Quantitative Chemistry

Question 1

Percentage mass of an element in a compound

Answer 1

Relative atomic mass of that element / relative atomic mass of the compound

X100

Question 2

Number of moles

Answer 2

Mass in g of an element or compound / relative atomic mass of the element or compound

Question 3

Conservation of mass

Answer 3

During a chemical reaction no atoms are created or destroyed so mass is conserved as there is the same number and types of atoms on each side of a reaction equation

If mass seems to change there is usually a gas involved

If the mass increases it’s probably because one of the reactants is a gas. When the gas reacts to form part of the product it becomes contained in the reaction and the mass increases

If the mass decreases it’s probably because one of the products is a gas. If the reaction isn’t enclosed then the gas can escape and the mass decreases

Question 4

Limiting reactants

Answer 4

Reactions stop when one reactant is used up
The amount of product depends on the limiting reactant

Question 5

Gases and solutions

Answer 5

Mass of gas / mr of gas x 24 = volume of gas

Concentration
= mass of solute / volume of solvent in dm*3

Concentration = number of moles of solute / volume of solvent in mol/dm*3

Question 6

Concentration calculations

Answer 6

Number of moles = concentration x volume

Converting mol/dm3 to g/dm3 you multiply by the MR

Question 7

Atom economy

Answer 7

% of reactants forming useful products

Relative formula mass of desired products / relative formula mass of all reactants
x100

Question 8

Percentage yield

Answer 8

Tells you the overall success of an experiment by comparing what you think you should get by what you actually get

Mass of product actually made / maximum theoretical mass of product x 100

Maximum theoretical mass is the mass in the mass mr mole equation