Moles and Chemical Calculations Flashcards
atomic number
the number of protons in the nucleus of an atom
atomic mass
the number of protons and neutrons in the nucleus of an atom
relative atomic mass (symbol Ar)
the average mass of the naturally occuring isotopes of the element relative to the mass of an atom of carbon-12, which is given a mass of exactly 12
it is found by comparing the mass of an atom with the mass of an atom of carbon-12
relative molecular mass, Mr (RMM)/Relative Formula Mass (RFM)
(the sum of the individual relative atomic masses of the atoms that make up a molecule/compound)
is the mass of a molecule of the compound relative to the mass of an atom of carbon-12, which is given a mss of exactly 12
isotopes
atoms of the same element which have the same number of protons but a different number of neutorns
relative isotopic mass
an isoptope of an element is the mass of one atom of that isotope relative to the mass of an atom of carbon-12
mole
the amount of substance that has the same number of particles (atoms, molecules, ions etc) as there are atoms in exactly 12g of carbon-12
mole = in refernece to a sentence
mol = unit
molar mass (gmol-1)
the mass of one mole of a compound, expressed in grams. Thus one mole of carbon-12 has a mass of exactly 12g. One moleof sodium-23 has a mass of 23g and so on
Avogadro’s number
the number of particles in a mole i.e. 6.023 x 1023 = 1 mole
what does the atomic number also equal?
number of protons
number of electrons
what is magnesium’s atomic mass in comparison to carbon
C : Mg
12 : 24
1 : 2
you divide by the smallest number or smallest factor of the two numbers
what is the relative molecular mass (RMM) of CO2?
C = 12
0= 16
12 + (16 x 2)
=44
what is the relative molecular mass (RMM) of Ca (OH)2
Ca = 40
O = 16
H = 1
Ca + (O x 2) + (H x 2)
=74
what is the relative molecular mass (RMM) of CuSO4 • 5 H2O
Cu = 63.5
S = 32
O = 16
H = 1
63.5 + 32 + (16x4) • (1 x 2 x 5) + (16 x 5)
159 • 90
(the • means for every CuSO4 there are five H2O)
=249.5
How do you find the % mass?
% mass = RMM of element ÷ RMM of compound x 100%
Find the percentage of oxygen in Ca (OH)2
O = 16
Ca = 40
H = 1
(2 x 16) ÷ 74 = 32 ÷ 74 x 100%
=43.2%
Calulate the relative molecular mass (Mr) of NH4NO3
N = 14
H = 1
O = 16
14 + (1 x 4) +14 + (16 x 3)
= 80
Calculate the relative molecular mass (Mr) of Al2(SO4)3
Al = 27
S = 32
O = 16
(27 x 2) + (32 x 3) + (16 x 4 x 3)
= 342
Calculate the relative molecular mass of H2C2O4 • 2H2O
H = 1
C = 12
O = 16
(1 x 2) + (12 x 2) + (16 x 4) • (2 x 1 x 2) + (2 x 16)
90 • 36
= 126
Calculate the percentage by mass of C in CH4
12 ÷ 16 x 100
= 75%
Calculate the percentage by mass of O in Ca(NO3)2
(16 x 3 x 2) ÷ 164 x 100
= 58.5 %
Calculate the percentage by mass of O in Fe(NO3)3
(16 x 3 x 3) ÷ 56 + (14 x 3) + (16 x 3 x 3) = 242
133 ÷ 242 x 100%
=59.5%
How many molecules are in one mole of CO2?
6.02 x 1023 molecules
How many atoms are in one mole of CO2?
because there are 3 atoms in 1 molecule of CO2?
3 x 6.02 x 1023
= 18.06 x 1023 atoms
How many molecules are in 1/2 mole of CH4?
1/2 x 6.02 x 1023 molecules / 3.01 x 1023 molecules
How many atoms are in 1.5 moles of CH4?
there are 5 atoms in the molecule
5 x 1.5 x (6.02 x 1023)
7.5 x 6.02 x 1023
=45.15 x 1023 atoms
What is the formula that connects moles, mass and Mr?
number of moles = mass (g) ÷ Mr
n = m ÷ Mr
this is in a solid
Caluculate the number of moles in 0.26g of Ca0
m = 0.26
Mr = 40 +16 = 56
n = 0.26 ÷ 56
= 0.0046 mol
How many molecules are present in this number of 0.0046 moles?
