Moles
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name the three subatomic particles
• protons • neutrons • electrons
describe the relative charges and positions of each of the subatomic particles
• proton relative charge = +1 • proton position = nucleus • neutron relative charge = 0 • neutron position = nucleus • electron relative charge = -1 • electron position = electron shells
define atomic number
the total number of protons in the nucleus of an atom
define mass number
the total number of protons and neutrons in the nucleus of an atom
define relative atomic mass
the weighted average mass of all the isotopes of an element compared to the mass of 1/12th of a 12C atom
relative atomic mass formula
(mass x abundance) + (mass x abundance) etc.. / 100
what is the symbol for relative atomic mass?
Ar
how do you calculate the relative formula mass (Mr) of a substance?
• find each of the relative atomic mass of each element present in the compound from the periodic table • multiply each of these values by the amount of times they are present in the compound (e.g in H2O, we’d times the RAM of hydrogen by 2 as there are 2 of them) • add these values together to obtain the RFM
what is the relative formula mass equivalent to?
• the relative formula mass (or the Mr) is equivalent to the molar mass (also the Mr) of a substance • the RFM doesn’t have a unit whereas the molar mass is the same number as the RFM, just with the unit g/mol next to it
what is the unit for amount of substance?
the mole (mol)
what does one mole weigh exactly the same as?
one mole weighs exactly the same as the number of atoms in exactly 12g of carbon-12, which is 6.02 x 1023
what is avogadro’s constant?
6.02 x 1023
how do you find the number of atoms in a mole of an element or a compound?
• elements: RAM of element x avogadro’s constant • compounds: RFM of compound x avogadro’s constant
how to calculate the % by mass of an element in a compound?
(RAM of element x number of its atoms present) / (RFM of compound) x 100
moles formula
moles = mass/molar mass
define empirical formula
the formula that shows the simplest whole number ratio of the atoms present in a compound
define molecular formula
the formula that shows the actual number of atoms of each element present in a compound
how do you calculate the empirical formula of a compound?
• write the masses or percentages out in a ratio x:y • divide each number in the ratio by its RAM or Mr • divide each number in the ratio by the smallest number in the ratio to obtain the mole ratio • multiply the ratio until a whole number ratio is obtained • apply this ratio to the compound to obtain its empirical formula
how do you calculate the molecular formula of a compound from its empirical formula?
• calculate the RFM of the empirical formula • calculate a multiplier by doing RFM of molecular formula (given in Q dw) / RFM of empirical formula • multiply the given empirical formula by the multiplier to get the molecular formula
what is the formula for the multiplier used in calculating the molecular formula of a substance from its empirical formula?
multiplier = RFM of empirical formula / RFM of given molecular formula
what are waters of crystallisation?
water molecules which are part of a crystal structure in which they are chemically bonded to a salt
what does the dot in the water of crystallisation mean?
loosely chemically bonded
what part of the formula represents the waters of crystallisation? What is the correct ratio for salt:waters of crystallisation?
• the .nH2O part where n shows the number of water molecules per unit of salt • your final ratio of salt:water should be 1:x otherwise it’s wrong
what is a hydrated salt?
a salt containing waters of crystallisation