Topic 1: Stoichiometric Relationships

Question 1

element

Answer 1

A sample substance composed of a single type of atom

Question 2

isotope

Answer 2

Atoms of the same element, having the same no. of protons, but with differing no. of neutrons

Question 3

compound

Answer 3

• molecules made up of atoms of various elements

- they chemically combine together in a fixed ratio

Question 4

mixture

Answer 4

• mixture of elements/compounds
• they are not chemically bonded
• so the individual components retain their individual properties

Question 5

homogeneous mixture

Answer 5

mixture in which all components are in the same state

e.g. all gases

Question 6

heterogeneous mixture

Answer 6

mixture in which components are in different states

e. g. - a solid and a liquid
- 2 immiscible liquids, like oil and water

Question 7

Outline the liquid-vapour equilibrium

Answer 7

A liquid in an enclosed chamber will form a dynamic equilibrium with its own vapor:

• fast-moving particles in the liquid escape and become vapor
• slow-moving particles in the vapor becomes part of the liquid

Question 8

Effect of temperature on vapor pressure

Answer 8

• as temp. increases, average speed of particles increases

- so as temp. increases, equilibrium vapour pressure also increases

Question 9

Define the enthalpy of vaporisation

Answer 9

A measure of the energy change when 1 mol of liquid converts to gas at standard pressure

Question 10

What is the relationship between the boiling point of a liquid and the vapor-atmospheric pressure equilibrium?

Answer 10

b.pt of a liquid is determined by the temp at which vapour pressure = atmospheric pressure

Elaboration: food in a pressure cooker is cooked faster because of the alteration in boiling point (70 degrees Celsius instead of 100)

Question 11

mole

Answer 11

The amount of substance containing the same number of atoms as in exactly 12g of the isotope C-12

Question 12

relative atomic mass

Answer 12

• unit: Ar
  The average mass of an atom of the element, taking into account all the isotopes and their relative abundance, as compared to 1 atom of C-12

Question 13

relative molecular mass

Answer 13

• unit: Mr

- sum of the relative molecular mass of all the atoms in a molecule

Question 14

relative formula mass

Answer 14

sum of the relative formula mass of all the atoms in one unit of an ionic compound

Question 15

types of molecular formulas

Answer 15

• empirical formula
• molecular formula
• structural formula

Question 16

empirical formula

Answer 16

• simplest whole number ratio of the atoms of all element in one molecule of the compound

Question 17

molecular formula

Answer 17

• shows the real number of atoms of all elements in one molecule of the compound

Question 18

how to obtain the empirical formula of a molecule

Answer 18

no. of moles = mass* / molar mass

* % mass can also be used here.

Question 19

structural formula

Answer 19

shows arrangements of atoms and bonds in a molecule

Question 20

how to experimentally determine empirical formula

Answer 20

1. Record the weight of a crucible
2. Place the compound (e.g. magnesium ribbon) inside the crucible and weigh it
3. Put the crucible on a clay triangle and strongly heat until the magnesium ribbon begins to burn
4. Lift the lid slightly to allow air to enter and continue until all the magnesium has burned (to form MgO2)
5. Allow the crucible to cool and weigh it
6. If you have common sense and can do basic arithmetic, please utilize those skills here.

Question 21

Avogadro’s Law

Answer 21

an equal vol. of gases, both measured at the same temp. and pressure, contain an equal no. of particles.

v1/n1 = v2/n2

v: volume
n: no. of particles/moles

Question 22

ideal gas concept

Answer 22

• no attractive forces between gaseous particles
• KE of particles are directly proportional to temp
• follows the kinetic particle theory 100%

Question 23

when are real gases closest to the ideal gas concept?

Answer 23

when the temp is high and pressure is low

Question 24

how temperature affects real gases

Answer 24

lower temp → less movement → stronger intermolecular forces

Question 25

how pressure affects real gases

Answer 25

higher pressure → molecules move closer to each other → stronger intermolecular forces

Question 26

ideal gas equation

Answer 26

PV = nRT

P: pressure
V: volume
n: no. of moles
R: ideal gas constant
T: temp (MUST BE IN KELVIN)

Question 27

Boyle’s law

Answer 27

at constant temp , pressure is inversely proportional to volume

P1V1 = P2V2

Question 28

gas law

Answer 28

P1V1 / T1 = P2V2 / T2
OR
PV/T = a constant

Question 29

how to calculate yield from reaction equations

Answer 29

1. Obtain the stoichiometric values of the reaction (molar coefficients of each compound)
2. Use mole equations to find the limiting reagent
3. You can find the max yield from there

Question 30

solutions and concentration equation

Answer 30

conc x vol = amount of solute

Question 31

dilution of solutions equation

Answer 31

c1v1 = c2v2
c = conc
V = vol

Question 32

mole equations

Answer 32

no. of moles = no. of particles / Avogadro’s constant
no. of moles = mass / molar mass
no. of moles = mass / molar vol
no. of moles = concentration x volume

Question 33

law of conservation

Answer 33

matter can neither be created nor destroyed, it can only be changed from one form to another

Question 34

gas and molar volume relationship

Answer 34

• all gases under same conditions have same molar volume

e. g. STP/RTP

Question 35

why is beer stored in cold places?

Answer 35

• they can burst if kept in hotter conditions!

higher temp = higher pressure

Question 36

back-titration

Answer 36

• technique used to analyse the unknown conc. of a solution

- indirectly measures amount of limiting reactant

Question 37

atom economy

Answer 37

• measure of the amount of reactants that become useful products
• ideally no atom should be wasted

Question 38

how does knowledge of atom economy help chemists?

Answer 38

higher atom economy → fewer resources used → less waste

Question 39

how to calculate atom economy

Answer 39

100 x (total mass of desired products) / (total mass of all products)