Chapter 3 - Amount of substance Flashcards
What is a mole? And give the equation for it
Measure of amount of substance. To count the number of particles in a substance.
Moles = mass/Ar
or
Moles = mass/Mr
What is the avogadro constant?
Gives the number of particles in one mole of a substance.
6.02 x 10^23 particles/molecules
Number of particles in one mole of carbon-12.
What is the molecular formula?
Number of atoms of each element in a molecule.
What is the empirical formula?
Simplest whole number ratio of atoms of each element in a compound.
Important for substances that do not exist as molecules e.g. ionic compounds or giant covalent.
How do you find molecular formula?
Step 1: Convert % by mass or mass in grams into moles using moles = mass/mr
Step 2:Divide all the values for moles by smallest mole value to give you empirical formula.
Step 3: Divide molecular mass of compound by molecular mass of empirical formula to find how many unit of empirical formula are in the molecular formula.
Step 4: Multiply empirical formula by value found to give molecular formula.
What is the water molecules in hydrated salts called?
Water of crystallisation.
How do you work out the concentration of a solution?
The concentration of a solution is the amount of solute (in moles) dissolved in each 1 dm^3 of a solution.
Concentration (moldm^3) = number of moles/volume (dm^3)
1 moldm^3 solution contains 1 mole of solute dissolved in each 1 dm^3 of solution
What is a standard solution?
A solution of known concentration
Prepared by dissolving an exact mass of the solute in a solvent and making up the solution to an exact volume.
What is the molar gas volume?
Volume per mole of gas molecules at a stated temperature and pressure
At RTP this is 24dm^3mol^-1
How do you work out moles of a gas?
Amount of moles = volume/molar gas volume
What are the standard conditions?
298K and 101Kpa (1atm)
What is the ideal gas equation?
PV = nRT Pressure (Pa) volume (m^3) number of moles of gas (mol) ideal gas constant (8.314mol^-1k^-1) temperature (K)
What assumptions are being made for the molecules making up an ideal gas?
Random motion
Elastic collisions
Negligible size
No intermolecular forces
What is stoichiometry?
Balancing numbers in balanced equations gives the ratio of amount, in moles, of each substance
What do chemists use balanced equations to find?
Quantities of reactants required to prepare a required quantity of a product.
Quantities of product that should be formed from certain quantities of reactants.
How do you find quantities from amounts and equations ? (reacting masses)
Step 1: Work out the amount of moles in whatever you can
step 2: Use equation to work out the amount in moles of the unknown chemical
step 3: work out unknown information
What is percentage yield?
Conversion of reactants into desired product expressed as a percentage.
Percentage yield = actual yield/theoretical yield x 100%
Why is theoretical yield hard to achieve?
Reaction may not go to completion
Side reactions alongside main reactions
Loss of product in separation/ purification/ handling
Reversible reaction
What is the limiting reagent?
The reactant that is completely used up first and stops the reaction.
What is atom economy?
Measure of how well atoms have been utilised
What do reactions with high atom economies have as an advantage?
Produce larger proportion of desired products - and so few unwanted waste products. This is important for sustainability - making best use of natural resources.
Makes industrial uses more efficient, preserves raw materials and reduces waste.
What is the definition of the term first ionisation energy?
Amount of energy required to remove one electron from one mole of gaseous atoms of an element, to form one mole of 1+ ions of an element.
What happens to first ionisation energies down a group? And why?
Ionisation energies decrease down a group. This is because the atomic radius increases due to more electrons - meaning more shells - meaning more internal electron shielding, so nuclear attraction (between nucleus and outer electrons) decreases. Even though nuclear charge increases, this is outweighed by he electron shielding and distance.
What happens to first ionisation energies across a period? And why?
Increases across a period as the atomic radius decreases due to nuclear charge increasing, as the shielding stays the same/ similar. This results in a stronger nuclear attraction (between nucleus and outer electrons).
