1.3 Chemical Calculations Flashcards
Define relative atomic mass
The average mass of one atom of the element relative to 1/12th the mass of one atom of carbon-12
Define relative isotopic mass
The mass of an atom of an isotope relative to 1/12th the mass of carbon-12
Define relative formula mass
The sum of the relative atomic masses of all atoms present in its formula
What is the mass spectrometer used to measure?
- The mass of each different isotope of the element
- The relative abundance of each isotope of the element.
What are the four main steps in the mass spectrometer?
- Ionisation
- Acceleration
- Deflection
- Detection
Describe what happens during ionisation (mass spectrometer)
The vapourised sample passes into the ionisation chamber
Particles in the sample are bombarded with a stream of electrons.
Some of the collisions are sufficiently energetic to knock one or more electrons to make the positive ions.
Most of the positive ions will carry a +1 charge because it is much more difficult to remove more electrons from an already positive ion.
Describe what happens during acceleration (mass spectrometer)
An electric field accelerates the positive ions to high speed.
Describe what happens during deflection (mass spectrometer).
Different ions are deflected by the magnetic field by different amounts.
Amount of deflection depends on:
1. Mass of ion. The lighter the ion, the more it is deflected.
2.The charge on the ion. Ions with 2 or more positive charges are deflected more.
These two factors are combined into the m/z.
Describe what happens during detection (mass spectrometer)
Beam of ions passing through the machine is detected electrically.
Only ions with the correct m/z make it to the end of the machine and reach the ion detector where they are detected.
Other ions collide with the walls, pick up electrons and are neutralised. Eventually they get rremoved from the mass spectrometer.
The signal is then amplified and recorded.
Define molecular ion.
A positive ion formed in a mass spectrometer from the whole molecule.
How do you get a wide range of lines from mass spectrum?
Molecular ions are energetically unstable, some of them break up into fragments.
All sorts of fragmentations of the original molecular ions are possible so you will get a wide range of lines in the mass spectrum.
What is a mole.
the amount of any substance that contains the same number of particles as there are atoms in exactly 12g of carbon-12.
What is Avogadro constant?
The number of atoms per mole.
What is molar mass?
The mass of one mole of substance.
How do you calculate the number of moles?
amount in moles = mass/molar mass (Mr)
How do we calculate reacting masses? e.g. mass of MgO produced after burning a certain amt of Mg
- Change mass of Mg into moles
- Use balanced eqn to state mole ratio of Mg:MgO, deduce moles of MgO
- Change amt of moles of MgO to mass.
What is empirical formula?
Simplest formula showing the simplest whole number ratio of the number of atoms of each element present.
What is molecular formula?
The actual number of atoms of each element present in the molecule.
How do you find the empirical formula?
- Find the amt in moles of each element present (mass/ percentage given in qn)
- Find ratio of no. of atoms presemt (divide by smallest value)
- Convert numbers into whole numbers.
(same method can be applied to find water of crystallisation)
What is one mole of any gas at standard temp and pressure (0 degrees C and 1atm)?
22.4 dm^3
What is one mole of any gas at room temp and pressure (25 degrees C and 1atm)
24.5 dm^3
How do you calculate the vol of gas produced?
- Change mass of element/compound reacted into moles.
- Use mole ratio from balanced symbol eqn
- Change moles into volume of gas.
State the ideal gas eqn.
pV = nRT
p = pressure in Pa
V = moles in m^3
n = no of moles
R = gas constant, 8.31JK^-1mol^-1
T = temp in kelvins
Write the formula to find concentration.
conc = amt of moles/vol of solution
