Thursday, October 05, 2006
Test Day
E- Regents: We're starting to move up although there is room for improvement; many of you still do not know how to take a test, which translates to lots of careless errors and inconsistencies. Come to extra help and we'll talk more about how to take a test to maximize your score.
Class average: 83 (7 more points, about two more correct answers per test, and I'll/we'll be happy)
D, G: I'll be grading your first Atomic Concepts exam over the next week. I was glad to hear that some found the test easy or easi-er than the last exam. I did note one disturbing and truly inexcusable mistake on some tests: recall the highly inappropriately named term, "MASS NUMBER". I did not invent that term and I wish that the name had never made its way into chemistry texts. However, I ranted on and on about how that term REALLY means "the number of PROTONS + NEUTRONS in the nucleus" of an isotope of an element and that the terms has NOTHING, per se ("by itself"), to do with the mass of an atom! Therefore, the "mass number" CANNOT POSSIBLY BE A DECIMAL (non-integer)! THERE ARE ONLY WHOLE NUMBERS OF PROTONS AND NEUTRONS in a nucleus or anywhere! YOU WILL NEVER, EVER SEE A "mass number" THAT IS NOT A POSITIVE INTEGER (even if you do crazy nuclear physics)! I went on and on about how the MASS NUMBER is NOT the MASS of the atom!
When I go out of my way to help you avoid the mistakes of legions of past students, I expect you to NOT make those mistakes. I could not have highlighted the mass number misnomer any further. If you did practice problems, you definitely would have repeatedly written and seen the difference between a mass number and an atomic mass.
Furthermore, we did problems for a WHOLE WEEK in which we calculated the (weighted average) ATOMIC MASS of an element. That number is THE MASS of an AVERAGE atom of a given element. ATOMIC MASS = MASS OF (average) ATOM!! These numbers ALWAYS HAVE DECIMALS (with ONE exception: the atomic mass of the Carbon-12 isotope is, BY DEFINITION, exactly 12 amu).
Here is an example of what I am talking about:
the MASS NUMBER ( =total of protons + neutrons) of a chlorine-37 atom/isotope is 37. The MASS of a SINGLE specific atom of chlorine-37 is 36.98 a.m.u. or grams per mole (measured to the hundredths place). YET, the ATOMIC MASS of chlorine, in general, is 35.453 a.m.u. (or grams per mole) because there are two isotopes of Cl, Cl-35 and Cl-37 and the WEIGHTED AVERAGE of those two isotopes is 35.453 a.m.u. (or grams per mole). The ATOMIC MASS of any element is the weighted average of the masses of the isotopes of that element. The ATOMIC MASS never turns out, even coincidentally, to be a whole number / integer (unless you are intentionally rounding your answer to the one's place).
If you made the above error, make SURE that you learn from that error and don't repeat it on future tests because you will have to know the distinction between mass number and atomic mass for the rest of this course.
AP: we almost wrapped up the gas unit as we applied the equation for the average (root-mean-squared) speed of a given type of gas molecule at a particular temperature. Remember that the formula given in the reference table is "WRONG": you have to multiply the numerator by 1000 grams per kilogram) in order to use "normal" molar mass units (grams per mole).
Also, of course, only Kelvin/Absolute temperatures "work". We then applied the two versions, relative rate and then relative time, of Graham's Law of Effusion. Be careful to always write the GENERAL formula first and then, from the given data, plug in the respective values for each gas. Graham's Law is a simple equation but, MARK MY WORDS, MOST students cannot apply it properly and consistently. They forget the square root or they write the gases in reverse order or they forget to "square" the square root when the variable is in the square root or they square only one side of the equation. Please QUALITATIVELY estimate your answers to these questions BEFORE you do ANY formal calculation. If you KNOW that a heavier gas effuses more slowly, then its rate is slower (lesser) BUT ITS EFFUSION TIME IS longer (greater!). Make sure that your answer is consistent with your guess. If you are solving for molar mass, remember, the lightest gas in the entire universe is MOLECULAR HYDROGEN! So, if you get a molar mass of less than 2.0158 grams per mole, you are wrong.
