3- Amount of a Substance Flashcards
What is Avogadro’s Constant (NA)?
The number of atoms in one mole of the carbon-12 isotope. = 6.02 X 10^23
What is a mole?
The amount of any substance containing as many particles as there are particles in 12g of the carbon-12 isotope
What is the equation for the number of moles of a substance?
n=m/Mr
n- number of moles (mol)
m - mass (g)
Mr - molar mass (g/mol)
What is relative formula mass?
The weighted mean mass of the formula unit of a compound compared with 1/12th of an atom of carbon-12
What is the molecular formula?
The number of atoms of each element in a molecule
What is the empirical formula?
The simplest whole-number ratio of atoms of each element in a compound
How is the empirical formula found?
- Mass (%) divided by Molar mass = find the moles of each element
- Divide all by the smallest answer to find the ratio
How is the molecular formula found?
- Find the empirical formula
- Divide molar mass of a molecule by molar mass of empirical formula
- Multiply empirical formula by the result
What is relative molecular mass?
The mass of a molecule compared with the mass of an atom of carbon-12
In hydrated salts, what is the name given to water molecules?
Waters of crystallisation
What is the concentration of a solute?
The amount of solute (in moles) dissolved in each 1dm^3 of solution
What is the equation for finding a solutions concentration?
c=n/v
c - concentration (mol/dm^3)
n - number of moles (mol)
v - volume (dm^3)
What is molar volume?
The volume per mole of a gas (dm^3/mol)
What is the value of molar volume at room temperature and pressure?
1 mole of any gas takes up 24 dm^3/mol at rip
What is the equation for amount in moles of gas when converting between moles and gas volumes?
n = V/mgv
mgv - molar gas volume (dm^3/mol)
n - number of moles (mol)
V - volume (dm^3)
V (dm^3) - n = V/24
V (m^3) - n = V/24000
What are the assumption made about an ideal gas?
Random motion
Elastic collisions
Negligible size
No intermolecular forces
What is the equation for ideal gas law?
pV = nRT
p- pressure (Pa)
V - volume (m^3)
n - number of moles (moles)
R - ideal gas constant (J/mol/K)
T - temperature (K)
What are the standard conditions?
298 K / 25C
101 kPa
What is stoichiometry?
The balancing of numbers giving the ratio of the amount, in moles, of each substance in a balanced equation
What are balanced equations used to find?
-the quantities of reactants required to prepare a required quantity of a product
-the quantities of products that should be formed from certain quantities of reactants
What is the percentage yield?
The actual yield divided by the theoretical yield expressed as a percentage
percentage yield = actual yield / theoretical yield x 100%
Why is theoretical yield so hard to achieve?
-the reaction is incomplete
-other reactions (competing side reactions) may have taken place alongside main reaction giving different products
-loss of product in handling and purification
-reversible reaction
What is a limiting reagent?
The reactant that is not in excess that will be completely used up first and stop the reaction
What is the atom economy?
atom economy = sum of molar masses of desired products / sum of molar masses of all products X100
What are the benefits of a high atom economy?
-produce a large proportion of desired products and few waste products
-important for sustainability as they make the best use of natural resources
What does the hydrated mean?
Water molecules are part of their (coloured crystals) crystalline structure
What does anhydrous mean?
structure that contains no water of crystallisation
How do you calculate the formula of a hydrated salt?
- calculate the amount (in mol) of anhydrous salt
- calculate the mass and amount in mol of water
- find the smallest whole-number ratio
- write down the value of x and the formula of the hydrated salt
What is the method for carrying out an experiment to determine the water of crystallisation in hydrated crystals?
- Weigh an empty crucible
- Add the hydrated salt into the weighed crucible. Weigh the crucible and hydrated salt
- Using a pipe-clay triangle, support the crucible containing the hydrated salt on a tripod.
- Heat the crucible and contents gently for about one minute.
- Then heat it strongly for a further 3 minutes
- Leave the crucible to cool, Then weigh the crucible and anhydrous salt
Why may the experiment to work out the water of crystallisation not be accurate?
We are assuming that:
-all of the water has been lost (if crystals have changed colour we can assume that all water has been removed but we can only see the surface of crystals and some water could be left inside. If hydrated and anhydrous are similar colours can be difficult to see colour change. SOLUTION = keep heating until the mass of the residue no longer changes)
-there is no further decomposition (many salts decompose further when heated. This can be difficult to judge if there is no colour change)
what are the volume measurements commonly used in chemistry?
1 cm^3 = 1ml
1dm^3 = 1000cm^3 = 1000ml = 1 litre
What is the value of the ideal gas constant?
8.31 J/mol^-1/K^-1
How do you convert from cm^3 to m^3?
X 10^-6
How do you convert from dm^3 to m^3?
X 10 ^-3
How do you convert from C to K?
+273
How do you convert from kPa to Pa?
X 10^3
