Stoichiometry: MCQ 2

Empirical Formulas

• The empirical formula of a compound indicates the simplest whole number ratio of elements present in the compound.
• To calculate the empirical formula, analyze the percentage of each element in the compound.
• Example: A compound contains 52.17% carbon, 13.04% hydrogen, and 34.79% oxygen. The empirical formula is C₂H₆O.

Calculating Empirical Formulas

• Calculate the moles of each element using the mass and molar mass (Mr).
• Find the ratio of moles by dividing the smallest number into each number.
• Example: Calculate the empirical formula of copper oxide using reactants and products.

Molecular Formulas

• The molecular formula indicates the actual number of atoms of each element present in a molecule.
• To find the molecular formula, both the empirical formula and molar mass (Mr) are needed.
• Example: Glucose has an empirical formula of CH₂O and an Mr of 180. The molecular formula is C₆H₁₂O₆.

Stoichiometry

• Stoichiometry involves the calculation of reactants and products in a chemical reaction.
• Example: Calculate the mass of MgO formed when 18g of magnesium is burned to produce magnesium oxide.
• Example: Calculate the volume of CO₂ produced when 45g of carbon is burned at s.t.p.

Balancing Chemical Equations

• Balancing chemical equations involves adjusting the coefficients of reactants and products to satisfy the law of conservation of mass.
• Example: Balance the equation for the combustion of methane in oxygen.
• Example: Balance the equation for the reaction of MnS with oxygen.

Calculations Based on Balanced Equations

• Calculate the moles of reactants and products using the balanced equation.
• Example: Calculate the moles of O₂ consumed and the moles of SO₂ formed in the reaction of MnS with oxygen.
• Example: Calculate the mass of MgO formed when 18g of magnesium is burned to produce magnesium oxide.

% Composition by Mass

• Calculate the percentage of an element in a compound using the mass of the element as a percentage of the total mass of the compound.
• Example: Calculate the percentage of nitrogen in ammonium manganate (NH₄MnO₄).

Structural Formulas

• The structural formula of a compound indicates the arrangement of atoms within a molecule.
• Example: The structural formula of ethane is CH₃CH₃.

Balancing Chemical Equations and Stoichiometry

• Use balanced equations to solve questions involving moles, volume, or molecules.
• Example: Calculate the moles of potassium dichromate used, the moles of water made, the volume of chlorine produced, and the number of molecules of chlorine produced in a reaction.

Calculate the empirical formula of a compound from percentage composition. Learn how to determine the simplest whole number ratio of atoms in a compound.