Moles Flashcards
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 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 element 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
what is an anhydrous salt?
a salt where the waters of crystallisation have been removed
how do you calculate the mass of water lost?
mass of hydrated salt - mass of anhydrous salt
what is the method for producing magnesium oxide?
- COMBUSTION OF Mg METHOD
- weigh the empty crucible w/o lid, and then w/ a ribbon of Mg in it
- set up apparatus w/ bunsen burner, crucible and tripod + gauze
- heat magnesium under a roaring flame and lift lid every few seconds to allow oxygen to enter to react w/Mg
- once a white powder has formed, turn off bunsen burner and allow apparatus to cool
- repeat the experiment X2 to heat contents to a constant mass to ensure experiment’s done
- weigh the apparatus again - there will be an increase in mass as Mg has gained mass of the oxygen to become MgO
what are some areas for error when making MgO from Mg?
- some of the MgO powder may have escaped from the crucible when the lid was lifted = final mass decreases
- not all Mg reacted with the oxygen so the increase in mass would be smaller than expected
- lid not lifted frequently enough
How do you calculate the empirical formula of magnesium oxide
- calculate the mass of Mg and mass of O
- write out the mass of Mg to the mass of O as a ratio in the form x:y
- divide each mass by its RAM
- divide the ratio by the smaller number
- multiply until you get a whole number ratio
- apply the ratio to get MgO
How do you calculate the mass of Mg and the mass of O2 when calculating the empirical formula of magnesium oxide?
- Magnesiummass = (initial mass of crucible + contents of Mg) - (mass of empty crucible)
- Oxygenmass = (final mass of crucible + contents) - (initial mass of crucible + contents of Mg)
what is the method for producing copper from copper(II) oxide?
- REDUCTION OF COPPER(II) OXIDE METHOD
- measure mass of empty reduction tube
- set up apparatus containing a reduction tube w/methane supplied into it, a clamp + stand and a bunsen burner
- flush out the reduction tube with methane to remove any oxygen to prevent an explosion
- ignite the reduction tube + heat CuO for several minutes until it changes from a black powder to pink-brown metal
- turn off the heat and flush out the tube with methane again to prevent the hot copper from reacting with any oxygen in the air
what are some reasons for error in the reduction of copper(II) oxide experiment?
- reduction tube not flushed out with with methane well enough at the start so explosion occurs
- reduction tube not flushed out with methane well enough at the end so final mass of copper would be greater than expected at hot copper would’ve reacted with oxygen in the air
how do you calculate the empirical formula of copper(II) oxide from its reduction experiment?
- calculate the mass of copper and the mass of oxygen
- place the mass of Cu to the mass of O in a ratio in the form of x:y
- divide each element by its RAM
- divide each number by the smallest one in the ratio
- if answer is in decimals, multiply until a whole number ratio is obtained
- apply the ratio to copper(II) oxide
what is percentage yield?
percentage yield shows how much product was actually made in a reaction compared to how much was expected to be made
what is theoretical yield?
the maximum mass of a product expected from a reaction
what is actually being asked of you when maximum mass is mentioned?
theoretical yield
what is actual yield?
the mass of product that is actually obtained from a chemical reaction
what is the formula for percentage yield?
% yield = actual yield / theoretical yield x 100
why is actual yield always lower than theoretical yield?
- gas can escape from the equipment
- reactants may be impure so side reactions may occur so unwanted products are made from reactants
- some products may get stuck inside the equipment so mass of product collected is lower
- spillages + loss of products during separation or purification processes (e.g filtration or distillation)
what are the two types of reactants in an experiment?
excess and limiting reactant
what is the excess reactant?
- the reactant that is in surplus
- not all of it will react - some of it will be left at the end of the reaction, when the limiting reactant is used up
what is the limiting reactant?
- the reactant that gets completely used up during the experiment
- it determines how long the reaction lasts for and how much product is made
what is avogadro’s law?
equal volumes of gases at the same temperature and pressure contain equal numbers of molecules
at constant temperature and pressure, what is one mole of any gas defined as?
the volume occupied by any one mole of gas is approximately 24dm3 or 24000cm3, the molar gas volume
formula for volume of gas
volume of gas = moles x molar gas volume
how many cm3 are in one decimeter cubed?
1dm3 = 1000cm3
if the Q is in cm3 or dm3, then how do you calculate the volume of gas?
- cm3 : volume = moles x 24000
- dm3 : volume = moles x 24
