Relative Atomic Mass

Question 1

Relative atomic mass

Answer 1

The relative atomic mass, Ar, is the average mass of an atom of an element on a scale where an atom of carbon-12 is 12.

An average so it’s not usually a whole number.

Question 2

Relative isotopic mass

Answer 2

Mass of an atom of an isotope of an element on a scale where an atom of carbon-12 is 12.

Usually a whole number.

Question 3

Relative molecular mass

Answer 3

The average mass of a molecule on a scale where an atom of carbon-12 is 12.

Question 4

How to find Mr

Answer 4

Add Ar all together.

If mass spectrometer, add all Ar and then divide by 100 or relative abundance.

Question 5

How can relative masses be measured

Answer 5

Using a mass spectrometer

Question 6

State the four steps of a mass spectrometer

Answer 6

1. Ionisation
2. Acceleration
3. Ion drift
4. Detection

Question 7

Explain what the first stage of mass spectrometer for high Mr compounds is

Answer 7

Electrospray ionisation- used for high Mr compounds
1. The sample is dissolved in a volatile solvent (like methanol and water) and injected through a fine hypodermic needle as a fine spray into a vacuum in the ionisation chamber.
2. The needle, being positively charged, has a very high voltage applied to the end where the spray emerges and so the particles gain a proton and become ions as a fine mist (They gain a H+ ion) and the sample turned into a gas is made of positive ions.
X(g)+H^+(g)->XH(g)
3. The solvent then evaporates leaving 1+ ions.

Question 8

Explain what the first stage of mass spectrometer for low Mr compounds is

Answer 8

Electron impact ionisation
1. An electron gun fires out high energy electrons at the sample inserted.
2. A 1+ ion is formed due to one electron from each atom/ molecule being knocked off.
X(g)->X^+(g)+e-

Question 9

If the real mass is a whole number and the relative atomic mass you have worked out is less, what does that show

Answer 9

Results show there’s isotopes of a higher number that brings it up

Question 10

Explain what is stage 2 of the mass spectrometer is

Answer 10

Acceleration of ions

1. Positively charged ions accelerated by electric magnetic field so that they all have the same kinetic energy. The ions are attracted to the magnetic field.
   The lighter ions travel faster (accelerate).

1/2mv^2=1/2mv^2

m/t^2=m/t^2

Question 11

Explain what is stage 3 of the mass spectrometer is

Answer 11

Ion drift - ions into flight tube which is a region with no electric field - drift through it.

1. Ions with different masses have a different time of flight allowing separation.
2. Lighter ions travel faster and take less time to reach the detector where:

t=d (square root of) m/2KE

3. Mass lighter = bigger velocity

Current = movement of electrons

Question 12

Explain what is stage 4 of the mass spectrometer is

Answer 12

Detection

1. Lighter ions reach detector in less time than heavier ions.
2. The detector is a negatively charged plate where a current is produced when the ions hit the plate.
3. The more ions that hit the detector the bigger the current.
4. Detector detects them by creating a current.

M^+ + e- ->M. The ion picks up an electron when hits the detector becoming unionised.

Question 13

What is a mass spectrum

Answer 13

Mass/ charge plotted against abundance