Unit 3 - Stoichiometry

Question 1

Molecular formula

Answer 1

The molecular formula of a compound is the number and type of different atoms in one molecule.

Question 2

Writing molecular formula

Answer 2

Subscript numbers represent the number of atoms of that element within the compound.

Question 3

Empirical formula

Answer 3

The empirical formula of a compound is the simplest whole number ratio of the different atoms or ions. Useful when very large molecules that contain hundreds, if not thousands of atoms.

Question 4

Ions

Answer 4

An ion is an electrically charged particle formed when an atom (or group of atoms) loses or gains electrons. When an atom becomes an ion it seeks to gain the very stable noble gas electron structure. The charge of the ion shows how many electrons it has gained or lost.

Question 5

Charge of ionic formula

Answer 5

The overall charge of the positive ions and the negative ions must be neutral.

Question 6

Valency

Answer 6

The valency of an element, also known as its combining power, is the number of electrons gained, lost or shared by an atom of the element when it forms a compound. The valency of an element is given by the number of electrons in the outer shell (known as valence electrons) and can be used to determine the charge on an ion.

Question 7

Group 1

Answer 7

All metals in Group 1 have one electron in their outer shell. They can lose this one electron to form ions with a charge of 1+.

Question 8

Group 2

Answer 8

All metals in Group 2 have two valence electrons, so have a valency of 2. They can lose 2 electrons to form ions with a charge of 2+.

Question 9

Group 3

Answer 9

The valency of Group 3 is 3 so in this group three electrons can be lost to form a 3+ charge.

Question 10

Group 4

Answer 10

Group 4 elements have a valency of 4. They do not usually form ions, instead elements tend to achieve a full outer shell by sharing electrons.

Question 11

Group 5

Answer 11

They have a valency of 3 and five outer electrons. These elements can achieve a full outer shell by gaining three electrons to form ions with a 3- charge.

Question 12

Group 6

Answer 12

They have six outer shell electrons and a valency of 2. They can gain 2 electrons to form ions with a 2- charge.

Question 13

Group 7

Answer 13

They have seven outer shell electrons and a valency of 1. They can achieve a full outer shell by gaining one electron.

Question 14

Group 8

Answer 14

They are the noble gases and already have a outer shell so they don’t need to gain or lose electrons.

Question 15

Chemical reactions

Answer 15

It is a process when one set of substances, the reactants, are converted into another set of substances, the products. Any chemical reaction can be represented by a chemical equation.

Question 16

Chemical equation

Answer 16

• A word equation in which the chemical names of all reactants and products are displayed.
• A symbol equation in which the names of the substances are replaced by their elemental symbols.

Question 17

Polyatomic ions

Answer 17

Some ions are composed of more than one element.

Question 18

Balancing equations

Answer 18

When creating a symbol equation the number of atoms present on the left and right for each element must be the same. The total mass of the reactants must equal the total mass of the products.

Question 19

Balancing complex equations

Answer 19

With more complicated equations it is useful to attempt to balance one atom at a time.

Question 20

State symbols

Answer 20

State symbols add extra information that allow the reader to know which physical state each substance is during the reaction.

Question 21

Aqueous solution

Answer 21

The is used to represent a substance fully dissolved in water. This is called the aqueous state and is represented by: (aq)

Question 22

State symbols (representation)

Answer 22

• Solid = (s)
• Liquid = (l)
• Gas = (g)

Question 23

Molecular ions (polyatomic ions)

Answer 23

Compounds comprised of atoms that have been covalently bonded together but contain unequal numbers of protons and electrons.
If there is more than one molecular ion in a compound, then brackets are required to make sure the reader understands which ions are involved. Also put brackets around it to show how many of them there are.

Question 24

Ionic compounds

Answer 24

Formed between two or more ionic molecules. The formula of an ionic compound can be involving a molecular ion can be worked out in the same way as of an ionic compound formed of a metal and non metal. Charges must balance each other an result in a overall charge of zero.

