Structure 1.4 Counting particles by mass: the mole

Structure 1.4.1 Structure 1.4.2 Structure 1.4.3 Structure 1.4.4 Structure 1.4.5 Structure 1.4.6

Avogadro’s constant

Avogadro’s constant

6.02 × 10²³

Calculating the amount of substance (N)

N= n (no.of moles) x Na (avagadro’s constant)

Define relative molecular mass (Mr)

The ratio of the mass of a molecule to one-twelfth of the mass of a carbon-12 atom.

Calculating Mr

Add all the Ar’s for all the atoms in that molecules

Define molar mass (M) (g/mol)

mass of 1 mol of that substance
[= to relative atomic mass]

relationship between no.of moles, mass and molar mass

n= m/M

Calculating the number of atoms of an element in a compound

1. Identify the no.of atoms of that element present in that compound

2. multiply the n of that element to the n of the compound (n=m/M)

3. Substitute the no.of moles into N=n/M

Molecular formula

shows the actual no.of atoms of each element in the molecule of that substance

Empirical formula

shows the simplest ratio of atoms of different elements that are present in the substance

Calculating the % composition of a substance

1. Find the mass of each of the elements present [m=n x M]

2. Find the mass of the entire compound

3. Divide the mass of the element with the mass of the compound and divide it by 100

Calculating the empirical formula

1. mass of the elements

2. find no.of moles of the elements

3. divide n by the smallest value of n

Calculating the empirical formula of the reactants from the product

1. mass of the elements

2. n of the elements

3. the n of the reactant= the no.of atoms of the reactant x the n of the compound

4. the mole ratio

Calculating empirical formula from the percentage composition of an element

1. Mass of each element

2. Find the mass of the other element [100-x]

3. Find n of both the elements

4. divide the values by the smallest number

5. Mole ratio of the elements

6. multiply by a factor for whole number

Determining molecular formula of a compound using the empirical formula using the molar mass (with given molar mass)

1. use the empirical formula values to find the molar mass of the empirical formula [M x no.of atoms of that element]

2. Calculate the molar mass of the empirical formula

3. Divide the given molar mass by a number to obtain the calculated molar mass

4. Multiply the empirical formula by the remainder of the division

Calculating concentration (mol/dm³)

concentration (c ) (solute)= no.of moles (n) (solute)/ volume (V) (solution)

relationship between mass concentration, molar concentration and molar mass

Molar concentration (P)= mass concentration (c ) x molar mass (M)

Calculating concentration of a solute in a new solution after transferring a sample of it from a standard solution (mass of solute, volume of volumetric flask (standard solution), volume of pipette, volume of volumetric flask (new solution given)

1. calculate the concentration of solute in standard solution [c=n/v]

2. calculate the n of the solute in the sample [c=n/v]

3. n(sample)= n(new solution)

4. calculate the concentration in new solution

Calculating concentration of a solute in a new solution

required:

1. original concentration of the solute

2. volume of the original solution

3. volume of the new solution
   [C1 x V1= C2 x V2]

What is spectrophotometry and what is it used for?

analytical technique based on measurement of the intensity of visible, UV and near-IR radiation.

Used for determining concentrations of colored substances in solutions

Absorbance

the amount of light absorbed by the sample

Caliberation curve

Absorbance against concentrations

used for determining the unknown concentration of the colored substance in a solution

Ideal Caliberation curves

• Liner

• Pass through the origin

• 45 degree tilt

Avogadro’s law

Equal volumes of any 2 gases at the same temperature and pressure contain equal numbers of molecules

Relationship between no.of moles and volume

(N1/N2)=(V1/V2)

Calculating volume of gases in combustion reactions (Volume of 1 substance given)

1. Identify the ratio of the stoichiometric coefficients

2. multiply the ratio with the volume given to find the volume of the other gas (eg. 2:3 = 3/2 x volume)