Moles, RAM & RMM

Question 1

What is a mole?

Answer 1

The mole is the amount of substance that contains as many elementary particles (atoms, ions or molecules) as there are carbon atoms in 12g of carbon-12 isotope

or
The mole is the amount of substance that contains 6.02 x 1023 elementary particles which may be atoms, ions or molecules.

Question 2

Define Relative atomic mass, Ar

Answer 2

This is the weighted average mass of atoms of an element divided by one twelfth of the mass of one atom of carbon-12 isotope.

Question 3

What is the overview of determination of relative atomic mass using a mass spectrometer?

Answer 3

• The isotopes of the element are separated by converting them into positive ions (cations) by bombarding them with high energy electrons which knock off electrons from the isotopes.
• The resulting ions are separated according to their mass-to-charge ratio and then resulting mass spectrum recorded.
• The number of peaks on spectrum gives the number of isotopes and the height of the peaks gives their relative abundance.

Question 4

Describe the mode of operation of a mass spectrometer in determining RAM of an element

Answer 4

VIADDAC

Vaporization stage
- The sample is injected into the instrument where it is heated and vaporized, producing gaseous atoms or molecules.

Ionization stage
- The ionization chamber is first evacuated of air molecules.
- The vapourised sample of the element is then fed into the ionization chamber where its atoms are bombarded with high energy electrons from the hot filament which knock electrons off the atoms forming positively charged ions.

Acceleration stage
- The positive ions from the ionization chamber are accelerated in an electric field by strong negatively charged plates into the magnetic field.

Deflection stage
- The positive ions emerging from the electric field are deflected by a magnetic field perpendicular to their path.
- The angle of deflection depends on the mass-to-charge ratio (the m/z ratio) with the cations with the smallest mass, m, and the highest charge, z, being deflected the most.
- The angle of deflection is inversely proportional to mass of the cation but directly proportional to the charge on the cation.

Detection stage
- All positive ions with the same charge to mass ratio are focused in turns through the slit and into the collector by varying the accelerating voltage while keeping the magnetic field constant.

Amplification and Recording
- At the amplifier the charge received by the collector is turned into sizable electric currents that are recorded as peaks on a trace with each peak corresponding to a cation of a particular mass-charge ratio.
- The height of each peak is proportional to the relative abundance of the corresponding isotope while the mass-charge ratio gives a direct measure of its relative isotopic mass since the charge on the cations is +1.

Calculation of the relative atomic mass
- The relative atomic mass of the element is calculated as the average mass of the isotopes according to their abundances using the formula:
𝑅. 𝐴. 𝑀 = ∑(% 𝑎𝑏𝑢𝑛𝑑𝑎𝑛𝑐𝑒 × 𝐼𝑠𝑜𝑡𝑜𝑝𝑖𝑐 𝑚𝑎𝑠𝑠) /
100

Question 5

What are the uses of a mass spectrometer?

Answer 5

a) Used in forensic science to obtain evidence from crime scenes.
b) Identification of isotopes of an element.
c) Determination of R.A.M of elements.
d) Determination of R.M.M of compounds.
e) Determination of structures of compounds.

Question 6

Give advantages of a mass spectrometer

Answer 6

• It requires a very small mass of the sample to be analyzed; as little as 10-12g.
• It is a very accurate method of determining the relative atomic and relative molecular masses if compared to colligative methods.
• It is a very precise, rapid and sensitive method of determining R.A.M and R.M.M.

Question 7

What are isotopes?

Answer 7

These are atoms of the same element having the same number of protons but different numbers of neutrons.

Question 8

Define Relative molecular mass

Answer 8

This is the mass of one molecule of an element or compound divided by one – twelfth the mass of one atom of carbon- 12 isotope.

Question 9

What is Relative abundance of an isotope?

Answer 9

This is the percentage of atoms with a specific atomic mass found in a
naturally occurring sample of an element.

Question 10

Define relative density/ vapor density of a gas

Answer 10

This is the mass of a given volume of gas divided by the mass of an equal volume of hydrogen measured at the same temperature and pressure.

Question 11

True or false
𝑽.𝑫. 𝒐𝒇𝒈𝒂𝒔=𝟐×𝑴𝒓 𝒐𝒇 𝒈𝒂𝒔.

Answer 11

False
𝑴𝒓 𝒐𝒇𝒈𝒂𝒔=𝟐×𝑽.𝑫 𝒐𝒇 𝒈𝒂𝒔.

Question 12

Describe the procedure using the victor mayor method of determining RMM of volatile liquids.

Answer 12

Procedure
- A fixed mass (Mg) of a volatile liquid whose relative molecular mass is required into the hypodermic syringe.
- Steam is passed into the steam jacket to heat and vaporize the volatile liquid into a vapour whose volume (Vcm3) is read on a scale of a calibrate syringe.
- The temperature (ToC) of the steam jacket is measured using a thermometer and the pressure (P Nm-2) at which the experiment is also measured.
- The molecular mass of the volatile liquid is calculated using the equation 𝑴𝒓 = 𝒎𝑹𝑻/ 𝑷𝑽