1 mol= 6.02 x 1023
0.0046 mol = 6.02 x 1023 x 0.0046
= 0.0277 x 1023molecules
Find the mass of 3.2 moles of SO2
mass = moles x Mr
3.2 x (32+(16x2))
=3.2 x 64
= 204.8
what happens when the proton number changes?
the identity of the atom changes
calculate the relative atomic mass of the isotope chlorine where:
25% 37Cl
75% 35Cl
25 ÷ 100 x 37 = 9.25
75 ÷ 100 x 35 = 26.25
9.25 + 26.25 = 35.5
= 35.5 (R.A.M.)
how can you check that your R.A.M. is correct?
you can check it on the periodic table
Calculate the R.A.M. of boron when:
% of abundance = 19.7 for the the relative isotopic mass of 10
% of abundance = 80.3 for the relative isotopic mass of 11
(10 x 9.7) + (11 x 80.3) ÷ 100 = 10.8 (1 d.p)
Calculate the relative atomic mass of Sulphur when:
32S = 95.0%
33S = 0.76%
34S = 4.22%
35S = 0.020%
(32 x 95.0) + (33 x 0.76) + (34 x 4.22) + (35 x 0.020) ÷ 100
=32.0926
how many moles are in 1.8g of H2O?
moles = mass ÷ Mr
mols = 1.8g ÷ 18
=0.1 mol
how many moles are in 80g of NaOH
moles = mass ÷ Mr
moles = 80 ÷ (26+16+1)
=80 ÷ 40
=2 mol
how many moles are in 19.6g of H2SO4?
moles = mass ÷ Mr
moles = 19.6 ÷ (1x2) + 32 + (16x4)
= 0.2 mol
how many moles are in 585g of NaCl
moles = mass ÷ Mr
moles = 585 ÷ 23 +35.5
= 585 ÷ 58.5
= 10 mol
how much does 0.5 mol C2H5OH weigh?
mass = moles x Mr
= 0.5 x (24 + 5 + 16 + 1)
= 23g
how much does 2.5 mol NaNO3 weigh?
mass = moles x Mr
= 2.5 x (23 + 14 + 16 x 4)
= 212.5 g
how much does 3 mol of K2SO4 weigh?
mass = moles x Mr
= 3 x (39 x 2 + 32 x 16 x 4)
= 522
how much does 0.25 mol of NaOH weigh?
mass = moles x Mr
= 0.25 x (23 +16 + 1)
= 10 g
calculate the percentage composition of of Cl in AlCl3?
106.5 ÷ 27 + (35.5 x 3) x 100%
= 79.8 %
calculate the percentage compostion of H in H3PO4?
3 ÷ 3 = 31 + (16 x 4) x 100% = 3.1%
how many atoms are there in 0.25 moles of sodium?
0.25 x 1 (there is one atom of sodium) x 6.023 x 1023
= 1.505 x 1023
how many molecules are there in 1.5 of CO2?
1.5 x 1 (there is one molecule) x 6.023 x 1023
= 9.023 x 1023
how many atoms are there in 1.5 moles of CO2?
1.5 x 3 (there are 3 atoms in the molecule) x 6.023 x 1023
= 27.09x 1023
what is the total number of atoms in 1 mole of H2SO4?
7 (there are 7 atoms in the molecule) x 6.023 x 1023
= 42.14 x 1023
what is the total number of molecules of 0.47 moles of SO3?
0.47 x 1 (there is only one molecule) x 6.023 x 1023
= 2.83 x 1023
what is the molecular formula?
the number of atoms of each different element in he molecule
e.g.
H H H H
H - C -C - C - C - H
| | | |
H H H H
= C4H10
| | |
what is the empirical formula?
the simplest whole number ratio of atoms of each elemnt present in the molecule
e. g. m.f = C4H10
e. f = C2H5
the e.f. doesn’t have fractions
what is special about carbon dioxide (C02)’s molecular formula and empirical formula?
they are the same
what is special about water (H2O)’s molecular and empirical formula?
they are the same
how do you work out the molecular formula?
Mr ÷ empirical formula
If the empirical formula is CH2 and the Mr is 42, what is the molecular formula?
molecular formula = Mr ÷ empirical formula
= 42 ÷ 14
= 3
C3H6
If the molecular formula is C5H10, what is the empirical formula and Mr?
