Physical 1 - Amount of Substance

Relative atomic mass (Ar)

The mean mass of an atom of an element, divided by one twelfth of the mean mass of an atom of the carbon-12 isotope.

Relative molecular mass (Mr)

The mean mass of a molecule of a compound, divided by one twelfth of the mean mass of an atom of the carbon-12 isotope.

Relative formula mass

The term used for ionic compounds in place of relative molecular mass.

Mole

A unit of measurement for substances that always contains the same number of particles.

Avogadro Constant (L)

The number that allows the number of particles present in a sample of a substance with known mass to be found.

Ideal gas equation

PV=nRT where P = pressure, V= volume, n = number of moles, R = the molar gas constant and T = temperature.

Moles equation

Moles = mass/Mr

Concentration equation

moles/volume

Concentration equation using mass

mass/volume

Volume of gas equation

Moles x 24

Empirical formula

The simplest whole number ratio of atoms of each element in a compound.

Molecular formula

The true number of each atom in the molecule.

Molar ratios

Used to find the empirical formula.

Percentage yield

A calculation that represents the efficiency of a reaction.

Atom economy

A measure of the amount of starting materials that become useful products in a chemical reaction.

High atom economy

Desirable in industrial chemical processes as it means little or no waste product.

Reacting ratios

Must be correct for chemical equations to be balanced before calculations.

Standard conditions

Conditions under which gases and volatile liquids follow certain trends.

Formula to work out number of particles?

Moles x Avogadro’s constant

How to make a standard solution?

1. Weigh by difference - weigh solid and the weighing boat on a balance, then tip solid into a beaker and weigh only the boat.

2. Add a little distilled water to the beaker, stir with a glass rod until it has dissolved

3. Use a funnel to transfer the solution into a volumetric flask. Rinse equipment and transfer rinsings to the volumetric flask.

4. Use distilled water to make flask up to the mark, and finalise with a dropping pipette - meniscus base should be on the line.

5. Stopper added to flask - invert 10 times to ensure and even conc of moles.

How to measure burette volume accurately?

Read volume from the bottom of the meniscus, and use a piece of paper behind it to cast a shadow onto the meniscus so it is easier to read, read at eye level.

Why do burettes contribute to the precision of the titre?

They have divisions of 0.1cm³ so precise readings are recorded which have little spread around the mean, and measurement precision is determined by limits of the scale on the instrument

what is uncertainty?

Plus or minus half of the smallest division.

Percentage uncertainty formula?

uncertainty/value x100

Why is atom economy useful?

Tells us the efficiency of the reaction so there is less waste

What factors other than atom economy need to be considered?

Raw materials, energy consumption, rate of reaction, conditions needed, purity of product and whether waste products can be used.