3 & 4: Mole Concept and Spectroscopy Flashcards
Number of molecules in a mole
6.022 * 10^(23)
Is the mass same or different for two exact mole amounts (e.g. 1 mole of apples and 1 mole of watermelons)
Same number of each of these fruits, but different masses! Watermelons are heavier!
Two interpretations of an atomic mass
- Average mass in amu of an atom
- Average mass in grams of a mole.
Percentage of Carbon atoms with a mass of 12.011 (found on the periodic table)?
0%. Remember the 12.011 was an average based on natural abundance?
Isotope
A form of an element (same number of protons) with a unique/specific number of neutrons. This causes the mass values of each isotope to be different.
Why do we take the average mass based on natural abundance?
There are multiple isotopes of one element (containing different amounts of neutrons) that need to be taken into consideration.
Empirical Formula
Ratio of number of molecules of each element that results in a “simplified formula” for a compound.
e.g. Glucose is C6H12O6, the empirical (simplified) formula would be CH2O.
Molecular Formula
The actual formula, related closely to the structural formula of the actual compound.
What you need to calculate empirical formula:
- The percentage of mass of the elements in the compound.
- Assume you have 100g of the compound (easy to work with when it comes to ratios)
- Use the percentages to determine the grams of each element you have in the compound and calculate the number of moles of each element.
- This forms a ratio that is the empirical formula.
What you need to calculate molecular formula:
- Calculate empirical formula.
- Calculate the mass for one mole of the compound (empirical molar mass).
- Given the molar mass, divide the actual molar mass by the empirical mass, which gives you the multiplier.
- Multiplier is applied to the empirical formula ratio to find the molecular formula.
Four processes of a mass spectrometer
Ionization, Acceleration, Deflection, and Detection
Ionization
- Electrons are knocked off sample particles to form (mostly) +1 ions.
Acceleration
- Ions move through a series of charges plates to form a narrow beam of high speed particles with equal kinetic energy.
Deflection
Ions are attracted t the negative side of an electromagnetic field causing separation of the mixture based on mass and charge.
Detection
Ions collide with a metal plate. Electrons are transferred from the metal to the ion, producing a current and thus a signal to a computer.
What was John Dalton incorrect about in his First Atomic Theory?
“Atoms of the same element are identical.” However, we later find that there can be several different isotopes that differ in mass.
Mass Spectroscopy
Principle technique used to study isotopes. Used to both “count” and “weigh” atoms in a sample, not just in the traditional sense.
When a sample is injected into the mass spectrometer, do the atoms or molecules turn into positive or negative ions?
Positive since electrons are knocked off in the ionization phase.
Positive ions are also attracted to the negative side of the electromagnetic field (opposites attract)
What causes sample mixture to become separated in mass spectroscopy?
Deflection phase - Ions will attract to the negative side of the electromagnetic field, sorting them by change and mass.
What is key in mass spectroscopy?
Particles go into the deflection chamber with the same kinetic energy but have different mass/charge ratio. They are all +1 ions though.
Thus, the amount of deflection they experience by the electromagnet is different and the strength of it can be varied so only particles with a particular mass/charge ratio makes it to the detector.
Rank ions in their degree of deflection by the electromagnet from least to greatest.
[19]F^(1+) , [16]O^(1+) , [17]O^(1+)
Least to Greatest:
[19]F^(1+) , [17]O^(1+), [16]O^(1+)
Lighter molecules deflect better
What pressure is needed for the mass spectrometer chamber under vacuum to work correctly?
Low pressure to make sure that there is no other elements like oxygen and nitrogen that can cause particles to collide and neutralize.
Mass Spectrum
Models the different isotopes of the element (mass/charge) on the x-axis versus the percent abundance on the y-axis.
Relative intensity Mass Spectra
Ions from the sample sorted into mass/charge ratio, and the ion that hits the detector most is assigned a relative intensity of 100. The other ions are given proportional relative intensities based on their abundance in the sample.
How to find percent abundance using a relative mass spectra
Add all the values together (100 + n + m) and divide the isotope value (e.g. 100 for the largest abundance ion) by the total.
e.g. Largest abundance Ion:
100 / (100 + n + m)
Fragmentation
Process used in mass spectroscopy where electrons are knocked off the atoms or molecules by high speed particles colliding with them, breaking apart a molecule.