C4: Quantitative chemistry Flashcards
Describe avoados constant
Reactions that are carried out in the laboratory or in an industrial process involve many millions of atoms/ions/molecules. Because these particles are very small, they are counted by weighing them.
Avogadro’s constant, NA, is the conversion factor between the atomic and macroscopic scales. It gives the number of particles in 1 mole (unit: mol) of a substance, and has a value (to 4 significant figures) of 6.022 × 1023 mol−1.
The amount of a substance is the number of moles of that substance.
Just as a collection of 12 eggs is called 1 dozen eggs, 6.022 × 1023 atoms of hydrogen is 1 mol of H atoms and 6.022 × 1023 molecules of H2O is 1 mol of water.
Describe Mr
The relative molar mass/ relative molecular/ formula mass/Mr is calculated by adding together the relative atomic masses, Ar , of all the atoms or ions present .
Because relative atomic masses, and therefore relative molar masses, are ratios of masses, they do not have any units.
The mass of an element is comprised of…
The mass of an element or compound is comprised of the masses of the atoms/ions in the substance.
In 1 mol of calcium carbonate, CaCO3, there is: 1 mol of calcium ions, 1 mol of carbon atoms and 3 mol of oxygen atoms.
1 mol of calcium ions has a mass of 40 g
1 mol of carbon atoms has a mass of 12 g
3 mol of oxygen atoms have a mass of 3 × 16 = 48 g
The total mass of 1 mol of CaCO3 = 40 + 12 + 48 = 100 g
Of that total, 40 g is due to the calcium ions present
therefore the percentage Ca = 40/100 × 100 = 40%
12 g is due to the carbon atoms present, therefore percentage C = 12/100× 100 = 12%
48 g is due to the oxygen atoms present, therefore percentage O = 48/100 × 100 = 48%
what is empirical formula?
The empirical formula of a compound is the simplest whole number ratio of atoms of each element present in the compound.
what is molecular formula?
The molecular formula of a compound is the actual number of atoms of each element present in one molecule of the compound.
The molecular formula is a multiple of the empirical formula, and can be found if the Mr of the compound is given.
What is the maximum mass of magnesium oxide that can be formed if 0.12 g of magnesium is completely burned in excess oxygen?
Mg(s) + 1/2O2(g) → MgO(s)
Mr (MgO) = 24 + 16 = 40
Number of moles of Mg reacting = mass/molar mass
= 0.12/24
= 0.005 mol
From the equation 1 mol Mg forms 1 mol MgO, therefore the maximum amount of MgO that can be formed from 0.0050 mol Mg is 0.005 mol.
Mass of MgO = amount × molar mass = 0.0050 × 40 = 0.20 g
describe gas volumes
The volume of gas molecules is very small compared to the volume occupied by the gas (this is one of the assumptions of an “ideal gas) is considered negligible.
A consequence of this is that the volume of a gas depends on the conditions of temperature and pressure, and not the identity of the gas.
This means that equal volumes of gases, when measured under the same conditions of temperature and pressure, contain the same number of moles (and molecules) of gas.
A car engine burns octane, C8H18 :
2C8H18(g) + 25O2(g) → 16CO2(g) + 18H2O(g)
What is the minimum volume of oxygen that is required to ensure complete combustion of 1 dm3 of octane vapour (all volumes measured at the same conditions of temperature and pressure)?
The equation states that 2 volumes of octane vapour require 25 volumes of oxygen for complete combustion.
2 volumes is equivalent to 1 dm3
therefore, 25 volumes = 12
× 25 = 12.5 dm3
12.5 dm3 oxygen are required to ensure complete combustion of the octane.
Describe molar volume
When the temperature and pressure of a gas is given, its molar volume has a specific value. At room temperature and pressure (rtp), this is often quoted as 24 dm3.
Number of moles of gas = volumemolar volume
(both volumes in the same unit)
describe concentration and the diff units
The concentration of a solution is a measure of the number of solute particles in a given volume of solution.
Because the absolute number of solute particles is usually very large, the concentration of a solution is often given as:
the amount in moles of a solute present in 1 dm3 of solution, unit: mol dm−3
or
the mass of solute in 1 dm3 of solution, unit: g dm−3
(Note that concentration in either unit is ‘in 1 dm3 of solution’ and not ‘in 1 dm3 of solvent’ because when a solute dissolves in a solvent, there is a volume change.)
What is the concentration, in g dm−3, of a solution of 8.0 g sodium hydroxide dissolved in 5.0 dm3 of solution?
Concentration (in g dm−3) = mass of solute (in g)/volume of solution (in dm3)
= 8.05.0
= 1.6 g dm−3
what happens in conc is given as cm3?
if the volume of solution is given in cm3, then the volume has to be converted into dm3.
what is a saturated solution?
A saturated solution is one in which no more solute will dissolve at a given temperature.
what happens to some substances as temp increases?
Not all substances become more soluble as the temperature of the solution increases. For example, gases become less soluble as temperature increases as shown by this graph of the solubility of carbon dioxide.
Why is it not always possible to obtain the predicted mass of a product?
Even though no atoms are gained or lost in a chemical reaction, it is not always possible to obtain the predicted mass of a product because:
the reaction will not go to completion if it is reversible
some of the product may be lost when it is separated from the reaction mixture
some of the reactants may also react in ways different to the expected reaction.
