1.3 chemical calculations Flashcards
Ar definition
the relative atomic mass of an atom is the ratio of the average mass of one atom of that element to 1/12th of the mass of one atom of carbon-12
what are all measurements of masses in the periodic table measured relative to?
carbon-12
what is the relative atomic mass?
the average mass of all the isotopes
(Ar)
what are isotopes?
atoms of an element that have the same number of protons, but different numbers of neutrons in the nucleus
Mr definition
the relative molecular mass (or formular mass) of a molecule is the ratio of the average mass of that molecule to 1/12th of the mass of an atom of carbon-12
what is the relative molecular mass? (Mr)
the sum of the relative atomic masses of its constituent atoms
e.g CO2 = 12.0 + 16.0 + 16.0 + 16.0 = 44.0
what is the Mr of KMg(SO4)Cl.3H2O?
K x 1 = 39.1
Mg x 1 = 24.4
S x 1 = 32.1
O x 4 = 16.0
Cl x 1 = 35.5
H2O x 3 = 3(18.02) = 54.06
= 201.16
what is a mass spectrometer used for?
calculating the masses of isotopes, and therefore working out the average mass of an element in a sample
(measures mass (Mr))
(also:
- identifying unknown compounds e.g testing athletes for prohibited drugs)
what are the 5 stages in the mass spectrometer?
VIADD
V = vaporisation
I = ionisation
A = acceleration
D = deflection
D = detection
what happens during the 1st stage in the mass spectrometer?
V = vaporisation
atoms in a sample are heated to turn the sample into a gas form
e.g Na (s) —> Na (g)
what happens during the 2nd stage in the mass spectrometer?
I = ionisation
a heated filament (or electron gun) emits electrons in order to ionise the gaseous atoms into positive ions
Na (g) —> Na + (g) + e -
oxidised/ionises
what happens during the 3rd stage in the mass spectrometer?
A = acceleration
negatively charged plates to cause an electric field, resulting in the ions to focus into a beam and become accelerated
what happens during the 4th stage in the mass spectrometer?
D = deflection
the electromagnet causes the charged ions to become deflected by the magnetic field and forced onto the detector
(introduce a magnetic field by an electromagnet) magnetic field deflects particles onto detector
during the 4th stage in the mass spectrometer, how is a magnetic field introduced?
by an electromagnet
what happens during the 5th stage in the mass spectrometer?
D = detection
electric current measured as ions land on plate. the greater the abundance of the isotope, the larger the current
only positive ions are recognised
during the 5th stage in the mass spectrometer, what ions are recognised?
positive ions only
during the 5th stage in the mass spectrometer, what does a large current mean?
the larger the current, the greater the abundance of the isotope
what does the label Y on the mass spectrometer represent?
a vaccum pump which sucks all of the air particles out of the mass spectrometer l
why is a vaccum pump needed in a mass spectrometer?
as it sucks all of the air particles out of the mass spectrometer, which ensures the beam does not come into contact with any particles (stops them blocking the ions)
what does the degree of deflection depend on?
the mass and the charge
the greater the mass, the less the deflection, and the greater the charge, the greater the deflection
what is the relationship between deflection and the mass/charge (m/z) ratio?
it can be shown as inversely proportional
the greater the mass, the less the deflection
the greater the charge, the greater the deflection
HOWEVER in most cases, the charge is +1, so the deflection depends essentially on the relative mass of the species in the mass spectrometer
in most cases, what is the charge of ions in the mass spectrometer?
+1
why can the relative abundance of different isotopes be measured using a mass spectrometer?
the greater the number of particles landing at a single point on the detector, the greater the electric current and the larger the peak
do different isotopes appear at differentiation points on the detector? why?
yes - since the position at which an ion appears on the detector depends on its mass
what does the magnitude of the peak from the mass spectrometer give is?
the relative abundance of the isotope
thus the relative atomic mass of the element can be calculated from its mass spectrum
what if the atoms in the sample in a mass spectrometer had a high mass?
there would be less deflection
(would need to increase the strength of the electromagnet)
what if the sample in a mass spectrometer had a 3+ charge after being ionised?
there would be more deflection
(need weaker electromagnet/weaken the magnetic field)
what if the atoms in the sample in a mass spectrometer had a low mass?
more deflection
(need a weaker electromagnet)
what if the electromagnet broke in a mass spectrometer?
the ions would crash into the side
what does the number of peaks on a mass spectra represent?
the number of different isotopes in the sample tested
what are the axis on a mass spectra?
y - relative abundance
x - M/Z (mass/charge ratio) usually +1
what does the axis relative abundance mean on a mass spectra?
how much of that isotope was detected compared to the others. this may also be expressed as a percentage
how do you work out the relative atomic mass of an element from its mass spectrum?
relative atomic mass:
(% abundance 1 x mass of isotope 1) + (%abundance 2 x mass of isotope 2) … /100
how do you work out the relative atomic mass of an element from its mass spectrum if the abundances do not add to 100?
divide by the number they add up to, rather than 100
what is important to remember when looking at mass spectra?
e.g chlorine
the first two peaks are in a 3:1 ratio due to the 75:25 % abundance
chlorine-35 has a 75% abundance
chlorine-37 has a 25% abundance
what is special about diatomic elements on a mass spectra?
(- these are molecular ion peaks, where the whole molecule has traveled through the mass spectrometer rather than individual atoms)
e.g chlorine is diatomic so some molecules break up into atoms, whilst some stay as molecules
there will be peaks at:
m/z 35
m/z 37
m/z 70 (35 + 35)
m/z 72 (35 + 37)
m/z 74 (37 + 37)
why are the peaks for e.g chlorine m/z 70,72 and 74 at ratios 9:6:1?
due to the 75:25 % abundance
35-35 = 3/4 x 3/4 = 9/16
35-37 = (3/4 x 1/4) + (3/4 x 1/4) = 6/16
37-37 = 1/4 x 1/4 = 1/16
in what ratio are the molecular ion peaks of bromine? (m/z 79, 81, 158, 160 and 162)
bromine-79 = 50% 1:1
bromine - 81 = 50%
(79-79) = 158 1:2:1
(79-81) = 160
(81-79) = 160
(81-81) = 162
what is happening in the ionisation part of the spectrometer?
the sample is bombared by high energy electrons to knock an electron from the atom to form a positive ion
how are the ions produced in the ionisation chamber accelerated?
using magnets
what is the empirical formula of a compound?
the simplest whole-number ratio
how can empirical formula be calculated?
Element 1 I Element 2
mass/percentage
/ Mr
= answer
/ smallest answer
= ratio
what is the molecular formula of a substance?
the formula which shows the number of each type of atom in one molecule of that substance
(the number of atoms that actually make up the compound)
what does the molecular formula only apply to?
molecular substances
what must be known to calculate the molecular formula?
the empirical formula and molar mass of the compound
how do you work out the molecular mass?
e.g empirical formula = BCl2
(Ar of B x 1) + (Ar of Cl x 2) = (10.8x1)+(35.5x2) = 81.8
relative molecular mass = 163.6
163.6/81.8 = 2
molecular formula = B2Cl4
mole definition
the amount of a substance that contains the same number of particles as there are carbon atoms in 12g of carbon-12