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
what is the mole traingle with moles, Mr and mass? units?
moles = mass/Mr
mol = g/gmol^-1
what does avogadro’s number stand for?
the number of particles in one mole of a substance
(6.02 x10^23)
what is the mole triangle with moles, avogadro’s number and number of particles?
moles = no of particles/avogadro’s number
what is the law of conservation of mass?
the total combined mass of the reactants must be the same as the total combined mass of the producta
what does the ratio (stoichiometry) in which specues react correspond to?
the number of moles
NOT THEIR MASS
meaning masses must therrfore all be converted into moles, then compared to each otber, then converted back
when given one reactant with more moles than the other (but the same ratio) which one do you use?
the smallest number
e.g A + B -> C + D
A = 1 mole
B = 0.8 mole
A is in excess
so use B (0.8)
when given one reactant with more moles than the other (but the same ratio) what does it mean?
one is in excess
what does the limiting reactant mean?
the reactant that isn’t in excess
what is a solution?
a mixture of two or more substances in which the proportions of the substances are identical throughout the mixture
what the major component of a solution called? the minor component?
major = solvent (e.g water)
minor = solute
what is the moles triangle with moles, volume and concentration? units?
moles = vol x conc/molartiy
mol = dm^3 x moldm^-3
what must you do to convert cm^3 into dm^3?
divide by 1000
what must you do to convert dm^3 into cm^3?
x 1000
how do you convert gdm^-3 into moldm-3?
divide by molar mass (Mr)
(gdm^-3 = moldm^-3 x Mr)
what does the volume occupied by a gas depend on?
- the temperature
- the pressure
- the amount of gas
how does the temperature affect the volume occupied by a gas?
the hotter the gas, the faster the particles are moving and the more space they will occupy
how does the pressure affect the volume occupied by a gas?
the higher the pressure, the more compressed the gas will be and the less space it will occupy
how does the amount of gas affect the volume occupied by a gas?
the more gas particles there are, the more space they will occupy
does the volume occupied by a gas depend on what gas it is?
no
will one mole of any gas, at the same temperature and pressure, have the same volume as one mole of any other gas?
yes
what is the gas equation?
PV = nRT
P = unit of pressure : pascals, Pa
V = unit of volume : m^3!!!!
T = unit of temperature : Kelvin, K
n = moles
R = molar gas constant, 8.31 Jk-1 mol-1
0.C in kelvin?
273K
what is 70.C in kelvin?
0.C = 273k
70.C = 343K
1KPa = ____Pa
1KPa = 1000Pa
1m^3 = _____ dm^3 = _____ cm^3
1m^3 = 1000 dm^3 = 10^6 cm^3
m^3 /1000 = dm^3 /1000 = cm^3
how is the relative atomic mass of an element calculated?
it’s the average of the abundance of each isotope and the isotopic mass
how can the abundance of each isotope in a sample be determined?
using mass spectrometry
how can the % by mass of an element in a compound be determined?
% by mass =
(molar mass of element x no. of that element) / molar mass of compound x100
e.g % of hydrogen in ammonia, NH3 =
3.03/17.03 x100 = 17.8%
what happens during the vaporisation stage in the mass spectrometer?
the sample is heated and turned into a gas before it enters the spectrometer
what happens during the ionisation stage in the mass spectrometer?
the gaseous sample is bombarded with high energy electrons using an electron gun. this knocks electrons off tbe particles to form positive ions
what happens during the acceleration stage in the mass spectrometer?
the positive ions are accelerated using an electric field (and a vaccum pump)
what happens during the deflection stage in the mass spectrometer?
the ions are deflected by a magnetic field.
(the amount of deflection depends on the mass and charge of the ion. lighter ions are deflected more than the heavier ones)
what does the amount of deflection depend on in the mass spectrometer?
the mass and charge of the ion
(lighter ions are deflected more than heavier ones)
what happens during the detection stage in the mass spectrometer?
the abundance and mass/charge (m/z) ratio of the ions is read, and a mass spectrum graph is produced
why is the inside of the spectrometer a vaccum?
so that air molecules don’t interfere with the movement of the ions
what are the two standard molar volumes of gas that you should know how to use?
- 22.4 dm^3 at a temp of 273K (O.C) at a pressure of 1 atm
- 24.5 dm^3 at a temp of 298K (25.C) at a pressure of 1 atm
what is the equation used to calculate the number of moles of a gas using the volumr and molar volume?
n = V / Vm
when the temperature or pressure is not 273/298K or 1 atm, what can you use to
work out the number of moles or volume of a gas?
PV = nRT
what does acid-base titration allow us to calculate?
