3 & 4: Mole Concept and Spectroscopy

Question 1

Number of molecules in a mole

Answer 1

6.022 * 10^(23)

Question 2

Is the mass same or different for two exact mole amounts (e.g. 1 mole of apples and 1 mole of watermelons)

Answer 2

Same number of each of these fruits, but different masses! Watermelons are heavier!

Question 3

Two interpretations of an atomic mass

Answer 3

• Average mass in amu of an atom
• Average mass in grams of a mole.

Question 4

Percentage of Carbon atoms with a mass of 12.011 (found on the periodic table)?

Answer 4

0%. Remember the 12.011 was an average based on natural abundance?

Question 5

Isotope

Answer 5

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.

Question 6

Why do we take the average mass based on natural abundance?

Answer 6

There are multiple isotopes of one element (containing different amounts of neutrons) that need to be taken into consideration.

Question 7

Empirical Formula

Answer 7

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.

Question 8

Molecular Formula

Answer 8

The actual formula, related closely to the structural formula of the actual compound.

Question 9

What you need to calculate empirical formula:

Answer 9

1. The percentage of mass of the elements in the compound.
2. Assume you have 100g of the compound (easy to work with when it comes to ratios)
3. Use the percentages to determine the grams of each element you have in the compound and calculate the number of moles of each element.
4. This forms a ratio that is the empirical formula.

Question 10

What you need to calculate molecular formula:

Answer 10

1. Calculate empirical formula.
2. Calculate the mass for one mole of the compound (empirical molar mass).
3. Given the molar mass, divide the actual molar mass by the empirical mass, which gives you the multiplier.
4. Multiplier is applied to the empirical formula ratio to find the molecular formula.

Question 11

Four processes of a mass spectrometer

Answer 11

Ionization, Acceleration, Deflection, and Detection

Question 12

Ionization

Answer 12

1. Electrons are knocked off sample particles to form (mostly) +1 ions.

Question 13

Acceleration

Answer 13

2. Ions move through a series of charges plates to form a narrow beam of high speed particles with equal kinetic energy.

Question 14

Deflection

Answer 14

Ions are attracted t the negative side of an electromagnetic field causing separation of the mixture based on mass and charge.

Question 15

Detection

Answer 15

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.

Question 16

What was John Dalton incorrect about in his First Atomic Theory?

Answer 16

“Atoms of the same element are identical.” However, we later find that there can be several different isotopes that differ in mass.

Question 17

Mass Spectroscopy

Answer 17

Principle technique used to study isotopes. Used to both “count” and “weigh” atoms in a sample, not just in the traditional sense.

Question 18

When a sample is injected into the mass spectrometer, do the atoms or molecules turn into positive or negative ions?

Answer 18

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)

Question 19

What causes sample mixture to become separated in mass spectroscopy?

Answer 19

Deflection phase - Ions will attract to the negative side of the electromagnetic field, sorting them by change and mass.

Question 20

What is key in mass spectroscopy?

Answer 20

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.

Question 21

Rank ions in their degree of deflection by the electromagnet from least to greatest.

[19]F^(1+) , [16]O^(1+) , [17]O^(1+)

Answer 21

Least to Greatest:

[19]F^(1+) , [17]O^(1+), [16]O^(1+)

Lighter molecules deflect better

Question 22

What pressure is needed for the mass spectrometer chamber under vacuum to work correctly?

Answer 22

Low pressure to make sure that there is no other elements like oxygen and nitrogen that can cause particles to collide and neutralize.

Question 23

Mass Spectrum

Answer 23

Models the different isotopes of the element (mass/charge) on the x-axis versus the percent abundance on the y-axis.

Question 24

Relative intensity Mass Spectra

Answer 24

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.

Question 25

How to find percent abundance using a relative mass spectra

Answer 25

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)

Question 26

Fragmentation

Answer 26

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.