Class average: 83 (7 more points, about two more correct answers per test, and I'll/we'll be happy)
D, G: I'll be grading your first Atomic Concepts exam over the next week. I was glad to hear that some found the test easy or easi-er than the last exam. I did note one disturbing and truly inexcusable mistake on some tests: recall the highly inappropriately named term, "MASS NUMBER". I did not invent that term and I wish that the name had never made its way into chemistry texts. However, I ranted on and on about how that term REALLY means "the number of PROTONS + NEUTRONS in the nucleus" of an isotope of an element and that the terms has NOTHING, per se ("by itself"), to do with the mass of an atom! Therefore, the "mass number" CANNOT POSSIBLY BE A DECIMAL (non-integer)! THERE ARE ONLY WHOLE NUMBERS OF PROTONS AND NEUTRONS in a nucleus or anywhere! YOU WILL NEVER, EVER SEE A "mass number" THAT IS NOT A POSITIVE INTEGER (even if you do crazy nuclear physics)! I went on and on about how the MASS NUMBER is NOT the MASS of the atom!
When I go out of my way to help you avoid the mistakes of legions of past students, I expect you to NOT make those mistakes. I could not have highlighted the mass number misnomer any further. If you did practice problems, you definitely would have repeatedly written and seen the difference between a mass number and an atomic mass.
Furthermore, we did problems for a WHOLE WEEK in which we calculated the (weighted average) ATOMIC MASS of an element. That number is THE MASS of an AVERAGE atom of a given element. ATOMIC MASS = MASS OF (average) ATOM!! These numbers ALWAYS HAVE DECIMALS (with ONE exception: the atomic mass of the Carbon-12 isotope is, BY DEFINITION, exactly 12 amu).
Here is an example of what I am talking about:
the MASS NUMBER ( =total of protons + neutrons) of a chlorine-37 atom/isotope is 37. The MASS of a SINGLE specific atom of chlorine-37 is 36.98 a.m.u. or grams per mole (measured to the hundredths place). YET, the ATOMIC MASS of chlorine, in general, is 35.453 a.m.u. (or grams per mole) because there are two isotopes of Cl, Cl-35 and Cl-37 and the WEIGHTED AVERAGE of those two isotopes is 35.453 a.m.u. (or grams per mole). The ATOMIC MASS of any element is the weighted average of the masses of the isotopes of that element. The ATOMIC MASS never turns out, even coincidentally, to be a whole number / integer (unless you are intentionally rounding your answer to the one's place).
If you made the above error, make SURE that you learn from that error and don't repeat it on future tests because you will have to know the distinction between mass number and atomic mass for the rest of this course.
AP: we almost wrapped up the gas unit as we applied the equation for the average (root-mean-squared) speed of a given type of gas molecule at a particular temperature. Remember that the formula given in the reference table is "WRONG": you have to multiply the numerator by 1000 grams per kilogram) in order to use "normal" molar mass units (grams per mole).
Also, of course, only Kelvin/Absolute temperatures "work". We then applied the two versions, relative rate and then relative time, of Graham's Law of Effusion. Be careful to always write the GENERAL formula first and then, from the given data, plug in the respective values for each gas. Graham's Law is a simple equation but, MARK MY WORDS, MOST students cannot apply it properly and consistently. They forget the square root or they write the gases in reverse order or they forget to "square" the square root when the variable is in the square root or they square only one side of the equation. Please QUALITATIVELY estimate your answers to these questions BEFORE you do ANY formal calculation. If you KNOW that a heavier gas effuses more slowly, then its rate is slower (lesser) BUT ITS EFFUSION TIME IS longer (greater!). Make sure that your answer is consistent with your guess. If you are solving for molar mass, remember, the lightest gas in the entire universe is MOLECULAR HYDROGEN! So, if you get a molar mass of less than 2.0158 grams per mole, you are wrong.
# posted by C @ 1:30 PM 
