Topic 11: Measurement, Data Processing, and Analysis

Question 1

What is a systematic vs. random error?

Answer 1

Systematic occurs as a result of the procedure. Random uncertainties and errors occur when there is an equal probability of the reading being too high or too low from one measurement to the next.

Question 2

What types of random errors are common?

Answer 2

Fluctuations in temperature, fluctuations in volume of glassware as temperature increases/decreases, decision on exactly when an indicator changes color during titrations

Question 3

What is the difference between absolute and percent uncertainty?

Answer 3

Absolute is the uncertainty associated with the instrument/measurment. Ex: uncertainty for a scale that measures 25.0 g of substance is +/- 0.1 g. Percent uncertainty is the absolute uncertainty compared to the measurement. So, 0.1/25.0 * 100 = 0.4%

Question 4

How do you determine overall uncertainty after adding/subtracting values?

Answer 4

If quantities are added/subtracted, absolute uncertainties are added together

Question 5

How do you determine overall uncertainty after multiplying/dividing values?

Answer 5

Use percent uncertainties and sum all of the percent uncertainties together. Once you calculate the final value of your measurement, convert the percent uncertainty back to an absolute uncertainty.

Question 6

How do you determine percent error?

Answer 6

(experimental-theoretical)/theoretical x 100

Question 7

What is the index of hydrogen deficiency?

Answer 7

The index of hydrogen deficiency (IHD) is a count of how many molecules of H2 need to be added to convert the molecule to the corresponding, saturated, non-cyclic molecule. In other words a degree of unsaturation

Question 8

If a hydrocarbon has x carbon atoms and y hydrogen atoms, how do you calculate IHD?

Answer 8

((2x+2-y))/2. Note that each unit represents one H2, not one H atom.

Question 9

An IHD of 1 means that the molecule has…

Answer 9

one double bond

Question 10

An IHD of 2 means that the molecule has…

Answer 10

one triple bond

Question 11

If other atoms besides H and C are present, how do you calculate IDH?

Answer 11

Sulfur and oxygen do not affect IHD
Halogens (F, Cl, Br and I) are treated like H atoms
For each nitrogen atom, add one to the number of carbon and hydrogen atoms

Question 12

An IHD of 0 means…

Answer 12

The hydrocarbon is fully saturated, all single bonds

Question 13

Break down the equation c=wavelength x frequency

Answer 13

c is the constant speed of light, 3.0 x 10^8 m/s. Wavelength in m, frequency in 1/s

Question 14

How does IR spectroscopy work?

Answer 14

When molecules absorb energy in the IR region of the electromagnetic spectrum, it causes the bonds between the atoms to vibrate
The intensity depends on the dipole moment of the bond:
Strongly polar bonds produce strong bands
Bonds with medium polarity produce medium bands

Question 15

A bond will only interact with IR radiation if…

Answer 15

it is polar covalent

Question 16

How can you use IR spectroscopy to identify an unknown element?

Answer 16

The fingerprint region can be used to identity an unknown compound by comparing with the IR spectra of known compounds. IR spectroscopy is generally not used to determine the entire molecular structure

Question 17

Mass spectrometry can be used for two purposes:

Answer 17

mass spectrometry is used to determine the relative atomic mass (Ar) of an element it can also be used to determine the structure of a compound

Question 18

How does mass spectrometry work?

Answer 18

Inside the mass spectrometer, some of the molecular ions break down to produce fragments
A fragmentation pattern is produced which gives useful information about the structure of the compound

Question 19

How do you determine the structure of a compound using mass spectrometry values?

Answer 19

• When determining the structure of a compound don’t use the mass/charge (m/z) value
• Instead take the largest value and subtract every other value from it

Question 20

What is 1H NMR?

Answer 20

• NMR or nuclear magnetic resonance spectroscopy is a technique used to determine a compound’s unique structure
• The position of the NMR signal is measured relative to the signal produced by TMS tetramethylsilane
• The chemical shift is measured relative to the point 0

Question 21

1 dm^3 is also equal to…

Answer 21

1 L

Question 22

1000 cm^3 is also equal to…

Answer 22

1 L