Chemistry Important Questions & Answers
Question #1(Lord Rayleigh’s Argon Experiment) (Ryan Rosniak answered the question
and received the bonus [4 % added to the final mark])
At the end of the 19th century, Lord Rayleigh discovered that atmospheric air contains a new
heavier gas (argon) in addition to nitrogen and oxygen. His work was based on measurements of
the density (mass) of nitrogen from the air and alternatively nitrogen from the decomposition of
ammonium nitrate after removing gas contaminants (oxygen, water vapor, etc.) from the gas
stream. Statistical treatment of experimental data (t-test) from two measurements led to the
discovery of argon.
A careful analysis of Lord Rayleigh’s data (Part I, slide 90) shows one unexpected and
unexplained feature. The mass measurement of nitrogen gas from ammonium nitrite occurred
with much lower precision than the mass measurement of nitrogen gas from the air. Intriguingly, a ~ 10-fold difference in precision (sample standard deviation) was observed for
virtually the same two methods. The reason for this observation is not understood. Your task is to
get familiar with Lord Rayleigh’s Argon Experiment and answer Bonus
Question #1:
What is the main reason for the low precision (relatively larges) of the measurement of the
mass/density of nitrogen gas from the decomposition of ammonium nitrate in Lord Rayleigh’argon experiment?
Because it contained a heavier gas from atmospheric air, atmospheric nitrogen was denser, and it was identified as a new monatomic element (argon).
What is the source of such high variability in this experiment?
2
The disparity between atmospheric and chemical nitrogen sources was hypothesized and later proven to be attributable to argon, the third most prevalent element in dry air.
Question #2 (Calibration of Analytical Methods) (the bonus for the correct answer is 3
% added to the final mark in the course)
An important aspect of analytical chemistry, covered in CHEM 2400/24880, is the calibration of
instrumental methods for quantitative analysis. A calibration curve provides a critical correlation
between the analyte concentration and the analyte response (signal). We have learned how to
calibrate the technique of Absorption Spectroscopy for the quantitative detection of proteins by
producing and using a calibration curve.
Almost all analytical methods/techniques require calibration because the actual correlation
between the concentration and the response of analytes is not known and must be determined
experimentally. However, there are few analytical methods/techniques which don’t require
calibration because the signal response to analyte concentration is well known (a priori) and
independent of experimental conditions. Your task is to find one analytical method which
doesn’t require calibration in quantitative analysis. You should be able to explain why the
calibration of this method is not necessary.
What is the analytical method which doesn’t require calibration in quantitative analysis?
The gravimetric approach is one of the few quantitative analytical methods that does not require calibration.
Why is calibration not necessary for this method?
The analyst in this example utilizes a balance as the measuring device. Because the temperature of the solvent may be neglected, the gravimetric method is inherently more accurate than the volumetric method. Temperature affects the amount of solvent contained in a volumetric flask, but it does not affect the weight of the solvent.
Question #3 (Are Equilibrium Constants Constant? Experimental
Determination of Equilibrium Constants) (the bonus for the correct answer is 3 % added to
the final mark in the course)
An important aspect of analytical chemistry, covered in CHEM 2400/24880, is a quantitative
treatment of chemical equilibria to determine concentrations of critical components of various
chemical systems. This approach relies on accurate values of equilibrium constants. I will
surprise you a little by stating that there is a problem with the experimental determination of
equilibrium constants concerning their accuracy.
There are different experimental techniques used to determine equilibrium constants and such
different techniques can produce different K values. To determine K values properly, we must
use highly accurate methods for measuring concentrations of species at equilibrium. However,
even such methods may still provide different K values from measurements of the same
equilibrium system, by the same technique, but under slightly different experimental conditions.
Even if we try to determine the dissociation equilibrium constant of a weak acid (Ka) using the
best quantitative technique, we will obtain different Ka values for different amounts of acetic
acid dissolved in water. Your task is to find out why two virtually the same analytical methods
can provide significantly different values of equilibrium constants. You are expected to explain
why this could be the case.
What are two very accurate analytical methods which can provide significantly different
values (even one order of magnitude difference) of equilibrium constants for the same
equilibrium process?
The numerical value of the equilibrium constant K can be obtained using a variety of approaches. Measuring the concentrations of each reactant and product is the most direct way.
- If the initial concentrations of all species involved in the reaction are known, the value of the equilibrium constant can be determined by measuring the equilibrium concentration of only one of the species involved in the reaction. The final concentrations can be determined using stoichiometry calculations.
- There’s another way to figure out what K is worth.
Titration will be used to determine the concentration of one of the reaction products at equilibrium. This number is used to compute the equilibrium concentration of the remaining species after it is discovered. The equilibrium constant can be computed once these concentrations are known
What is the reason for such big differences in experimental values of investigated equilibrium constants?
Such a large disparity is due to an inaccuracy in estimating the beginning or final concentrations, which will result in errors in the constant computation.
The product concentration will be modest and the percentage error will be considered if the expected equilibrium constant is 1.
If the predicted equilibrium constant is >1, tiny inaccuracies in reactant concentration estimates will result in a substantial percentage error.
The large disparity in equilibrium constants could be attributed to temperature differences.
There are numerous ways for calculating the equilibrium constant. The electrochemical method is the most essential, while the spectroscopic method is the second. These are practical procedures that produce the most precise outcomes. Even so, there could be discrepancies in the results acquired using these procedures.
Because it is dependent on several minor things (the majority of which we are unconcerned about in most scenarios). Personal error, instrumental error, climatic change, humidity, other atmospheric conditions, reactions or anything occurring nearby, quality of reagents used in the reaction (their impacts are minor, therefore we ignore them). There are numerous examples of this. The value of the equilibrium constant may change as a result of various circumstances. It’s impossible to achieve 100 percent perfection. Each object has a level of uncertainty associated with it. In addition to these factors, the temperature is a significant factor that can influence the value of the equilibrium constant. The tiny temperature variations result in diverse results.
