Pakistan MCQs

Chemical reactions rearrange atoms but do not create or destroy them.
Therefore total mass remains constant in a closed system.

A pure compound has constant composition.
This is also called the law of constant composition.

Percent composition is calculated from formula mass contributions.
The element percentages sum to approximately 100%.

An empirical formula shows the lowest integer ratio of elements.
It may differ from the molecular formula.

Stoichiometry uses balanced equations to relate amounts.
It supports mass, mole, particle, and gas calculations.

A molecular formula is a whole-number multiple of the empirical formula.
It applies to substances composed of discrete molecules.

Moles equal mass divided by molar mass.
18 g ÷ 18 g/mol = 1 mol.

Balancing follows conservation of atoms and mass.
Coefficients change amounts, not chemical formulas.

Mass equals moles multiplied by molar mass.
2×58.5 = 117 g.

Equation coefficients give stoichiometric ratios.
They may be interpreted as molecule or mole ratios.

Once the limiting reactant is used up, no more product can form.
Other reactants may remain in excess.

An excess reactant is supplied beyond the stoichiometric requirement.
Some remains when the reaction is complete.