s1.4 the mole

Question 1

what is a mole?

Answer 1

the mole, symbol mol, is the SI unit for amount of substance (n). it is a measure of the number of specified elementary entities. one mole contains exactly 6.02 x10²³ elementary entities - this is numerically equal to the avogadro constant (L or Nₐ)

Question 2

what is the equation linking number of entities and avogadro’s constant?

Answer 2

number of entities (M) = number of moles (n) x avogadro’s constant (Nₐ)

Question 3

what is relative formula mass?

Answer 3

relative formula mass, Mr, is the weighted average mass of a substance relative to 1/12 the mass of an atom of carbon-12

Question 4

what is molar mass?

Answer 4

the molar mass (M) is the mass of one mole of a substance in grams. the unit for molar mass is g mol⁻¹.

Question 5

what is the equation linking moles, mass and molar mass?

Answer 5

number of moles (n) = mass (m) / molar mass (M)

Question 6

what is water of crystallisation?

Answer 6

epsom salt contains hydrated magnesium sulfate which has the formula MgSO₄.XH₂O. hydrates are ionic compounds that have a definite amount of water as part of the structure. X is known as the water of crystallisation. the value for x can be determined by removing the water by heating and then calculating the number of moles of the water. this can be determined by removing the water by heating and then calculating the number of moles of the water. this can be done by using the change in mass before and after heating the sample. once all the water has been removed the compound is known as anhydrous

Question 7

what equipment do you need for water of crystallisation?

Answer 7

• boiling tube
• boiling tube holders
• balances
• glass beaker
• bunsen burner
• mat

Question 8

what is the method for water of crystallisation?

Answer 8

1. find the mass of an empty boiling tube - record this and all other masses in a suitable table
2. place about 2cm depth of hydrated magnesium sulphate in the boiling tube and find the mass
3. heat the tube to constant mass - if steam is still being given off from the tube, there is still water of crystallisation to be lost

Question 9

how do you analyse the results of water of crystallisation?

Answer 9

1. calculate the moles of anhydrous MgSO₄ at the end (n = m / M)
2. find the number of moles of water (n = m / M)
3. find the molar ratio of MgSO₄ : H₂O (divide by the smaller number of moles)

Question 10

what is molecular formula?

Answer 10

the molecular formula gives the actual number of atoms of each element present in a molecule

Question 11

what is empirical formula?

Answer 11

the empirical formula of a compound gives the simplest ratio of atoms of each element present in that compound

Question 12

how do you calculate empirical formula from percentage composition by mass?

Answer 12

1. check that the percentages add up to 100%
2. divide the percentage of each element by its relative atomic mass
3. divide each number by the smallest ratio - this will give you the empirical formula of the compound
4. to find the molecular formula from the empirical formula, determine the mass of the empirical formula and divide the molecular formula by the mass of the empirical formula

Question 13

how do you calculate percentage composition by mass?

Answer 13

when a compound is put into a mass spectrometer, it can be used to determine the percentage composition of each element by mass. the percentage composition by mass can be used to determine the formula of a compound
& by mass = (Ar of element x number of atoms) / Mr of compound x 100

Question 14

how do you calculate empirical formula from combustion analysis?

Answer 14

1. calculate the mass of carbon in CO₂ and convert to moles
2. calculate the mass of hydrogen in H₂O and convert to moles
3. calculate the mass of oxygen by subtracting the mass of carbon and mass of hydrogen from the original mass of menthol - convert to amount in moles
4. divide each amount by the smallest to get the lowest whole number ratio

Question 15

what equipment do you need to determine the empirical formula of magnesium oxide?

Answer 15

• crucible and lid
• magnesium ribbon
• bunsen burner
• mat
• tripod
• clay triangle
• tongs
• balance

Question 16

what is the method for determining the empirical formula of magnesium oxide?

Answer 16

1. measure the mass of the crucible and lid without magnesium
2. measure the mass of the crucible, lid and magnesium
3. set up the apparatus and heat the crucible, with the lid on, using the roaring flame
4. tilt the lid occasionally with the tongs to allow air in, but don’t let the magnesium oxide out
5. when the magnesium stops flaring, take the lid off and heat strongly for another 3 minutes
6. turn off the bunsen burner and leave the crucible to cool
7. reweigh the crucible and calculate the mass of the solid after oxidation

Question 17

what is a solution?

Answer 17

a solution is a homogenous mixture comprised of a solvent and a solute. solutions are classified by the mass or mole ratio between the solute and solvent.
a concentrated solution will have a large number of solute particles dissolved in the solvent. a dilute solution will have a small number of solute particles dissolved in the solvent

Question 18

what is concentration?

Answer 18

concentration is the amount of solute per unit volume. two common units are used for concentration: g dm⁻³ and mol dm⁻³. the units mol dm⁻³ are sometimes abbreviated to M.

Question 19

what is molar concentration?

Answer 19

the molar concentration is determined by the amount of solute and the volume of solution.
concentration = amount of solute / volume of solution
the molar concentration can also be determined by the mass of solute and the volume of solution.
concentration = mass of solute / volume of solution
molar concentration can be represented by []. eg. [NH₃] = 0.5M

Question 20

what is a standard solution?

Answer 20

a standard solution is a solution of a known concentration. it is also known as the stock solution

Question 21

how do you prepare a standard solution?

Answer 21

1. weigh an empty weighing boat
2. add a mass of copper (II) sulphate and weigh the weighing boat again
3. transfer the sample to a 100cm³ beaker and weigh the weighing boat again
4. record the mass of copper (II) transferred to the beaker
5. dissolve the copper (II) sulphate in approximately 100cm³ of distilled water
6. transfer the solution into a 250cm³ volumetric flask using a funnel
7. make up the volume to the mark with distilled water, using washings from the beaker. invert the contents of the volumetric flask repeatedly

Question 22

how do you calculate percentage uncertainty?

Answer 22

percentage uncertainty = absolute certainty / measured value x 100

Question 23

how do you prepare a secondary solution?

Answer 23

another method of preparing a solution involves making a dilution using a solution that is already prepared. this is sometimes referred to as making a ‘secondary solution’, using a more concentration ‘primary solution’. the amount of solute (n) does not change - therefore using C₁V₁ for the primary solution and C₂V₂, we can combine the two equations

Question 24

how do you identify transition metal ions in solution?

Answer 24

two methods that can be used to quantify the concentration of transition metal ions in solution based on the colour they appear are:
1. spectrophotometry; technique used to study the absorbance of light for a sample across all wavelength of the visible spectrum
2. colorimetry; technique used to study the absorbance of light for a sample at specific wavelengths

if we know the identity of the sample, we can make up solutions of known concentrations. we can analyse the absorbance of light for each of the concentrations and compare the absorbance of the unknown concentration to the known concentrations. this enables us to deduce the unknown concentrations

Question 25

what are the factors affecting gases?

Answer 25

when a sample of gas is seal in a vessel, the particles would:
- possess kinetic energy, so be in constant motion
- collide with each other as well as the sides of the vessel
- the number of particles contained would be the same unless certain factors changed
the factors that can change the number of gas particles include:
1. the volume of the container
2. temperature of the gas
3. pressure of the gas

Question 26

what is avogadro’s law?

Answer 26

the three variables (volume, temperature and pressure) determine the number of gaseous elementary particles that fit into a container.
avogadro’s gas law states that an equal number of gas particles occupy the same volume measured at the same temperature and pressure. it can be used to determine the quantity of gas involved in a chemical reaction
amount of substance ∝ number of particles (or volume)