Molecular Formulae Flashcards
what is the molecular formula of a compound
the actual numbers of atoms of each element in a compound
what is the molecular formula of butane if its empirical formula is C2H5
C4H10
what do you need in order to determine the molecular formula of a compound
- the empirical formula
- the molecular mass
if the empirical formula of a compound was HO and you know the relative molecular mass of it is 34, how would you calculate its molecular formula
- you would add up the atomic masses of the elements composing the empirical formula
- in this case it is 1 + 16 = 17
- you would then divide the total molecular mass of 34 by this number
- giving you 34 /17 = 2
- meaning you are multiplying the empirical formula by 2
- H2O2
what is the disadvantage that comes with using relative molecular masses when calculating molecular formulae
- it cant be used for ionic compounds
- as the atoms dont have covelant bonds between them
- therefore it is not a molecule nor does it contain any
what do we use rather than relative molecular mass when we want to work with both covelant and ionic compounds
- relative formula mass
- every compound has a mass that is made up of all the element composing it
what is molar mass and what is its unit
- the mass per mole of any substance
- it had the units gmol-1
- its symbol is M
what formula would you use to calculate the amount in moles of a substance
- amount in mol = mass of substance (g) / molar mass (gmol-1)
- n = m / M
what would be the amount in moles of CH4 if 4g of it was available
- 1 mole of CH4 is 12 + 4 = 16g
- so the molar mass (mass per mole) is 16g
- using the formula n = m / M
- you would have n = 4 / 16
- = 0.25
a compound contains the composition by mass: Na = 34.3%, C = 17.9%, O = 47.8% and has a molar mass of 134gmol-1. how would you calculate the empirical formula of the compound
- you firstly need to calculate the empirical formula of the compound
- Ar of Na = 23, C = 12, O = 16
- you divide the percentage by the Ar
- 34.3 / 23 = 1.5, 17.9 / 12 = 1.5, 47.8 / 16 = 3
- simplify those into ratios of 1:1:2
- giving you the empirical formula NaCO2
using the empirical formula, how would you calculate the relative formula mass of the compound
- the Mr of NaCO2: 23 + 12 + (16 x 2) = 67g
- of the molar mass = 134gmol-1
- 134 / 67 = 2
- 2 x (NaCO2) = Na2C2O4
what is the ideal gas equation (which can also be used for volatile liquids above their boiling temperature)
pV = nRT
what is the ideal gas equation used for
finding the amount of a substance in moles
what are the SI units you should be using for pV = nRT
- p = pressure (Pa)
- V = volume (m^3)
- n = amount in moles (mol)
- R = the gas constant (8.31Jmol-1K-1)
- T = temperature (K)
what do you do to convert kPa to Pa
multiply by 10^3
