C4 - Chemical Calculations

The sum of the relative atomic masses of all the atoms shown in its formula

percentage mass of element in compound = (Ar x number of atoms of element) / Mr of the compound X 100

6.02 x 10^23

Mass (g) = Mr x Number of moles

• 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

One of the reactions which gets used up and causes a reaction to stop

A reactant added in excess to make sure the other (limiting) reactant is used up

• 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

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

Volume of gas (dm^3) = mass of gas / mr of gas X 24

Moles = volume (dm3) / 24 dm3 Moles = volume (cm3) / 24000cm3

The amount of a substance in a certain volume of solution. The more solute, the more concentrated the solution

Concentration (g/dm) = amount of solute (g) / volume of solvent (dm3)

concentration = amount of solute (mol) / volume of solvent (dm3)

x 1000

divide by 1000

times by the Mr

• 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

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

• 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

• 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)

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

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.

atom economy (%) = Mr of desired product / sum of Mr of all reactants X 100