Module 2.1.3 - Amount of a substance Flashcards
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
How do you prepare a standard solution? (5)
- Accurately weigh the solid (mass by difference method)
- Dissolve the solid in a beaker using a small amount of distilled water
- Carefully transfer the solution into the volumetric flask (& rinse beaker with distilled water and add to flask)
- Fill flask until the bottom of meniscus lines up to the graduation mark
- Invert flask several times (with stopper) to mix the solution
What is molar gas volume, Vm?
The volume per mole of gas molecules at a stated pressure and temperature
What is the molar gas volume at RTP?
24.0 dm3/mol OR 24000 cm3/mol (one mole of gas is 24.0 dm3)
How do you calculate amount of substance of gases AT RTP?
amount, n = volume (dm3/cm3) / Vm (24.0 dm3/mol or 24000 cm3/mol)
What are the two methods of collecting gas?
- Delivery tube + measuring cylinder in a water tray
- Collecting a gas with a gas syringe
What is the equation to find out volume of gas NOT at RTP?
pV = nRT
p = pressure , Pa
V = volume of gas , m3
n = amount of gas , mol
R = gas constant , 8.314
T = temperature , K
What are the conversions needed for ideal gas equation?
1 K Pa = 1000 Pa
1 m3 = 10^-3 dm3 = 10^-6 cm3
Celsius -> kelvin,, add 273
What is the empirical formula?
The simplest whole number ratio of atoms of each element present in a compound
What is the molecular formula?
The actual number of atoms of each element in a molecule
How do you calculate empirical formula?
Find the moles of each element and divide by lowest number of moles. Must be a whole number
(must be +/- 0.1 to be rounded up or down)
How do you calculate the molecular formula?
Use the empirical formula and relative molecular mass
Can add all relative atomic masses of all atoms from empirical formula and divide the empirical mass by the molar mass and then multiply the answer
What does hydrated mean?
When water of crystallisation is present in a crystal compound
What does anhydrous mean?
When all the waters of crystallisation have been removed from a compound
What is water of crystallisation?
The water present in a compound giving the compound a crystalline appearance (. x H2O)
What is uncertainty?
Each piece of equipment has uncertainty, gives more accuracy. More precise the equipment, lower uncertainty.
What assumptions are made about uncertainty on equipment when there is no information present? (2_
- Graduated apparatus: uncertainty assumed to be +/- half of smallest graduation. e.g. graduation 0.1 cm3 -.> uncertainty +/- 0.05 cm3
- Digital apparatus: uncertainty presumed to be resolution of apparatus. e.g 2.d.p balance has uncertainty of +/- 0.01g
How do you calculate % error / % uncertainty?
uncertainty / (recorded measurement) * 100
When calculating % uncertainty, what do you have to take into account?
How many times the apparatus has been used for measurement.
e.g. temperature (change), titration (burette readings), mass readings -» *2 uncertainty
How can % uncertainty be improved (lowered)? (2)
- Increase the mass measurement, percentage uncertainty is lower
- Measure using a more precise equipment (more d.p., +/- 0.001)
