Chapter 4 - Stoichiometry Flashcards
What is the structural formula
The structural formula tells you the way in which the atoms in a particular molecule are bonded. This can be done by either a diagram (displayed formula) or written (simplified structural formula)
E.g CH3CH2CH2CH3 simplified
What is the empirical formula
The empirical formula tells you the simplest whole-number ratio of atoms in a compound
E.g simplified from C4H10 to C2H5
What is the molecular formula
The molecular formula tells you the actual number of atoms of each element in one molecule of the compound or element e.g. H2 has 2 hydrogen atoms, HCl has 1 hydrogen atom and 1 chlorine atom
E.g C4H10
What does the forward arrow imply in chemical reactions
The arrow implies the conversion of reactants into products
In naming compounds with metal and non metal what do you do
If one is a metal and the other a nonmetal, then the name of the metal atom comes first and the ending of the second atom is replaced by adding –ide. Though this still works with hydrogen being a non metal. E.g hydrogen chloride
What happens to ionic compounds in aqueous solutions
In aqueous solutions ionic compounds dissociate into their ions, meaning they separate into the component atoms or ions that formed them
E.g. HCL —> H + CL
Write the ionic equation for the reaction of aqueous chlorine and aqueous potassium iodide.
It goes from 2KI + CL2 —> 2KCL + I2
to 2K + 2I + 2CL —> 2K + 2CL + 2I then it goes
To 2I + CL2 —> 2CL + I2
How do you calculate relative atomic mass
This is calculated from the mass number and relative abundances of all the isotopes of a particular element
Basically finding the average weight that a element has with all their isotopes
How do you calculate relative formula (molecular mass)
To calculate the Mr of a substance, you have to add up the Relative Atomic Masses of all the atoms present in the formula.
E.g H20 = H( 2 x H) + O( O x 1) = 18
What is the mole
This is the mass of a substance containing the same number of fundamental units as there are atoms in exactly 12.000 g of Carbon
What is the Avogadros constant
6.02 x 10^23 is known as the Avogadro Number
One mole is the amount of a substance that contains 6.02 x 10^23 particles (Atoms, Molecules or Formulae) of a substance.
Examples: 1 mole of Sodium (Na) contains 6.02 x 10^23 Atoms of Sodium
1 mole of Hydrogen (H2) contains 6.02 x 10^23 Molecules of Hydrogen.
1 mole of Sodium Chloride (NaCl) contains 6.02 x 10^23 Formula units of Sodium Chloride
What is the mole and relative atomic mass close
One mole of any element is equal to the relative atomic mass of that element in grams
For example one mole of carbon, that is if you had 6.02 x 10^23 atoms of carbon in your hand, it would have a mass of 12g
For compounds we add up the relative atomic mass to find how many grams in one mole.
If hydrogen has one atomic mass how much is the mass of carbon
Hydrogen which has an atomic mass of 1 is therefore equal to 1/12 the mass of a 12C atom
So one carbon atom has the same mass as 12 hydrogen atoms
What’s the molar volume
This is the volume that one mole of any gas (be it molecular such as CO2 or monoatomic such as helium) will occupy
It’s value is 24dm^3 or 24,000 cm^3 at room temperature and pressure (r.t.p.)
