C4 - Chemical Calculations Flashcards
What is the relative formula mass of a substance
The sum of the relative atomic masses of all the atoms shown in its formula
Equation for the percentage mass of an element in its compound
percentage mass of element in compound = (Ar x number of atoms of element) / Mr of the compound X 100
What is the avogadro number of particles
6.02 x 10^23
Equation linking mass Mr and moles
Mass (g) = Mr x Number of moles
Reasons for mass changing in a reaction
- Mass increases if one of the reactants is a gas and all products are solids, liquids or aqueous
- Mass decreases if one of products is gas and all reactants are solids, liquids or aqueous
What is the limiting reactant
One of the reactions which gets used up and causes a reaction to stop
What is an excess reactant
A reactant added in excess to make sure the other (limiting) reactant is used up
How to calculate mass of product formed by using the limiting reactant and balanced reaction equation
- Work out the Mr of reactant and product you want
- Find out how many moles there are of substance you know the mass of (Mol = mass/mr).
- Use balanced equation to work out how many moles there will be of the other substance
- Use number of moles to calculate the mass
Rules for volume of gas
At the same temperature and pressure, equal numbers of moles of any gas will occupy the same volume. One mole of any gas occupies 24dm^3 or 24000cm^3
Equation linking volume of gas to mass and mr
Volume of gas (dm^3) = mass of gas / mr of gas X 24
Equation linking moles of gas to volume
Moles = volume (dm3) / 24 dm3
Moles = volume (cm3) / 24000cm3
what is the concentration of a solution
The amount of a substance in a certain volume of solution. The more solute, the more concentrated the solution
Equation for concentration
(g/dm3)
Concentration (g/dm) = amount of solute (g) / volume of solvent (dm3)
Equation for concentration (mol/dm3)
concentration = amount of solute (mol) / volume of solvent (dm3)
How to get to from dm3 to cm3
x 1000
How to get from cm3 to dm3
divide by 1000
How to get from (mol/dm3) to (g/dm3)
times by the Mr
How to increase the concentration for an aqueous solution
- Add more solute and dissolving it in the same volume of solvent
- Evaporate off some of solvent so you have the same mass of solute in a smaller volume of solvent
What are titrations used for
to determine the volume of acid and alkali that react together completely to form a neutral salt solution.
OR
to determine the concentration of an acid or alkali of the concentration of the reactant is known
Method for titrations
- unknown concentration (usually alkali) is added to a conical flask. usually 25cm3 is added using a pipette to measure
- Add a few drops of phenolpthalein indicator to the solution
- Use a funnel to fill a burette with (acid) of known concentration. Do below eye level and wear safety goggles. Record the initial volume in burette
- Use burette to add the known (acid) solution to the unknown, slowly, giving the conical flask a regular swirl
- When indicator changes colour (from pink -alkali- to colourless -acidic-) the (alkali) has been neutralised
- Record the final volume of the burette and use it, along with initial reading, to calculate the volume of acid needed to neutralise the alkali. Use solution calculation
Considerations for titration
- A white tile should be used to observe colour change
- A test run should be performed first to give a rough idea of how much acid or alkali should be added. After this, a more accurate titration should be done
- Repeat until you get consistent results (within 0.1cm3 of each other)
What does the atom economy of a reaction tell you
How much of the mass of reactants is wasted when manufacturing a chemical and how much ends up as the desired product.
100% atom economy means all atoms in reactants have turned into the desired product.
The amount of reactants that end up as useful products is called the Atom Economy
Disadvantages of reactions with a low atom economy
They are unsustainable and make lots of waste products which can also be expensive to remove and dispose of responsibly. Pollution can also be caused by waste products.
Equation for atom economy
atom economy (%) = Mr of desired product / sum of Mr of all reactants X 100
What does the percentage yield of a chemical reaction tell you
How much product the reaction actually produces compared with the maximum amount that could possibly be produced (100%)
Equation for percentage yield
Percentage yield = mass of product produced / maximum theoretical mass of product X 100
Reasons for NOT getting a 100% yield
- Reaction is reversible
- Some reactants may react to give unexpected or unwanted products in alternative reactions
- some of product may be lost in handling or left on the apparatus
- Reactants may not be pure
- Some of desired product may be lost during the separation from its reaction mixture