Question 25

Relative atomic mass

Answer 25

The standard unit comes from another atom, carbon-12. Carbon was chosen because it has a mass of exactly 12. The mass of every other atom is give relative to it. It is defined as the number of times heavier an average atom of a specific element is than 1/12th of the mass of carbon. It depends on the abundance (how much) 0f each isotope of that element exists.

Question 26

Relative molecular mass

Answer 26

It is the combined relative atomic mass of all the atoms in a compound or molecule.

Question 27

Relative formula mass for ionic compounds

Answer 27

Ionic compounds do not exist as molecules so when dealing with their mass, we use relative formula mass. Calculated same way as relative molecular mass.

Question 28

Calculating reacting masses

Answer 28

In a balanced equation, the number of each element must be the same on both sides of the equation. This means that the total mass of the reactants equals the total mass of the products. You can predict the relative masses of products and reactants involved in a chemical reaction with appropriate data.

Question 29

Calculating reacting masses (method)

Answer 29

1. Create a balanced symbol equation for the reaction.
2. Work out the relative atomic masses and relative molecular masses of all substances present. Add the total mass of the reactants together and check it equals the total mass of the products.
3. Use the given data to solve the problem.

Question 30

The Avogadro constant

Answer 30

In a chemical reaction there isn’t just one atom or molecule there is actually a huge number of them. These numbers a so big we refer to them as a mole.

Question 31

Mole

Answer 31

I mole contains exactly 6.02 x 10^23 elementary entities. An elementary entity can be an atom, a molecule, an ion, etc. This number is also referred to as the Avogadro constant. Constant because the number never changes.

Question 32

Mole (Relation to Ar/Mr)

Answer 32

1 mole of a substance is related to its Ar/Mr. I mole of a substance is it’s Ar/Mr in grams.

Question 33

Mole (equation)

Answer 33

Mass (g) = Ar/Mr x Moles (mol)

Question 34

Molar gas volume

Answer 34

Under the same temperature and pressure conditions, equal volumes of different gases would contain an equal number of molecules. 1 mole of a gas occupies a volume of 24 dm^3 (24 000 cm^3).

Question 35

Mole (formula with gas)

Answer 35

The number of moles of a gas at room temperature and pressure can be calculated using the formula:

moles (mol) = gas volume (V)/24

Question 36

Molar mass

Answer 36

The molar mass is the mass of one mole, so it is the same as relative molecular mass, Mr, expressed in grams:

moles (mol) = mass (g)/molar mass (g/mol)

Question 37

Mass of chemical equation

Answer 37

The concept of the mole being a constant number of atoms to enable us to work out the mass of a substance formed during a chemical reaction.

Question 38

Concentration

Answer 38

Method that shows the proportion of the amount of substance contained in a solution. Formula:

concentration (g/dm^3) = mass(g)/volume(dm^3)

Question 39

Molar concentration

Answer 39

The amount of moles of a substance dissolved in 1 dm^3. Formula:

concentration (mol/dm^3) = moles (mol)/volume (dm^3)

Question 40

Titration

Answer 40

A titration is performed to determine the concentration of a solution.

Question 41

Molecular formula

Answer 41

Molecular formula indicates the exact number of atoms in the case of the Alkenes (example is ethane).

Question 42

Limiting reagents

Answer 42

In chemical reactions sometimes there are reactants in excess. The limiting reagent is the element in the chemical reaction that is completely used up. The amount of product formed is limited by this element.

Question 43

Percentage yield

Answer 43

The number of mole of product formed according to an equation is the theoretical yield. The actual yield is the number of moles (or total mass) of a substance obtained experimentally. The percentage yield is the percentage of theoretical yield that is actually obtained when the reaction is carried out during a controlled reaction:

percentage yield = actual yield/theoretical yield x 100

Question 44

Percentage purity

Answer 44

percentage purity = mass of pure product/mass of impure product x 100