2.1.3: amount of substance Flashcards
definition of a mole?
the amount of substance in grams that has the same number of particles as there are atoms in 12 grams of carbon-12
what is avogadro’s constant?
the number of particles in 1 mole. there are 6.02 x 10(23) atoms in 12 grams of carbon-12, therefore 1 mole of any specified entity contains 6.02 x 10(23) of that entity
can be used for atoms, molecules and ions
definition of molar mass?
mass in grams of 1 mole of a substance - given in the unit g mol-1
how do you calculate the molar mass of a compound?
add up all the mass numbers of each element in the compound
what is molar gas volume?
the volume of 1 mole of a gas at a given temperature and pressure (dm3 mol-1)
all gases have this same volume
at what temperature/pressure is the molar gas volume 24 dm3 mol-3?
room temperature = 25•C
room pressure = 1 atm
how do you work out the amount of moles in solids?
moles = mass / molar mass (Mr)
e.g. what is the amount, in mol, in 35.0g of CuSO4?
35 / (63.5 + 32 + (16x4))
= 0.219 mol
how do you work out the amount of moles in gases?
moles = volume / 24
what equation do you use to work out gas volumes that aren’t at room temperature and pressure?
the ideal gas equation
PV = nRT
what are the correct units we use for the ideal gas equation?
Pressure = Pa
KPa -> Pa = x1000
Volume = m3
cm3 -> m3 = / 1000000
Temperature = K
C• -> K = +273
n = moles
R = 8.314 J mol-1 K-3
how do you work out the amount of moles in solutions?
moles = concentration x volume
how do you work out the number of particles in a substance?
number of particles = amount of substance (mol) x avogadro’s constant
e.g. how many atoms of tin are there in a 6.00 g sample of tin metal?
moles = 6/118.7 = 0.05055
atoms = 0.05055 x 6.02 x 10(23)
= 3.04 x 10(22)
what is empirical formula?
the simplest ratio of atoms of each element in a compound
how do you work out the empirical formula of a compound?
- divide each mass (or % mass) by the atomic mass of the element
- divide the answers to step 1 by the smallest answer to get ratio
what is molecular formula?
the actual number of atoms of each element in a compound
how do you work out molecular formula from empirical formula?
work out how many times the mass of the empirical formula fits into the Mr
e.g. work out the molecular formula for the compound with an empirical formula if C3H6O and a Mr of 116
C3H6O = mass of 58
116 / 58 = 2
molecular formula = C6H12O2
what is percentage yield?
how much of the starting materials converted into the desired product (measures the practical efficiency of a process)
what does having a high percentage yield mean?
a high conversion of reactions to products
how do you work out percentage yield?
% yield =
(actual yield /theoretical yield) x 100
25.0g of Fe2O3 was reacted and produced 10g of Fe. what is the percentage yield?
Fe2O3 + 3CO -> 2Fe + 3CO2
- n(Fe2O3) 25/159.6=0.1566
- Fe2O3:2Fe
- 1566:0.313
- mass(Fe)=0.313x55.8=17.48g
- (10/17.48)x100=57.2%
why would % yield be lowered / why’s it not always possible to get the calculated amount of product in a reaction?
incomplete reactions
side reactions
losses during transfers of substances
losses during purification stages
what is atom economy?
the amount of atoms wasted in a reaction - a measure of the proportion of reactant atoms that become part of the desired product
what does having a high atom economy mean?
less waste products/wasted atoms
how do you work out percentage atom economy?
(Mr of useful products/Mr of all products) x 100
what is the % atom economy for the following reaction where Fe is the desired product assuming the reaction goes to completion?
Fe2O3+3CO -> 2Fe+3CO2
(2x55.8)/(2x55.8)+(3x16)+3(12+16)
x 100
=45.8%
what is the atom economy for the production of 1,2-dibromoethane (CH2BrCH2Br) in the following reaction:
H2C=CH2 + Br2 -> CH2BrCH2Br
100% because there’s no waste products, only one product
what are potential errors in using a gas syringe?
- gas may escape before bung is replaced
- syringe may stick
- some gases like CO2 and SO2 are soluble in water so the true amount of gas isn’t measured
what’s the water in a hydrated salt called?
water if crystallisation