How do you calculate amount of gas(mol)
Amount of gas (mol) = Volume of gas (dm^3) ÷ 24
or
Amount of gas (mol) = Volume of gas (cm^3) ÷ 24000
What is dm^3
It’s a volume unit equivalent to a litre
How do you calculate the volume of gas that a particular amount of moles occupies
Equation:
Volume of gas (dm^3)= Amount of gas (mol) x 24
or
Volume of gas (cm^3)= Amount of gas (mol) x 24000
How to Calculate the moles in a particular volume of gas
Equation:
Amount of gas (mol) = Volume of gas (dm3) ÷ 24
or
Amount of gas (mol) = Volume of gas (cm3) ÷ 24000
How to calculate moles
Moles = mass of substance in grams / molecular mass N= m/M
How to calculate mass of substance
Mass of substance = moles x molecular mass
Or m = N x M
How to calculate relative formula mass
Molecular mass = mass of substance in grams / moles of substance
How to calculate percentage composition
calculating the percentage by mass of each particular element in a compound
How do you calculate percentage of oxygen in CO2
Step 1 – Calculate the molar mass of the compound
Molar mass CO2 = (2 x 16) + 12 = 44
Step 2 – Add the atomic masses of the element required as in the question (oxygen)
16 + 16 = 32
Step 3 – Calculate the percentage
% of oxygen in CO2 = 32/44 x 100 = 72.7%
How to calculate concentration of solutions
Concentration (mol / dm^3) = Amount of substance (mol) ÷ Volume of solution (dm^3)
Or C = N/V
How to calculate moles using concentration and volume of substance
Equation:
Amount of Substance (mol) = Concentration x Volume of Solution (dm3)
How to calculate concentration
Equation:
Concentration (mol / dm3) = Amount of substance (mol) ÷ Volume of solution (dm3)
Or C = n/V
What to always remember with cm^3 and dm^3
You need to remember to always when it says out it in the form of /dm^3 you convert the cm^3 into dm^3 by dividing by 1000
What is the limiting reactant
The limiting reactant is the reactant which is not present in excess in a reaction
It is always the first reactant to be used up which then causes the reaction to stop
How much is a tonne in grams
10^6g or 1000000g
An example: 9.2g of sodium is reacted with 8.0g of sulfur to produce sodium sulfide, NaS. Which reactant is in excess and which is the limiting reactant?
Step 1 – Calculate the moles of each reactant
Moles = Mass ÷ Molecular mass
Moles Na = 9.2/23 = 0.40
Moles S = 8.0/32 = 0.25
Step 2 – Write the balanced equation and determine the molar ratio
2Na + S → Na2S so the molar ratios is 2 : 1
Step 3 – Compare the moles. So to react completely 0.40 moles of Na require 0.20 moles of S and since there are 0.25 moles of S, then S is in excess. Na is therefore the limiting reactant.
What is the molar ratio
A mole ratio is the ratio between the amounts in moles of any two compounds involved in a chemical reaction. Mole ratios are used as conversion factors between products and reactants in many chemistry problems.
The mole ratio is also called the mole-to-mole ratio.
For the reaction: 2 H2(g) + O2(g) → 2 H2O(g)
What is the mole ratio
The mole ratio between O2 and H2O is 1:2. For every 1 mole of O2 used, 2 moles of H2O are formed.
The mole ratio between H2 and H2O is 1:1. For every 2 moles of H2 used, 2 moles of H2O are formed. If 4 moles of hydrogen were used, then 4 moles of water would be produced.
Calculate the Mass of Magnesium Oxide that can be made by completely burning 6 g of Magnesium in Oxygen
Magnesium (s) + Oxygen (g) → Magnesium Oxide (s)
Step 1 – Calculate the moles of Magnesium Used in reaction
Moles = Mass ÷ Mr Moles = 6 ÷ 24 = 0.25
Step 2 – Find the Ratio of Magnesium to Magnesium Oxide using the balanced Chemical Equation which is 1:1
Step 3 – Find the Mass of Magnesium Oxide
Moles of Magnesium Oxide = 0.25
Mass = Moles x Mr Mass = 0.25 x 40 = 10 g
Mass of Magnesium Oxide Produced = 10 g
Calculate the Mass, in Tonnes, of Aluminium that can be Produced from 51 Tonnes of Aluminium Oxide
Aluminium Oxide (s) → Aluminium (s) + Oxygen (g) Or 2Al2O3 → 4Al + 3O2
Step 1 – Calculate the moles of aluminium oxide used
Mass of Aluminium Oxide in Grams = 51 x 106 = 51,000,000 g
Moles = Mass ÷ Ar Moles = 51,000,000 ÷ 102 = 500,000
Step 2 – Find the ratio of aluminium oxide to aluminium using the balanced chemical equation which is aluminium oxide to aluminium 1:2
Step 3 – Find the mass of aluminium
Moles of aluminium = 1,000,000
Mass in grams = Moles x Ar Mass = 1,000,000 x 27 = 27,000,000
Mass in Tonnes = 27,000,000 ÷ 106 = 27 Tonnes
Mass of Aluminium Produced = 27 Tonnes
What is the empirical formula
gives the simplest whole number ratio of atoms of each element in the compound
Calculated from knowledge of the ratio of masses of each element in the compound
What is the empirical formula for 10 grams of Hydrogen and 80 grams of Oxygen
Empirical Formula of H2O. This can be shown by the following calculations: you have to first find out the moles
Amount of Hydrogen Atoms = Mass in grams ÷ Ar of Hydrogen = (10 ÷ 1) = 10 moles
Amount of Oxygen Atoms = Mass in grams ÷ Ar of Oxygen = (80 ÷ 16) = 5 moles
The ratio is hydrogen to oxygen 10:5 or 2:1
What’s the molecular formula
gives the exact numbers of atoms of each element present in the formula of the compound
Divide the relative formula mass of the molecular formula by the relative formula mass of the Empirical Formula
Multiply the number of each element present in the Empirical Formula by this number to find the Molecular Formula
Find out the question?
