chemistry a levels - foundations in chemistry

molar mass

(Mr) (g mol^-1) this is the mass per mole of a substance

equation for moles

moles = mass/Mr

avogadros constant

number of particles in one mole of a substance 6.02 × 10²³

mass defect

the small amount of mass that is lost

one atomic mass unit is

the standard mass which is 1/12th of C-12

relative atomic mass

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

relative isotopic mass

the mass of an atom of an isotope compared to 1/12of the mass of an atom of carbon 12

mass spectrometer

1. vaporisation

2. ionisation

3. acceleration

4. deflection

5. detection

calculating relative atomic mass using isotope abundance

mass number x percentage/100

electron relative mass is

1/1845

isotopes

atoms of an element with different number of neutrons and therfore different masses

physical properties of isotopes

higher mass means the isotope have a higher melting, boiling and density point

empirical formula

the simplest whole number ration of atoms of each element present in a compound

molecular formula

the actual number of atoms of each element in a molecule

empirical formula example

water crystallisation

colored crystals that are hydrated - water molecules are part of their crystalline structure.

example of water crystallisation

blue crystals of hydrated copper sulfate are heated, bonds holding the water within the crystals are broken and the water is driven off, leaving behind anhydrous copper sulfate

formula for water crystallisation

CuSO₄ . 5H₂O —> CuSO₄ + 5H₂O

a sample of hydrated calcium sulphate, CaSO₄ . xH₂O, has a relative formula mass of 172. what is the value of x

mr CuSO₄ = 40.1 + 32.1 + 60 = 136.2

172 - 136.2 = 35.8

1H₂O = 18

2H₂O = 36 35.8 is nearest to 36 so x = 2

CaSO₄ . 2H₂O

11.25g of hydrated copper sulphate, CuSO₄.xH₂O, is heated until it loses all of its water. Its new mass is found to be 7.19g. What is the value of x?

mass of water = 11.25 - 7.19 = 4.06 g

CuSO₄ H₂O

mass 7.19 4.06

mr 159.6 18

moles 0.0451 0.225

/ smallest 0.0451 0.0451

= 1 4.99 = 5

CuSO₄ . 5H₂O

a sample of hydrated magnesium sulphate, MgSO₄ . xH₂O, is found 51.1% water. what is the value of x ?

MgSO₄ xH₂O

mass 48.9 51.1

mr 120.4 18

moles 0.406 2.838

/ smallest 0.406 0.406

= 1 6.99 = 7

MgSO₄ . 7H₂O