Laboratory Skills 3 : Moles and Molarity

Question 1

what is atomic weight measured in?

Answer 1

AMU - Atomic Mass unit

Question 2

hydrogen atom

Answer 2

• H
• 1 proton
• 1 AMU

Question 3

Carbon Atom

Answer 3

• C
• 6 protons
• 6 neutrons
  12 AMU

Question 4

What is the formal definition of Relative Molecular Mass?

Answer 4

the mass of one molecule of that substance, relative to the unified atomic mass unit u (equal to 1/12 the mass of one isotope of carbon-12 )

Question 5

What is the practical definition of Relative Molecular Mass?

Answer 5

The numbers for molecular weight or atomic weight, but without units. mr is unitless because it is a ratio

Question 6

What do we work in other than AMU?

Answer 6

Grams - kg, g, mg, ng

Question 7

What is the formal definition of Mole ?

Answer 7

“the amount of substance of a system that contains as many elemental entities as there are atoms in 12g of carbon-12”

Question 8

What is Avogadro’s number?

Answer 8

• 6.023 x 1023 (Avogadro’s number) – number of particles in a mole
• 1 mole of C = 6.023 x 1023 atoms
• 1 mole of H2O = 6.023 x 1023 molecules
• One mole of one substance contains the same number of molecules as one mole of another substance

Question 9

Visualising Moles and Molecules

Answer 9

• A mole of one substance has the same number of molecules as a mole of another substance
• If one substance has a higher MW, then it will have a greater mass, mole for mole

Question 10

Practical Application of visualising moles and molecules

Answer 10

if we want to react equimolar amounts of the two substances, we will need a much greater mass of the substance with the higher MW

Question 11

If we have an equal mass of two substances

Answer 11

there will be many more moles of the substance with the lower MW

Question 12

Molar Mass

Answer 12

The Mass of 1 mole (in grams)

• Equal to the numerical value of the average atomic mass (periodic table)
• E.G
  1 g of hydrogen contains the same number of atoms as 12 g of carbon, or 16 g of oxygen

Question 13

What is Molecular Mass/ Molecular Weight ?

Answer 13

If you have a single molecule, mass is measured in amu’s instead of grams. But, the molecular mass/weight is the same numerical value as 1 mole of molecules. Only the units are different. (This is the beauty of Avogadro’s Number!)

Question 14

What is Formula Mass/Formula Weight?

Answer 14

Same goes for compounds. But again, the numerical value is the same. Only the units are different.

Question 15

What is Molarity?

Answer 15

Molarity is the number of moles of a solute that are dissolved per litre of total solution

• A 1M solution contains 1 mole of solute in a final volume of 1 litre.
• Example
  A 1M solution of H2SO4 contains 98.06g of sulphuric acid in a final volume of 1l
• ‘mole’ is an expression of AMOUNT
• ‘molarity’ is an expression of CONCENTRATION (mol/l)

Question 16

What is the equation for Molar concentration?

Answer 16

M = mol / V

Where mol is amount in moles and V is volume in litres

Question 17

How to convert directly from concentration in g/l to molar concentration use?

Answer 17

Cm = C/ M.W

where C = concentration in g/l
Cm= concentration in mol/l

Question 18

What is Normality?

Answer 18

Normality is how acids and bases are usually expressed

• Defined as the gram equivalent weight of the solute per litre of the solvent

Question 19

What is Gram equivalent weight?

Answer 19

Gram equivalent weight is the MW divided by the number of H+ or OH- ions released from 1 molecule of the acid or base, respectively , in solutions

Example

• 1N sulphuric acid MW=98g
• Each molecule of acid releases 2 H+ ions in solution
• Equivalent weight therefore is =MW/2=49g
• So 1l of a 1N H2SO4 solution contains 49g of H2SO4