1 1.4 Counting particles by mass: mole

the SI unit for the amount of substance. - a mole is the amount of a substance that contains exactly 6.022 x 10^23 (avogadros constant) elementary entities

The Avogadro constant (L) is the number of particles (atoms, molecules, or ions) in one mole of a substance. - Equivalent to the number of elementary entities in a sample whose mass in grams equals its relative atomic mass The Avogadro constant applies to atoms, molecules and ions The value of the Avogadro constant is 6.02 x 1023 mol-1

• An elementary entity is the smallest particle of a substance • it could be an atom, molecule, ion, electron or formula unit

the mass of a substance with 6.02 x 10^23 number of particles

0.500 x 6.02 x 10^23 molecules 0.500 mol= 3.01 x 10^23 molecules of water

divide by avogadros constant

multiply by avogadros constant

• The relative atomic mass (Ar) of an element is the weighted average mass of one atom compared to one twelfth the mass of a carbon-12 atom • The relative atomic mass is determined by using the weighted average mass of the isotopes of a particular element • The Ar has no units as it is a ratio and the units cancel each other out

• The relative formula mass (Mr) is used for compounds containing ions • It is calculated in the same way as relative molecular mass

the mass in g of one mole of a substance - this is exactly equal to the substance’s relative atomic mass or relative formula mass expressed in grams. - g mol^-1

Organic compounds often have different empirical and molecular formulae

the increase in relative atomic mass of group 1 elements H, Li, Na and K as we descend the group. Successive elements in the group also have an additional energy level of electrons, which increases the atomic radius and therefore the distance of the outermost electrons from the nucleus. The larger atoms tend to lose electrons more easily, increasing their reactivity as metals.

gives the simplest whole number ratio of atoms of each element present in a compound - it can be determined from percentage composition by mass data

elements are converted into new compounds, typically their oxides, by reaction with oxyge. So the primary data obstained are the masses of carbon dioxide, water, sulfur dioxide and so on which are measured by infrared absorption

The formula of an ionic compound is always written as an empirical formula

multiply/ round ex. ratio of atoms Fe: O= - 1:1.5 - multiplying by 2 gives the ratio 2:3 - Fe2O3

often from combustion data ex. to know the atoms of magnesium that combine with how many atoms of oxygen. - calculate the mol of Mg. the mol of oxygen that reacted from increasing mass by heating - express the ratio

compounds that contain a fixed ratio of water molecules within the crystalline strcuture of a compound - used to analyse a hydrated salt - used to calculate the ratio of water molecules to the anhydrous salt ex. CaCl2 - 4H2O

• divide the percentage by mass of each element by its molar mass, M, to convert it to moles (subtract from 100) • divide the moles of each element by the smallest number to give the ratio of moles • if its molar mass, use the one of the whole compound • approximate to the nearest whole number

• calculate the molar mass • for each element find the number of atoms and divide M then multiply by 100

gives the actual number of atoms of each element in a compound or molecule

• calculate mass of empirical formula • set Mr times X = Mr

• divide the compound’s relative mulecular mass by the relative mass of the empirical formula • thid gives a whole number multiplyers

percentage data effectively gives us the mass present in a 100g sample of the compound.