4.3 - Quantitative Chemistry

What is the law of conservation of mass

Mass of reactants = mass of products

Define relative atomic mass (Ar)

The mass of an atom relative to 1/12 of a carbon 12 atom

Define relative formula mass (Mr)

The sum of all the relative atomic masses of atoms in a substance

What is a mole (mol)

6.02x10^23 particles (the avogadro constant)
number of particles = moles x avagadro’s constant

Mass in grams of 1 mole of atoms/molecules= the Mr of the substance

What is the formula for the number of moles

number of moles = mass ÷ relative formula mass

What are the reasons for an apparent decrease in mass during a reaction

If gas is made during the reaction, and the chamber isn't sealed, gas will diffuse out, so that mass will no longer be accounted for, causing an apparent decrease in mass

What are the reasons for the apparent increase in mass during a reaction

If gas from the surroundings is involved in the reaction, such as oxygen from the air, and the produce a non gas product, there will be an apparent increase in mass, as mass from the surroundings will now be accounted for

Describe the steps to form a balanced symbol equation using the masses

- Divide mass by Mr for each each substance to find moles
- Find the simplest whole number combination of the molar ratios
- Put the numbers in front of the substances in the equations

Describe the steps to find the mass of a substance using the mass of another substance and the balanced symbol equations

- Divide mass by Mr to find moles of the first substance
- Use molar ratios in the equation to substitute in and find the ratio of the other substance, using the known substances moles
- Times the moles by Mr to find mass

What is the limiting reactant

The reactant that is totally consumed during a chemical reaction

What is the term for the reactant which is in the majority

In excess

Describe how to find the limiting reactant

-Find the moles of each substance by dividing mass by Mr
- Work out which is lacking and which is in excess using the molar ratios

Describe how to find the mass of product produced by using the limiting reactant

- Once the limiting has been found use the ratio to find out the moles of the product which will be produced
- Times moles by Mr to find mass

How do you find the volume of gas at room temperature and pressure

volume= mol x 24dm³

How to find atom economy

[note- in this instance multiply the Mr by the molar ratios]

What are the advantages of higher atom economies

- Use up resources at a slower rate

- Don't produce a lot of waste

- More profitable

[more sustainable]

How to find percentage yield

[yield in mass]

Percentage yield is always below 100%. Why?

- Reaction is not completed e.g. its reversible
- Side reactions have occur, so some reactants or products have been used up
- Some product is lost from being left in the reaction medium

How to convert between cm³ to dm³ to m³

cm³ × 1000= dm³

dm³ × 1000= m³

How many dm³ in a litre

1

What is the formula for concentration

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

Concentration(mol/dm³) = moles(mol) / volume(dm³)

How do you convert between g/dm³ and mol/dm³

mol/dm³ x M_r = g/dm³