The Empirical Formula of X is C4H10S1 and the Relative Formula Mass of X is 180. What is the Molecular Formula of X?
Step 1 – Calculate Relative Formula Mass of Empirical Formula
(C x 4) + (H x 10) + (S x 1) = (12 x 4) + (1 x 10) + (32 x 1) = 90
Step 2 – Divide Relative Formula Mass of X by Relative Formula Mass of Empirical
Formula
180 / 90 = 2
Step 3 – Multiply Each Number of Elements by 2
(C4 x 2) + (H10 x 2) + (S1 x 2) = (C8) + (H20) + (S2)
Molecular Formula of X = C8H20S2
What’s percentage yield
Percent yield is the percent ratio of actual yield to the theoretical yield.
It is calculated to be the experimental yield / theoretical yield x 100%.
If the actual and theoretical yield are the same, the percent yield is 100%. Usually, percent yield is lower than 100% because the actual yield is often less than the theoretical value
Formula for percentage yield
Equation:
Percentage Yield = (Yield Obtained / Theoretical Yield) x 100
In an experiment to displace copper from copper sulfate, 6.5 g of Zinc was added to an excess of copper (II) sulfate solution. The copper was filtered off, washed and dried. The mass of copper obtained was 4.8 g. Calculate the percentage yield of copper?
Step 1: Calculate the Amount, in Moles of Zinc Reacted
Moles of Zinc = 6.5 ÷ 65 = 0.10 moles
Step 2: Calculate the Maximum Amount of Copper that could be formed from the
Molar ratio
Maximum Moles of Copper = 0.10 moles (Molar ratio is 1:1)
Step 3: Calculate the Maximum Mass of Copper that could be Formed
Maximum Mass of Copper = ( 0.10 x 64 ) = 6.4 g
Step 4: Calculate the Percentage of Yield of Copper
Percentage Yield = ( 4.8 ÷ 6.4 ) x 100 = 75%
Percentage Yield of Copper = 75%
What’s percentage purity
Often the product you are trying to fabricate may become contaminated with unwanted substances such as unreacted reactants, catalysts etc.
Equation:
Percentage Purity = (Mass of pure substance / Mass of impure substance) x 100
Calculate the question in an experiment 7.0g of impure calcium carbonate were heated to a very high temperature and 2.5g of carbon dioxide were formed. Calculate the percentage purity of the calcium carbonate.
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Equation Of Reaction:
CaCO3 (s) → CaO(s) + CO2 (g)
Step 1: Calculate the relative formula masses
1 mole CaCO3 → 1 mole CO2
40+12+(3×16) 12+(2×16)
100 → 44
Step 2: Calculate the theoretical mass of calcium carbonate used if pure
From 2.5g CO2 we would expect 2.5/44 x 100 = 5.68g
Step 3: Calculate the percentage purity
(Mass of pure substance / mass of impure substance) x 100
= 5.68/7.0 x 100
= 81.1%
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