Topic 2- Amount of Substance

Question 1

What is the full definition of relative atomic mass (Ar)?

What is the formula for calculating relative atomic mass (Ar)?

What units does relative atomic mass (Ar) have?

Answer 1

The relative atomic mass (Ar) is the weighted average mass of an element, taking into account its naturally occurring isotopes, relative to 1/12th the relative atomic mass of an atom of carbon-12

Ar= Average mass of one atom of an element -:- mass of an atom of carbon-12 X12

Relative atomic mass (Ar) has no units.

Question 2

What is the full definition of relative molecular mass (Mr)?

What is the formula for calculating relative molecular mass (Mr)

What units does relative molecular mass (Mr) have?

How do we find the relative molecular mass (Mr)?

How do we find this?

Answer 2

The relative molecular mass (Mr) of a molecule is the mass of that molecule compared to 1/12th the relative atomic mass of an atom of carbon-12

Mr= Average mass of one molecule -:- mass of an atom of carbon-12 X12

Relative molecular mass (Mr) has no units

By adding up the relative atomic masses of all the atoms present in the molecule

We find this from the formula (ie periodic table).

Question 3

What is the full definition of relative formula mass?

What symbol is used for relative formula mass?

What units does relative formula mass have?

Answer 3

We use the term relative formula mass for ionic compounds because they don’t exist as molecules

Relative formula mass = Mr

Relative formula mass (Mr) has no units.

Question 4

What two things does using the mole tell you?

What is the full definition of molar mass?

What are the units for molar mass?

How do we find the molar mass for the following:

For an element

For a compound

What is the equation for calculating the number of moles in a substance?

What are the units for each part of the equation?

Answer 4

• How many particles you get in a certain mass
• The mass of a certain number of particles

The molar mass of a substance is the mass of a mole of any element or compound (ie if you take one mole of a substance and weight it, you’ll get the molar mass)

g/mol (preferred unit is g mol (-1)

Directly from the periodic table

Add up the atomic masses

Number of moles (n) = Mass (M) -:- Molar or molecular mass (Mr)

n= number of moles (mol)
M= mass of substance (g)
Mr= molar mass of substance (g mol-1).

Question 5

What is the ideal gas equation?

What does each part of this equation stand for and what is it measured in?

What is the temperature of a gas proportional to?

What is the definition of a newton?

What is pressure usually given in and what is the conversion formula)?

What is the SI unit of volume?

What is the conversion formulas for cm3, dm3 and m3?

What is the SI value for R (gas constant)?

What is temperature given in?

What do you do when converting degrees Celsius to Kelvin?

What are the five other equations for ideal gas equation?

Answer 5

PV = nRT

P= Pressure (Nm-2, Pa)
V= Volume (m3)
n= number of moles (mol)
R= the ideal gas constant (8.31 J K-1 mol-1)
T= Temperature (K)

The temperature of the gas is proportional to the average kinetic energy of the molecules

How much force is required to make a mass of one kilogram accelerate at a rate of one metre per second 2

KPa (kilopascals) (1 KPa = 1000 Pa)

Metres cubed (m3)

1 m3= 1000 dm3= 1,000,000 cm3 and 1 cm3= 10-6 m3

8.31441 J K-1 mol-1

Kelvin (K)

Add 273

V= nRT -:- P ; P= nRT -:- V ; n= PV -:- RT ; T= PV -:- nR ; R= PV -:- nT

Question 6

What is the definition of empirical formula?

What are the three steps for finding the empirical formula?

What is the definition of molecular formula?

What is the molecular formula not necessarily the same as?

What are the three steps for finding the molecular formula?

What does a solution consist of?

How many cm3 are there in 1 dm3?

What are the main four steps for titration calculations?

What question might the exam ask for at the end of the titration calculation?

Answer 6

The simplest whole number ratio of atoms of each element present in a compound

• Find the masses of each element present
• Work out the number of moles of each element (mol= mass / Mr)
• Convert the number of moles

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

The molecular formula is not necessarily the same as the empirical formula

• Find the empirical mass (that’s just the relative mass of the empirical formula)
• Divide the relative molecular mass by the empirical mass. This tells you how many multiples of the empirical formula are in the molecular formula
• Multiply the empirical formula by the number to find the molecular formula

A solvent with a solute dissolved in it

1000 cm3 = 1 dm3

• Number of moles in solution = Conc x Vol (n = cv)
• Moles of X : n =cv
• Moles of X = Moles of Y (sometimes more/less than) = Answer (use from balanced equation)

Might ask for dm3 or cm3 at the end (usually the same as whichever was used at the start).

Question 7

What five things cause a lower percentage yield?

What is the definition of atom economy?

What is atom economy very important in?

What is the formula for calculating atom economy?

What is the formula for calculating percentage yield?

Answer 7

• Impure reactants
• Reversible reactions
• Losing substances in the apparatus
• Not all the products are used (by-products)
• Incomplete reaction

The percentage mass of atoms which end up in the useful product (s)

Very important in working out industrial pathways to manufacture chemicals, as the less waste the better

Mass of useful product (from periodic table) -:- Mass of all reactants (from periodic table) X 100

Actual yield -:- Theoretical yield X 100.