. When lead storage batteries discharge, they produce hydrogen. Suppose the void volume in the battery is 100 mL at 1 atm of pressure and 25C. What would be the pressure increase if 0.05 g H2 were produced by the discharge of the battery? Does this present a problem? Do you know why sealed lead storage batteries were not used in the past?
PV= nRT = 6 atm assuming no gas in the void volume. Clearly this does present a problem, as H2 gas is extremely explosive. Because lead-storage batteries produce H2, sealing them would be dangerous.
A sample of nitrogen occupies a volume of 500 mL at 60C and 500 mm Hg of pressure. What will be its volume at STP?
STP is 273K and 1 atm, so that = 270 mL
Why should silver chloride be protected from light? Will your result be high or low if you don't protect your silver chloride from light?
Silver chloride is photosensitive and reacts with light to produce silver metal and chloride gas, which will lead to a low result if the sliver chloride is not protected from light.
Why does the vapor pressure of water contribute to the total pressure in the bottle?
Since gaseous and liquid water are in dynamic equilibrium, there will always be some water vapor above a sample liquid water. Since the vapor pressure of water is reasonably high at ambient temperature, it makes a significant contribution to the total pressure.
If your silver chloride undergoes extensive photocomposition before you determine its mass will your results be high or low?
Since photodecompostion liberates Cl2 from AgCl according to the equation 2AgCl --> 2Ag + Cl2, the Ag will weigh less than the AgCl and your results will be low.
Distinguish between solute and solvent.
Solute is the lesser component and solvent the greater component in a solution.
Does standard deviation give a measure of accuracy or precision?
Standard deviation measures precision.
What does stoichiometry mean?
Stoichiometry is the ratio of atoms in a compound or compounds in a chemical reaction and refers to the amounts of substances involved in reactions.
