Moles, Mass and Rearranging Equations for Quantitative Chemistry
Quantitative chemistry • Amount of substance and mass
Flashcards
Test your knowledge with interactive flashcards
Key concepts
What you'll likely be quizzed about
Definition of the mole and Avogadro constant
The mole serves as the unit for amount of substance and represents a fixed number of particles. One mole contains 6.02 × 10^23 entities (particles, atoms, molecules or formula units). The Avogadro constant gives the number of particles per mole and links microscopic particle counts with macroscopic amounts.
Relative formula mass (Mr) and relative atomic mass (Ar)
Relative atomic mass (Ar) is the weighted average mass of an element’s atoms compared with 1/12 of a carbon-12 atom. Relative formula mass (Mr) equals the sum of the Ar values for all atoms in a formula unit or molecule. Mr has no units and provides the mass of one mole of a substance expressed in grams per mole (g mol^-1). Calculations use tabulated Ar values and include whole-number stoichiometric sums; isotopic abundances appear only in more advanced contexts.
Calculating number of moles from mass (n = m ÷ Mr)
The number of moles (n) equals the mass of the sample (m, in grams) divided by the relative formula mass (Mr, in g mol^-1). This relationship arises because Mr equals the mass of one mole, so dividing the sample mass by Mr yields how many moles the sample contains. Significant figures and sample purity limit the precision of the result; reported answers include appropriate units (mol).
Changing the subject of an equation
Algebraic rearrangement isolates a required variable by applying inverse operations to both sides of an equation. Common steps include dividing or multiplying both sides, and swapping numerator and denominator positions when necessary. Brackets and fractions require careful attention: both sides must undergo the same operation, and each step must maintain equality. Symbolic substitution follows rearrangement for numeric calculation.
Calculating mass from number of moles (m = n × Mr)
Mass (m, in grams) equals the number of moles (n, in mol) multiplied by the relative formula mass (Mr, in g mol^-1). This equation directly reverses n = m ÷ Mr. Practical calculations use the correctly rearranged formula, consistent units and appropriate rounding. Mass results reflect composition and sample purity; limiting reactants and yield considerations appear in extended quantitative problems.
Key notes
Important points to keep in mind