3.1.2 Amount Of Substances

Question 1

Define relative atomic mass

Answer 1

The average mass of an atom of a particular element compared to 1/12 of the mass of a carbon-12 atom

Question 2

Define relative isotopic mass and give its equation

Answer 2

The average mass of all the isotopes of a particular element compared to 1/12 of the mass of a carbon-12 atom.

Relative isotopic mass = ∑(isotope abundance x isotope mass number) / 100

Question 3

Define relative molecular mass

Answer 3

The average mass of a molecule of a particular molecular compound compared to 1/12 of the mass of a carbon-12 atom

Question 4

What is an empirical formula?

Answer 4

The simplest whole number ratio of the elements present in one molecule

Question 5

Give the equation for percentage yield

Answer 5

Percentage yield = (actual yield / expected yield) x 100

Question 6

Give the equation for molar mass

Answer 6

Molar mass = mass / moles

Question 7

How can you find the limiting reagent of a reaction?

Answer 7

Calculate the number of moles in each individual reactant, taking into account the ratio of the reactants shown in the equation. The reactant that has the smallest number of moles is the limiting reagent.

Question 8

Give the equation for percentage atom economy

Answer 8

Percentage atom economy = (molecular mass of desired product / sum of molecular masses of all reactants) x 100

Question 9

Define water of crystallisation

Answer 9

When a compound can form crystals which have water as part of their structure. A compound that contains water of crystallisation is called a hydrated compound, whereas a compound that doesn’t contain water of crystallisation is called an anhydrous compound.

Question 10

Define the Avogadro constant and give its value and equation

Answer 10

The number of particles equivalent to the relative atomic mass or molecular mass of a substance.

6.02 x 10^23 g/mol

Number of particles = moles x Avogadro constant

Question 11

Give the equation for concentration

Answer 11

Concentration = moles / volume

If volume is in cm^3, then divide by 1,000 to convert it into dm^3.

Question 12

State the kinetic theory of gases and its assumptions

Answer 12

The molecules in gases are constantly moving.
- The gas molecules are moving very fast and randomly
- The molecules hardly have any volume
- The gas molecules don’t attract or repel each other
- No kinetic energy is lost when the gas molecules collide
- The temperature of the gas is related to the average kinetic energy of the molecules

Question 13

Give the ideal gas equation

Answer 13

Pressure (Pa) x Volume (m^3) = moles x gas constant (8.31J/K/mol) x temperature (K)