atoms, ions, and compounds

what is an isotope?

atoms of the same element with different numbers of neutrons with the same number of electrons and protons

why do different isotopes of the same element react in the same way?

neutrons have no effect on the reactions of an element
chemical reactions only involve electrons; isotopes have the same number of electrons in the same arranagement

what is the unit used to measure atomic masses called?

unified atomic mass unit, u

what is relative isotopic mass?

the mass of an isotope relative to 1/12th of the mass of an atom of carbon-12

the relative isotopic mass is the same as which number?

the mass number of the isotope 

what two assumptions are made when calculating the mass number?

the contribution of the electron is neglected, mass of both proton and neutron is taken as 1u

what is relative atomic mass?

the weighted mean mass of an atom of an element relative to 1/12th of the mass of an atom of carbon-12

what does ‘weighted mean mass’ mean?

it takes into account the percentage abundance and relative isotopic mass of each isotope

what are the uses of mass spectrometry?

identify unknown compounds, find the percentage abundance of the isotopes in a sample of an element, determine structural information

how does a mass spectrometer work?

1. the sample in the mass spectrometer is vapourised and ionised to form positive ions

2. the ions are accelerated; heavier ions move more slowly and are more difficult to deflect - so ions of each isotope are separated

3. the ions are detected on a mass spectrum as a mass-to-charge ratio; the greater the abundance, the larger the signal (relative mass/relative charge)

what is the equation for relative atomic mass?

sum of (isotope mass number x isotope abudance) / 100

what is the avagadro’s constant?

6.02 × 10²³ - the number of particles in each mole of carbon-12

what is used to measure the number of particles in a substance?

a quantity called ‘amount of substance’ (n) is used, which is measured in mols

what is the equation for amount of substance (concerning mass)?

amount = mass / molar mass
n = m/M

what is the molecular formula?

the number of atoms of each element in a molecule

what is the empirical formula?

the simplest whole number ratio of atoms of each element in a compound 

what does relative molecular mass do?

it compares the mass of a molecule with the mass of an atom of carbon-12

what does relative formula mass do?

it compares the mass of a formula unit with the mass of an atom of carbon-12

what is analysis?

investigating the chemical composition of a substance

how do you work out the empirical formula from mass?

1. convert mass into moles of atoms using n = m/M

2. to find the smallest whole number ratio, divide by the smallest whole number

3. write the empirical formula

how do you work out the molecular formula?

1. convert % by mass into moles of atoms using n = m/M

2. find the smallest whole number ratio and empirical formula

3. write the relative mass of the empirical formula

4. find the number of the compound units in one molecule e.g. relative molecular mass given in question / relative mass of empirical formula

5. write the molecular formula (empirical formula x answer to step 4)

what is the structure of many coloured crystals?

hydrated - water molecules are part of their crystalline structure
this water is known as water of crystallisation (represented with a dot e.g. CuSO4 • 5H2O)

what is the definition of anhydrous?

a substance that does not contain water

what is the definition of hydrated?

a crystalline compound containing water molecules

what is the definition of water of crystallisation?

water molecules that are bonded into a crystalline structure of a compound

how do you calculate the water of crystallisation as a molar ratio?

1. calculate the moles of anhydrous salt

2. calculate the moles of water

3. find the smallest whole number ratio

4. write down the formula of the hydrated salt

what is assumed when finding the formula of a hydrated salt?

all of the water has been lost, no further decomposition occurred 

what are the volume measurements commonly used?

1cm3 = 1ml
1dm3 = 1l

what is the concentration of a solution?

the amount of solute dissolved in each 1dm3 of solution

what is the equation for amount of substance (concerning concentration)?

amount = concentration x volume
n = c x V

what is a standard solution?

a solution of known concentration

what is the molar gas volume?

Vm - the volume per mole of gas molecules at a stated temperature and pressure, which is 24dm3

what is the equation for amount of substance (concerning volume of gas)?

n = volume of gas / 24

what are the ideal ways in gases can behave?

they are in continuous, random motion
no intermolecular forces 
exert pressure when particles collide with each other or with container
no kinetic energy lost in the collisions

what is the ideal gas equation?

pressure x volume = mol x ideal gas constant x temperature
p (m3) V (Pa) = n (mol) R (8.31 J mol^-1 K^-1) T (K)
it is used when gases in an experiment are at different temperatures or pressures

what are the conversions usually used for the ideal gas equation?

cm3 - m3 = x10^-6
dm3 - m3 = x10^-3
C - K = +273

what is percentage yield?

the efficiency of which reactants are converted into products

what is theoretical yield?

the maximum possible amount of produc

why is theoretical yield hard to achieve?

the reaction may not have gone to completion
other reactions may have taken place alongside the main reaction
purification of the product (e.g. filtration) may result on the loss of some product

what is the calculation for percentage yield?

actual yield / theoretical yield x 100

what is atom economy?

a measure of how well atoms have been utilised

what are two features of reactions with high atom economies?

produce a large proportion of desired products and few unwanted waste products
are important for sustainability as they make the best use of natural resources

what is the equation for atom economy?

sum of molar masses of desired products / sum of molar masses of all products x 100