- the concentration of an acidic or basic solution
- the concentration of one solution is known and it is reacted with the other solution until the exact moment of neutralisation occurs (the endpoint)
(can use moles = conc x volume)
what can a back titration be used to find out?
the amount of a substance in an impure solid
e.g reacting an acid with an indigestion tablrt in order to find the amount of calcium carbonatr in the tablet
how does back titration work?
an excess amount of acid is used, and the excess is then titratrd with an alkali
you can find out how much acid was used in the reaction and calculate from this how much (e.g calcium carbonate) was in the (tablet)
what are two ways of assessing the efficiency of a reaction process?
- atom economy
- percentage yield
what is atom economy?
- if products are formed other thsn the one needed, then these are wasted
- atom economy calculations determine this waste as a percentage
what is the atom economy equation?
atom economy =
Mr of desired product / total mass of reactants x100
what does percentage yield show?
the mass or number of moles obtained as a percentage of that expected
a very efficient reaction has a ___ atom economy and percentage yield
high atom economy and percentage yield
what is the percentage yield equation?
percentage yield =
mass or no. of product obtained / maximum theoretical mass or no. x100
a reaction with only one product has an atom economy of ___%
100%
in chemistry, any error in results is estimated from what?
the limitations of the equipment used
percentage error example :
a thermometer with a precision of 2.C has a potential error of up to 1.C. if a reading of 78.C is taken, what is the % error?
% error = 1/78 x100 = 1.3%
how is the percentage error calculated?
% error = estimate of error / value measured x100
if you have used more than 1 piece of equipment, what is the total error percentage?
the sum of the percentage error for each piece of equipment
relative isotopic mass of an isotope of an element definition
the avergae mass of an isotope relative to 1/12th of the mass of one atom of carbon-12
(refers to the mass of a particular isotope)
will a mole of atoms always occupy the same volume when under standard conditions?
yes
when under standard conditions, what is the relationship between pressure and temperature for gases and volatile liquids?
they are proportional
when under standard conditions, what is the relationship between volume and temperature for gases and volatile liquids?
they are proportional
when under standard conditions, what is the relationship between pressure and volume for gases and volatile liquids?
they are inversely proportional
what are some reasons why the yield may be lower than expected?
- the starting products can have impurities in them
- the reaction may not go to completion
- if working on a small scale, a small amount of material may be left behind when transferring between flasks (smaller numbers = larger % error)
- there may be competing reactions that give other products, reducing the yield of the desired one
- in separation and purification, some product is lost on filter papet
density equation
density = mass/volume
(for water, we assume 1g=1cm^3)
what do they say the error in any measurement is?
one-half of the smallest division of the apparatus scale
e.g on a burette, the smallest division is 0.1cm^3 so the error is +/- 0.05cm^3
when calculating a mean titre, what must the titres be within?
0.20cm^3
how do you calculate the % error?
% error =
( +/- error / measurement made on apparaturs (mass in g or vol in cm^3)) x100
if you make two readings in an experiment (reading before and aftet titrating), you multiply thr error by 2 beforr using the calculation - applies to multiple weighing’s on a scale too)
larger measurements = ___ % error
lower % error
how do you reduce errors in titration?
- replace measuring cylinders with pipettes or burettes which have lower apparatus uncertainty will lower the error
- make the titre a larger volume (increasing vol and conc of substance in conical flash or decreasing conc of the substance in the burette)
how do you reduce error in measuring mass?
- using a more accurate balance or a larger mass (for a solid)
- weighing sample before and after addition and then calculating difference will ensure a more accurate measurement of the mass added
gas equation for when conditions change:
P1V1/T1 = P2V2 / T2
state the difference, if any, between the chemical properties of the isotopes 6-Li and 7-Li, giving a reason for your answer
no difference (1)
same number of electrons (in the outer shell) (1)
explain how the relative atomic mass differs from the relative isotopic mass
relative isotopic mass only coniders one isotope, but the relative atomic mass considers a weighed average of the isotopes present
how to convert from cm^3 to m^3? or other way around
e.g 0.5m^3 = cm^3
divide the number of cm^3 by 1,000,000
e.g 0.5m^3 = 500,000 cm^3
LOOK AT SIGNIFICANT FIGURES IN QUESTION
explain, how the relative atomic mass differs from the relative isotopic mass [1]
relative isotopic mass only considers one isotope, but the relative atomic mass considers a weighted average of the isotopes present
the mass spectrum of bromine trifluoride, Br^19F3, shows two molecular ion peaks of equal intensity at m/z 136 and 138.
state what can be deducted about the relative isotopic masses of the bromine atoms present and their percentage abundances [2]
- the relative isotopic masses of the bromine atoms are
136-(3x19) = 79 and 138-(3x19) = 81 - (as they have equal intensities) each is 50% of the bromine atoms present
