Module 2.1.3 - Amount of a substance
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What is the relative molecular mass?
The weighted mean mass of a simple covalent molecule compared with 1/12 of the mass of an atom of carbon-12.
How do you calculate the relative molecular mass?
Add up individual relative atomic masses of elements combined within molecule
What is the equivalent of relative molecular mass for giant structures (lattices)?
relative formula mass
What is amount of a substance used for?
Counting atoms and molecules
How is amount of a substance signified and measured?
Signified using symbol, n, and measured using unit of mole
What is a mole?
The amount of a substance containing as many particles as there are carbon atoms in exactly 12g of Carbon-12.
What is molar mass?
Can be applied to any chemical substance (element,molecule,ion) Mass of one mole in grams, units are g/mol Calculated same way as relative molecular/formula masses
How is n, mass and relative molecular mass (Mr) related?
amount, n = mass (in grams) / Mr
What are the conversions to grams?
1 tonne = 10^6 g 1kg = 1000g 1 microgram = 10^-6 g
What is Avogadro’s constant?
The number of particles in one mole of that particle. Given symbol Na. Value of 6.02 * 10^23 mol. In one mole of any substance there are 6.02 * 10^23 particles
How do you work out number of particles?
Na * moles
What is stoichiometry?
The ratio between chemical substances in a reaction
What is need to calculate reacting masses of solids? (3)
• Fully balanced chemical equations for the reaction • The mass of one of the reactants/products • Amount of substance (moles), n = mass/Mr
What are the steps in calculating reacting masses? (4)
• Under balanced equation, fill out the known and unknown values • Work out the number of moles in the known substance (using moles equation, exclude balancing number) • Work out the moles of the unknown (using molar ratio, aka balancing number) • Convert moles to answer the question (using moles equation)
What is percentage (%) yield?
A measure of the efficiency of converting reactants into products Yield - mass of a product obtained from a reaction
How do you work out % yield?
Actual mass of product obtained / Theoretical mass of product * 100 OR Actual mol of product / Theoretical mol of product * 100
Why are 100% yields in practice rarely obtained? (3)
• Reaction may be incomplete, as a reversible reaction occurs, and reaction reaches an equilibrium • Reactants may be impure • Some reactants/products may be lost when preparing/purifying a compound
What are the 4 steps to calculating yield?
• Calculate the known amount of moles (moles equation) of reactant/product used • Use reaction equation to calculate amount in mols of expected amount (theoretical) • Convert theoretical moles into theoretical mass • Compare actual mass to theoretical mass to calculate the % yield.
What is atom economy?
How much of total mass of reactants is converted into desired product. High atom economy = less waste product
How do you calculate atom economy?
INCLUDE BALANCING NUMBERS molecular mass of the desired product / molecular mass of all products * 100
How do you work out the moles in a solution?
Amount, n = concentration (mol/dm^3) * volume (dm^3) volume must be in dm^3 , if volume in cm^3, n = C*V /1000
How do you convert m^3 and dm^3 and cm^3?
1 m3 = 1000 dm3 1 dm3 = 1000 cm3 1 m3 = 10^6 cm3
How do you convert between mol dm3 and g dm3?
Mol dm3 * relative formula mass = g dm3
What is a standard solution?
A solution of known concentration