# Chemical Calculations

## Chemical Calculations

**Quantitative Chemistry and the Mole Concept**

- The
**mole**is the standard scientific unit for measuring the amount of a substance. It is defined as the amount of a substance that contains as many entities (e.g. atoms, molecules, ions) as there are in 12 grams of carbon-12. - The
**Avogadro constant**(6.022 x 10^{23}mol^{-1}) is the number of entities in one mole of a substance. - The
**molar mass**of a substance is the mass of one mole of that substance. It has units of g mol^{-1}. - The molar mass of an element is numerically equivalent to the relative atomic mass of that element.
- The molar mass of a compound is equal to the sum of the molar masses of its constituent elements.

**Mol, Mass and Concentration Calculations**

- The number of moles of a substance can be calculated using the equation
**n = m/M**, where n is the number of moles, m is the mass in grams, and M is the molar mass. - The
**mass of a substance**can be calculated from the number of moles and the molar mass using the equation m = nM. - The
**concentration of a solution**(in mol L-1) can be calculated using the equation c = n/V, where c is the concentration, n is the number of moles, and V is the volume in litres. - The number of moles in a solution can be calculated from the concentration and the volume using the equation n = cV.

**Percentage Composition and Empirical Formulas**

- The
**percentage composition**of an element in a compound can be calculated by dividing the mass of the element by the total mass of the compound and multiplying by 100%. - The
**empirical formula**of a compound gives the simplest whole number ratio of atoms of each element in the compound. - To determine the empirical formula, convert the mass or percentage composition of each element into moles, then divide by the smallest number of moles to obtain the mole ratio.

**Stoichiometric Calculations in Chemical Equations**

- A
**balanced chemical equation**provides the stoichiometric relationship between reactants and products – that is, the ratio in which the reacting substances are consumed and the products are formed. - The
**stoichiometric ratio**can be used to calculate the moles, mass or volume of reactants or products from the moles, mass or volume of another reactant or product. - In limiting reagent problems, the reactant that is completely consumed and thus determines the maximum amount of product that can be formed is the
**limiting reagent**.