Lab Practical Study Guide Chem 200 (Discussion Questions)

What important assumptions did you make in this experiment to determine the composition of the Al-Zn alloy? Lists all assumptions cleats and in a sentence explain for each why it is important.

- That a 100% theoretical yield of H2 gas was obtained during the reaction of the alloy, if all of the alloy was reacted. You cannot claim that the entire alloy has reacted - The reasonableness of the assumption in a above depend upon the percent yields obtained by the students in this experiment. It is very difficult , even unde the most controlled environments, to achieve 100% yield and yields over 100% can be interpreted as water o other (additional/ unwanted) reagent in the experiment (such as initial DRY reagents that were weighed in wet glassware) that bubbled off during the experiment and created a yield over 100%. - TEMPERATURE: That the ambient (environmental) temperature was reported correctly and that ambient temperatures can change vein during the experiment, affecting the results of the experiment - VAPOR PRESSURE OF WATER PH2O:: That the vapor pressure of the water from Table 1 on Pg 58 was reported correctly and that this measurement was dependent upon a stable ambient (environmental) Temperature

Consider the effect of exposing the NaOH solution to the air. A. Would the calculated molar mass be too high or too low if your standardized NaOH were exposed to the air for a long period of time after the standardization? Explain how you reached that conclusion B. Would the calculated Molar mass be too high o too low if your NaOH solution were exposed to the air for a long period of time before the standardization? Explain how you reached that conclusion.

A. The molarity of the NaOH solution wold be a little less, because the solution would be partially neutralized by the carbon dioxide, so the volume required to neutralize the acid would be higher than it should be. Therefore, the measured number of moles of the acid would be too high and the molar mass would be too low. B. It doesn't matter because you still need to standize the base. (We do not have any measured molarity yet, and therefore we do not have a standardized base. It does not matter if the base is exposed to air until after we have standardized the base and have an exact molarity measurement)

An initial product of the reaction of HCL(aq) with sodium hydrogen carbonate is carbonic acid, H2CO3, which rapidly decomposes to give carbon dioxide gas and water. Write a balanced equation for the decomposition of carbonic acid.

H2CO3(g) = CO2(g) + H2O(l)

What if you had dissolved the acid in 50 mL of water instead of 25 mL? Would the accuracy of the molar mass be significantly affected by that change in procedure? Explain how you reached that conclusion.

No. You are still using the same number of moles of acid. The solutions could potentially be more dilute making it more difficult to see the endpoint but nothing in the procedure should change Only the NaOH base volume changes might affect accuracy of the molar mass of the acid. The titration procedure and math will change, but NOT the accuracy.

The enthalpy of formation of Cu2+(aq) is -219 kJ/mol. If you were to subnsititure the same amount of copper for zinc in this experiment, would you expect the temperature change to be higher, equal to, or lower than what you observed for zinc? Explain your answer.

Since the enthalpy of formation of Zn2+ (aq) -152 kJ/mol, the reaction with copper would yield more heat per more. Since the molar masses of zinc and copper are nearly equal, the reaction wth copper would yield more temperature change on a per gram basis

Find a spectra of different ions you observed in Part C and explain why one ion was seen while the other ion was not visible during the flame test. Is the flame color a characteristic property from your observations?

The heat of the flame excites the electrons of the metal ions, causing them to emit visible light. Whole the anion they have to lose their extra electron to become neutral atoms again. This means having to find extra energy rather than losing it, so no light would be emitted. The flame color is a characteristic property of metal ions (cations) which was seen in this experiment

Which Spectrum (H or He) is more complicated? Why?

The helium shoul be plainly more complicated. Reasonable answers for why: He is a two electron atom, it does not satisfy the Bohr model and/or Rydberg equation, the high ionization energy means many more quantum states appearing potentially in the spectrum

Suppose some of the solid is not washed out of the beaker and into the filter. How will the mass of solid collected be affected? Explain how this affects the mass percentage for the limiting reagent.

The mass of the solid collected will be deficient. The mass percentage for the limiting reactant would decrease die top the lose of the solid not being accounted for.

Suppose you touched your filter paper with wet fingers immediately before weighing it. What happens to its mass measurement? After you collected your precipitate on the filter paper, you dried the solid band filter paper thoroughly before obtaining the final weight. Explain how this mistake affects the calculation for the solid's mass.

The measurement of the mass will be affected if you touch the filter paper with wet fingers before weighing by eventually giving the inaccurate true amount of solid that was produced and dried. The wet grieve paper will have an increase mass measurement, bu when you burn off the aqueous solution the extra weight will burn off leaving a decrease in the true amount of the solid.

Would the calculated molar mass be too high or too low if the Buret had been rinsed with deionized water and then filled with NaOH solution without having been rinsed with the NaOH solution first? Explain how ou reached that conclusion

The molarity of the NaOH solution would be a little less because of the extra water in the Buret, so the volume required to neuratlize the acid would be higher than it should be. Therefore, the measured number of moles would be too high

How might using tap water in place of de-ionized water affect the outcome of your qualitative analysis?

The possibility of false positive tests due to the presence of impurities in the tap water.

From the percent yield of Al and Zn that you obtained from Parts A and B Discuss how these percent yields would affect the results for your Al-Zn alloy in this experiment.

Use the assumptions. Temperature and therefore Vaport Pressure may both be different and therefore affect the outcome.

What changes in your experimental data would result from using 200.0 mL of 3.0 M HCL in the calorimeter? Would this change affect the result for enthalpy of formation of Zn2+ (aq)?

Using a larger volume of 200.0 mL of 6.0 M HCl in the calorimeter would increase the heat capacity of the calorimeter, resulting in a lower final temperature and a smaller Tcalorimeter for the experiment. (The Tcalorimeter would be 1/2 large). However, the results will be the same for the enthalpy calculation.

What changes in your experiment (Expt. 6 Calorimetry) would result from using 200.0 mL of water in the calorimeter for the aluminum experiment?

Using a larger volume of 200.0 mL of water n the calorimeter would increase the heat capacity of the calorimeter reuniting in a higher final temperature and a smaller Tcalorimerer for the experiment. (Think: a drop of cold water or small piece of cool aluminum in the ocean makes a lot less change in temperature than a drop of cold water or a small piece of cold aluminum in a very tiny container)

Can scientists determine specific elements based off a spectrum. Why or why not?

Yes, because every element has its own unique spectrum.

Can you use the values from the ocean optics spectrometer to assign precise wavelengths to the transitions that you observed with the STAR spectrometer?

Yes. Ocean optics is a far more precise instrument that gives clear wavelengths, while STAR Spectrometer is a little harder to read and some students might have a hard time seeing all the wavelengths.