Chapter 3- Amount of Substance
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Avagadro’s Constant
the number of atoms in one mole of carbon-12 isotope - 6.02X10^23
Equation for the number of moles of a substance
n=m/M n=number of moles m=mass (g) M= Molar mass (g/mol)
Define a mole
amount of any substance containing as many particles as there are particles in 12g of the carbon-12 isotope
Molar mass
mass (g) of 1 molee of a substance in g/mol
Equation for number of atoms using avagadro’s constant
no, of atoms= (m/ M) x 6.02x10^23 m=mass (g) M= molar mass (g/mol)
Why must you be careful when the question says “amount of substance/moles”
Amount of substance and moles can refer to anything: - 1 mol of H: 1 mole of hydrogen ATOMS - 1 mol of H2: 1 mole of hydrogen MOLECULES
Relative molecular mass
the number of atoms of each element in a molecule
Relative formula mass
the weighted mean mass of the formula unit of a molecule compared to 1/12 the mass of a carbon-12 atom
Empirical formula
the simplest whole-number ratio of atoms of each elements in a compound
How to workout empirical formula from mass
• Convert mass into moles using n=m/M • Find the smallest whole number ratio by dividing both sides by the smallest whole number • Write empirical formula
How to work out the molecular formula
• Convert mass into moles using n=m/M • Find the smallest whole number ratio and then the empirical formula • Write relative mass (M) of empirical formula • Find no of empirical formula units in one molecule (M of molecule/ M of em.form) • Multiply empirical formula by result of step 4
How to convert percentage composition by mass to moles
Think of whole compound as 100%. e.g. 40% C; 6.67% H; 53.33% 0 n= %composition/ Molar mass n(C)= 40%/ 12 =3.33 mol
Components of a hydrated salt
water= water of crystallisation solute= anhydrous salt
How to work out the formula of a hydrated salt
• calculate no. mole for the anhydrous salt using n=m/M • Calculate the no. mole of water using n=m/M • Find smallest whole ratio (tip: make anhydrous salt=1 so ratio is 1:n) e.g. 0.04:0.2= 1:5
Problems to experimental formula of hyrated salts
Assumption 1: All water has been lost Assumption 2: No further decomposition
Equation for the no. of moles using conc & volume
n=cv aka (v=n/c or c=n/v) n= moles c= concentration (moldm^-3) v= volume dm3
Chemistry Conversions (liquid)
1cm3=1ml 1dm3=1000cm3 1000cm3=1000ml=1L
Chemistry conversions (ideal gas equation)
cm3 x(10^-6) = m3 dm3 x (10^-3) = m3 cm3 x (10^-3) = dm3 Celsius +273= Kelvin kiloPascals x(10^3)= Pascals
Molar gas volume (Vm)
the volume per mole of gas molecules at stated temperature and pressure
Conditions for RTP
20 celsius/293 K 101kPA=1ATM At RTP, 1 mole of gas molecules has the volume of 24dm3=24000cm3
Equation for molar gas volume Vm
n=V/ Vm volume (dm3 OR cm3) Molar volume (24dm3 or 24000cm3)
Ideal gas equation
pV=nRT p=pressure (Pa) V= Volume (m3) n=moles R= ideas gas constant (8.314 J/mol/K T= Temperature (Kelvin)
Assumptions on ideal gas equation
random motion elastic colliisons negligible size no intermolecular forces
Electron shell capacity
2.8.18.32.50.72