Quantitative Chemistry (paper 1)

What is the law of conservation of mass?

No atoms are lost or made during a chemical reaction, so the mass of the products equals the mass of the reactants.

What metals form several ions?

Transition metals e.g. Iron (II) or Iron (III)

Negative non metal ions

OH- = hydroxide

NO3 - = nitrate

SO4 2- =sulfate

CO3 2- =carbonate

Positive non metal ions

H+ = hydrogen ion

NH4 + = ammonium

What must the charges of the ions have to cancel out to in an ionic compound?

0

How to find formula of ionic compounds?

Ca2+ OH-

switch and drop to get

Ca(OH) 2

we write the 2 outside of brackets to multiply the hydroxide not just the hydrogen in it’s own.

What is the relative atomic mass? Ar

The average mass of the isotopes of that element weighted to take into account the abundance of each isotope.

What is relative formula mass? Mr

• The sum of the relative atomic masses of the atoms in the numbers shown in the formula.

• Has no units

• Never involves big numbers - numbers at the front

Formula for percentage by mass of an element

Percentage by mass of an element =

(Total Ar of the atoms of that element/ Mr of the compound) x 100

What does 1 mole=?

6.02 × 10^ 23

Formula for the no of moles in an element

Number of moles= Mass(g)/ Relative atomic mass Ar

Formula for the no of moles in a compound

Number of moles= mass (g) / Relative formula mass Mr

What’s Avogadro’s constant?

• The number of atoms, molecules or ions in a mole of a given substance.

• 6.02 × 10^ 23

Calculate the no of moles of atoms in one mole of calcium hydroxide

Ca (OH)2

5 atoms in one mole of calcium hydroxide so

contains 5 moles of atoms.

How to calculate the number of atoms in one mole of Na2O?

3 atoms so

3 x Avogadro’s constant= no of atoms

Calculate the no of atoms in 28 g of lithium. Ar Li= 7

No of moles= 28/7=4

Then do 4 x Avogadro’s constant= no of atoms

Calculate the no of atoms in 56 g of calcium oxide. Ar Ca= 40 Ar O= 16

CaO

Mr=40+16= 56

no of moles= 56/56 = 1

1 mole has 2 atoms so

2 x Avogadro’s constant= no of atoms

What’s a limiting reactant?

The reactant which is completely used up before the other reactants which limits the amount of product that can be made.

What is meant by concentration.

The mass of a solute in a given volume of solution.

OR

The number of moles of a solute in a given volume of solution

What is a solute?

A chemical dissolved in a solvent.

(water is often used in chem as a solvent)

Units for concentration

g/dm³

OR

mol/dm³

Formula for concentration

concentration (g/dm³)= mass (g) / volume (dm³)

OR

concentration (mol/dm³)= no of moles/ volume (dm³)

How does increased mass of solute effect concentration if volume is kept the same

Increases concentration

How does increased volume of solution effect concentration if mass is kept the same?

Decreases the concentration

Reasons why 100% yield isn’t always possible

• Some of the product may be lost when it is separated from the reaction mixture.

• Some of the reactant may react in different ways to the expected reaction so we do not get the product we expect.

• Reaction may be reversible

Formula for percentage yield

% yield= mass of product actually made/ (maximum theoretical mass of product) x 100

Why can’t a yield or atom economy be greater than 100%?

Because that would mean that atoms were created which isn’t possible.

What is atom economy?

A measure of the amount of starting materials that end up as useful products.

Why is atom economy important?

• Because minimising the production of unwanted products saves money. (economic advantage)

• We also increase sustainability by not wasting resources. (environmental advantage).

Formula for atom economy

Atom economy=

Mr of desired products (from equation)/

[sum of Mr of all reactants (from equation).] x100

FROM EQUATION MEANS INCLUDE BIG NUMBERS

How do chemists try to prevent money being wasted?

They try to find a use for the side-products that are produced e.g. in other reactions.

Gas volumes

• One mole of any gas occupies a volume of 24dm³ (at room temperature and pressure)

• Room temperature= 20 degrees C

• Room pressure= 1 atmosphere

Formula for volume of a gas

Volume (dm³)= Number of moles x 24