C3 - Quantitative Chemistry Flashcards
Relative formula mass, the mole, conservation of mass, mole and equations, limiting reactants, gases and solutions, concentration calculations, atom economy, percentage yield.
Relative Formula Mass -
What is the relative formula mass?
Relative atomic masses of all the atoms in the molecular formula added together.
Relative Formula Mass -
What is the symbol for relative formula mass?
Mr
Relative Formula Mass -
What is the equation to calculate % by mass of an element in a compound?
Ar x number of atoms of that element
——————————————————- X 100
Mr of the compound
The Mole -
What is The Mole?
The name given to an amount of a substance.
The Mole -
What is Avagadro’s constant?
6.02 x 10 to the power of 23
The Mole -
What is one mole of a substance?
An amount of that substance that contains an Avagadro number of particles - 6.02 x 10 to the power of 23
The Mole -
What can the particles in a mole of avagadros constant be?
atoms, molecules, ions or elements
The Mole -
Why is the number for Avagadros constant so long?
One mole of atoms or molecules of any substance will have a mass in grams equal to the relative formula mass for that substance. eg Carbon has an Ar (relative atomic mass) of 12 and so one mole of carbon weighs exactly 12g
The Mole -
What is the equation to calculate the number of moles in a given mass?
Mr (of element or compound)
Conservation of Mass -
What happens to the mass in a chemical reaction?
Mass is always conserved because no atoms are destroyed or created meaning same number + types of atoms on each side of a reaction equation - mass conserved
Conservation of Mass -
What is involved if the mass of a chemical reaction appears to have changed?
A gas
Conservation of Mass -
Why might the mass of a chemical reaction have appeared to increase?
- One reactant is a gas (eg oxygen) found in air and all products are solids, liquids or aqueous.
- Before reaction, gas floating in air not in vessel so can’t account for its mass
- gas reacts form product, contained in vessel - total mass increase
eg metal + oxygen = metal oxide
Conservation of Mass -
Why might the mass of a chemical reaction have appeared to decrease?
- One of the products is a gas and all reactants are solids, liquids or aqueous.
- before reaction, all reactant in vessel
- vessel not enclosed gas escape - can’t account for mass so mass decreases.
eg metal carbonate = metal oxide + carbon dioxide
(gas expands to fill container in, so if not sealed will expand out from vessel and escapes into the air around).
The Mole and equations -
What do the large numbers in front of chemical formulas of the reactants and products tell you?
How many moles of each substance takes part/ formed during the reaction
The Mole and equations -
What do the little numbers tell you after the chemical formulas of reactant and products tell you?
How many atoms of each element there are in each of the substances
The Mole and equations -
How can you balance equations using reacting masses?
- divide mass each substance by its relative formula mass to find number of moles
- divide number of moles of each substance by smallest number of moles in each reaction.
- if any number not integers, multiply all numbers by same amount so become integers.
- write balanced symbol equation for reaction by putting these numbers in front of the chemical formulae.
Limiting reactants -
What is a limiting reactant?
One reactant that is completely used up in a reaction before the rest, the reaction will stop.
Limiting reactants -
What are the other reactants that aren’t the limiting reactant in?
excess
Limiting reactants -
Why are the other reactants added in excess?
Make sure that the other reactant is used up.
Limiting reactants -
Why is it called the limiting reactant?
Limits the amount of product that’s formed.
Limiting reactants -
What is the amount of product formed directly proportional to?
The amount of the limiting reactant.
(because if you add more reactant there will be more reactant particles to take part in reaction, means more product particles).
Limiting reactants -
What is the mass of the product called in a reaction?
The yield.
Limiting reactants -
How can you calculate the mass of a product formed in a reaction by using the mass of limiting reactant and the balanced equation?
- Write balanced equation
- Work out Mr of reactant and product wanted
- find how many moles there are of the substance you know the mass of.
- use balanced equation work out how many moles of the other substance (in this case how many moles of product made of this many moles of the reactant)
- use number of moles to calculate mass
(called theoretical mass)
(also use this method find mass of reactant needed to produce known mass of a product).
Gases and solutions -
What does one mole of any gas occupy and at what temperature?
24 dm cubed at 20 degrees celcius
Gases and solutions -
What will happen to any gas at the same temperature and pressure?
Equal number of moles will occupy the same volume
Gases and solutions -
What happens to any gas at room temperature and pressure?
One mole will occupy 24 dm cubed.
Gases and solutions -
What is the equation to calculate the volume of gas (dm cubed) ?
mass of gas (g)
—————– x 24
Mr of gas
Gases and solutions -
What is concentration?
The amount of a substance in a certain volume of a solution.
Gases and solutions -
What happens with the more solute (substance that’s dissolved) there is in a given volume to the concentration of a solution?
The concentration of the solution with increase
Gases and solutions -
What are the way to measure the concentration of a solution?
Calculate mass of substance in given volume of solution.
Gases and solutions -
What are the two equations to calculate the concentration of a solution?
volume of solvent
volume of solvent
Concentration Calculations -
What are titrations?
Experiments let you find volume needed for two solutions to react together completely.
Concentration Calculations -
How do you find a concentration in g/dm cubed?
By first finding concentration in mol/dm cubed:
1. Calculate Mr of acid
2. convert concentration into concentration in grams by using equation
Mass in grams = moles x Mr
Atom Economy -
What does the atom economy of a reaction tell you?
- How much of mass of reactants is waste when manufacturing a chemical
- how much ends up as useful products
Atom Economy -
What is the equation to calculate atom economy?
Mr of desired products
———————————– x 100
Mr of all reactants
Atom Economy -
What does 100% atom economy mean?
All atoms in reactants turmed into useful (desired) products.
Atom Economy -
What does higher atom economy mean?
The greener the process
Atom Economy -
What are the problems of having a reaction with a low atom economy?
- lots of waste needs to be disposed of
- use up resources quickly
- unsustainable
- not profitable
- raw material expensive
Atom Economy -
What are the reactions with the highest atom economy usually the ones with?
One product (have 100% atom economy)
Atom Economy -
What does having more products mean?
The more products, the lower the atom economy
Atom Economy -
What other factors should be considered when choosing which reaction to use to make a certain product?
- yield
- rate of reaction
- position of equilibrium for reversible reactions
(reaction with low atom economy that produces useful by - products also be used)
Percentage Yield -
What does percentage yield tell you about?
Overall success of an experiment.
Compares theoretical and actual yield
Percentage Yield -
What is the yield?
The amount of product you get.
Percentage Yield -
How does the amount of reactants you start with affect actual yield?
More reactants, the higher the actual yield
Percentage Yield -
What does 0% yield mean?
No reactants were converted into products
Percentage Yield -
What is the equation to calculate percentage yield?
mass of product actually made
———————————————- x 100
maximum theoretical mass of product
Percentage Yield -
What are common problems with percentage yield experiments?
- Not all reactants react to form products
- reversible reactions products turn back into reactants so yield never 100% eg Haber process - Side effects
- reactants sometimes act differently to expected (react with gases in air or impurities in reaction mixture form extra products) - Lose product when separate it from reaction mixture.
- when filter a liquid to remove solid particles, lose bit of liquid + solid