Mr = (12 x 5) + (1 x 10) = 70
empirical formula = CH2 (cancel down)
if the molecular formula is C4H8, what is the Mr and empirical formula?
Mr = (12 x 4) + (1 x 8) = 56
empirical formula = CH2 (cancel down)
If the empirical formula is C3H8 and the Mr is 44, what is the molecular formula?
molecular formula = Mr ÷ empirical formula
= 44 ÷ 44
= 1
C3H8
if the empirical formula is CH2O and the Mr is 180, what is the molecular formula?
molecular formula = Mr ÷ empirical formula
= 180 ÷ 30
= 6
C6H12O6
what happens when salts crystallise from solution
they often form hydrated salts containing loosely bonded water molecules
water of cyrtallisation
what is used in the test for water?
anhydrous (no water) copper sulphate
CuSO4(s) + 5H2O –> CuSO4(s) • 5H2O
white powder of anyhdrous copper dulphate –> blue crystals of hydrated copper suplhate
the water in the blue crystals is called the water of crystallisation
what does gentle heating do to water in crystallisation?
heating will drive the water molecules from the crystal lattice
water of crystallisation is lost and the anyhdrous salt
will form again
what is the formula of hydrated magnesium sulphate crytsals that contain MgSO4 (48.8%) and H2O (51.2%)
When 4.76g of hydrated cobalt chloride crystals were heated they gave 2.6g of anyhydrous cobalt chloride (CoCl2). What is the formula of the hydrated salt? (Co = 59 Cl = 35.5 ; H = 1 ; O = 16)
What three steps do you go through to calculate the reacting mass?
- look at the chemical equation and circle the two substances that are discussed in the question. Using the information given, work out the moles of the reactant
- compare the molar ratio; Reactant:Product (or substance required) - is it 1:1? 2:1? 3:2? etc… then determine the moles
- now that you have the moles of the product (or substance requires), again, using the moles equation, work out what is asked for in the question i.e. mass(g), concentration (moldm-3), volume (cm3) etc…
once you know the mass of the reactant, you can work out the moles of everything
what does 1 mol O2 molecules equate to?
2 mol O atoms
what is yield?
amount of product
what is percentage yield?
the efficiency of a reaction can be assesed by finding the percentage yield
i.e. how much product did we actually make? was it as much as we predicted?
what is the percentage yield equation?
how much product we actually made ÷ how much product we should have obtained (theoretical number) x 100%
what are the units used in percentage yield?
it doesn’t matter, the % yield will be the same regardless
e.g. grams, kg, moles etc…
what is the equation for atom economy?
Mr of ‘desired’ product ÷ sum of Mr of all products x 100%
what is the difference between atom economy and percentage yield?
the % yield is how efficient the reactant is (yield = amount of product)
atom economy is how much of the yield is actually useful
why is the % yield never at 100%?
reactions always have yields lower than 100% for the following reasons:
loss of product in transfer and separation
formation of by-products
decomposition of starting materials
incomplete reaction and reversibility
what range of% yields might be economically viable?
industrial chemists need their reaction yields to be as high as possible
generally, they want reations with a % yield above 75% so that they do not waste too much starting material
there are exceptions where lower products are acceptable
e.g. if the product is particularly valuable, starting material is very cheap, unused reactants can be re-used or recycles
why is atom economy important?
atom economy determines the efficiency of a reaction, how many atoms of starting material are usefully converted into product
it is an important measure of the enviromenta friendliness of a reaction
the more efficient/ higher atom economy the less wasteful and therefore more green
in less efficient/lower atom economy reactions most atoms are converted to useless by-products
A hydrocarbon contains 62% C and the rest hydrogen. Find the empirical formula
Calculate the empirical formula for iron bromide containing 3.78g of iron and the overall ass being 20g
A hydrocarbon contains 68% C and 12% H and the rest is made of oxygen. Calculate the empirical formula
A group were given 10g of hydrated copper sulphate to heat. After heating, 6.4g of anhydrous coper sulphate remained. How many water molecules are there in hydrated copper sulphate?
1.5 H2O
moles of H2O is 164.1
Dibutyl succinate is a domestic insect repellent. Its composition is 62.58% C, 9.63% H, 27.79 O. Its relative mass is 230. What are the empirical and molecular formulae of dibutyl succinate.
