6. Relative Formula Mass

Law of Conservation of Mass

no atoms are lost or made during a chemical reaction, so the mass of the products is equal to the mass of the reactants, which is why symbol equations must be balanced

Relative Formula Mass (Mr)

the sum of the relative atomc masses (Ar) of the atoms in the numbers shown in the formula, find the number on top (atomic mass) of each element in the formula and add them all together

in a balanced chemical equation the sum of the relative formula masses of the reactants in the quantities shown..

equals the sum of the relative formula masses of the products in the quantities shown

mass changes when

a reactant or product is a gas

some reactions might appear to have a change in mass but

this can usually be explained to be because a reactant or product is a gas, and its mass is not taken into account

metal + oxygen

mass of oxide produced is greater than the mass of the metal

In thermal decomposition of metal carbonates

carbon dioxide is produced, which escapes into the atmosphere and leaves the metal oxide as the only solid product

percentage by mass

number of atoms an element has x relative atomic mass of element / relative formula mass of compound (whole thing x 100)

atom economy

a measure of the amount of starting materials that end up as useful products. It’s important for sustainable development and economic reasons (save money)

sustainable

being able to meet the needs of the age without sacrificing future generation’s abilities to meet their own needs

% atom economy

Relative formula mass of desired product from equation / sum of relative formula masses of all reactants from the equation (whole fraction x100)

important to have higher atom economy

more sustainable, cheaper, less waste, more efficient, waste products could be pollutants or greenhouse gases

reaction pathway

series of reactions leading to a desired product

factors for choosing a reaction pathway

percentage yield, atom economy, rate of reaction, equilibrium position, usefulness of by-products

why is a specific pathway chosen

higher percentage yield and atom economy, most efficient and economic pathway is chosen