What mass of potassium oxide is fromed when 9.75g of potassium is burned in oxygen?
4K + O2 —> 2K2O
What mass of sodium is needed to reduce 1kg of titanium chloride?
Ti Cl4 + 4Na —> Ti + 4NaCl
What mass of silver chloride is formed when 83.25g of CaCl2 is reacted with AgNO3 solution?
bromeothane can be prepared by reacting hydrogen bromide with ethanol according to the followng equation:
HBr + C2H5OH —> C2H5Br +H2O
( C2H5OH is ethanol)
mass of ethanol is 9.20
work out the percentage yield
when James reacted 5.6g of ethane with bromine water he obtained 16.9g of C2H4Br2. Calculate his % yield
C2H4 + Br2 —> C2H4Br2
If the percentage yield fora reaction is 100%, 60g of reactant A would make 80g of product C. How much reactant A is needed to make 80g of product C if the percentage yield of the reaction is only 75%?
Calculate the atom economy of CaCO3 —> CaO + CO2
What is a good example to use to explain the difference between % yield and atom economy?
ammonia
i) What mass of copper (II) oxide will be formed if 10g of copper(II) carbonate is decomposed?
CuCO3 —> CO2 + CuO
ii) When 10g of copper (II) carbonate produced 4.5g of copper(II) oxide. What is the % yield?
ii) 4.5 ÷ 6.36 = 69.9%
b. 49.6 ÷ 4 x 10 = 124
what is the equation for moles in a gas
n = v ÷ 24000
what is the equation for volume in a gas?
v = n x 24000
how much does one mole of gas occupy a volume of?
one mole of any gas occupies a voulme of 24000cm3
this is only applicable at room temprature which is 250 (298K) and at the atmospheric pressure of 1 atm
what is the Kelvin scale
00 = 273 K
add on the degrees to the kelvin number 273 to find the Kelvin temprature
e.g. 250 = (273+ 25) = 298 K
find the voume of CO2 gas produced hen 4.5g of CaCO3 is decomposed
CaCO3 (s) —> CaO (s) + CO3 (g)
n = v ÷ 24000
v = n x 24000
n = m÷ Mr = 4.5 ÷ (40 + 12 + (16x3)) = 4.5 ÷ 100
v = 0.045 x 24000
= 1080 cm3
a hydrocarbon contains 18g of carbon and 3.5g hydrogen and has an RFM (Mr) of 86
calculate the empirical formula of the hydrocarbon
what should a student do to be certain that the reaction had occured fully and they achieved the maxiumum final mass of anhydrous solution?
gently heating a water of crystalisation soultion will drive the water away from the crystal lattice thus breaikng water of crystallisation and the anyhdrous soultion will form again
heat to +1000C (boil)
wiegh on a mass balance to measure change
which of the following contains the greatest number of hydrogen atoms?
A) 2 moles of water, H2O
B) 1.5 moles of ammonia, NH3
C) 1 moles of hydrogen gas, H2
D) 0.5 moles of methane, CH4
suggest why 100% atom economy is an advantage
no waste or associated disposal costs (hazardous/radioactive waste)
more efficient in terms of money and energy saving
which of the following gas samples occupies the greatest volume at the same temperature and pressure?
Relative atomic masses: H = 1; C = 12; O = 16; F = 19; Ne = 20
A) 1 gram of ethane
B) 1 gram of oxygen
C) 1 gram of flourine
D) 1 gram of neon
ethane, oxygen and flourine are diatomic but neon is monatomic
A = 0.8 dm3
B = 0.75 dm3
C = 0.63 dm3
D = 1.2 dm3
17.1g of aluminium sulphate, Al2(SO4)3, was dissolved in water
calculate the number of sulfate ions, SO42-, present in the solution formed
(assume the molar mass of Al2(SO4)3 is 342g mol-1 and the Avogadro Constant is 6 x 1023 mol-1)
A) 3 x 1023
B) 1 x 1023
C) 3 x 10 22
D) 9 x 10 22
D) 9 x 1022
consider the following reaction:
Na2CO3 + 2HCl –> 2NaCl + H2O + CO2
11g of sodium carbonate is reacted with 7.3g of hydrochloric acid to produce sodium chloride, water snd carbon dioxide. Has too much sodium carbonate been used? Explain
