C4 - Quantitative Chemistry
Chemistry Study Notes
Relative Atomic Mass
Definition: The relative atomic mass is the weighted average mass of an atom of an element compared to 1/12 the mass of an atom of carbon-12.
Relative Formula Mass:
Calculated by adding together the relative atomic masses of all the atoms in the molecular formula.
Example Calculation (MgCl₂):
Relative atomic mass of Mg (Ar) = 24
Relative atomic mass of Cl (Ar) = 35.5
Formula:
ext{Relative Formula Mass of MgCl}_2 = 24 + (2 imes 35.5)
Calculation: 24 + (2 imes 35.5) = 24 + 71 = 95
Therefore, ext{Mr(MgCl}_2) = 95
Calculate the Mass of an Element in a Compound
Percentage Mass Formula:
ext{Percentage mass of an element in a compound} = \frac{\text{Ar of element} \times \text{number of atoms of that element}}{\text{Molar mass of the compound}} \times 100
Example Calculation:
A mixture contains 20% iron ions by mass. Find the mass of iron chloride (FeCl₂) needed to provide the iron:
Ar of Fe = 56
Find mass of iron in a 50g mixture:
Calculation: 50g \times 0.20 = 10g ext{ of iron}
Calculate the mass of iron in iron chloride:
ext{Molar mass of FeCl}_2 = 56 + (2 imes 35.5) = 56 + 71 = 127
Percentage of iron in FeCl₂:
\frac{56}{127} \times 100 = 44.1 \%
Amount of iron chloride needed for 10g iron:
\text{Mass of FeCl}_2 = \frac{10g}{0.441} = 22.6g
Thus, you would need approximately 23g of iron chloride.
the Mole
Avogadro's Constant:
Defined as 6.02 \times 10^{23} particles per mole.
Definition of a Mole:
1 mole of any substance contains Avogadro's array of particles, which can be atoms, ions, or molecules. The unit used to measure the size of particles in chemistry.
Mass Equivalence:
The mass of one mole of atoms or molecules is numerically equal to its relative atomic or formula mass in grams.
Example:
Carbon (C) has an atomic mass (Ar) of 12, thus:
1 mole of carbon = 12g.
Formula for Moles Calculation:
\text{Number of moles} = \frac{\text{mass in grams}}{\text{Molar mass (Mr)}}
Conservation of Mass
Law of Conservation of Mass:
Mass is conserved during chemical reactions; therefore, the mass of the reactants equals the mass of the products.
Example Reaction:
2\text{Li} + \text{F}_2 \rightarrow 2\text{LiF}
Calculation for Left Side:
\text{Li:} 2 \times 7 = 14
\text{F:} 2 \times 19 = 38
Total = 52g.
Right Side:
2 \times (7 + 19) = 2 \times 26 = 52g
Implications of Mass Changes:
Mass increase indicates gas from air is absorbed; gas reacts and remains in the reaction vessel.
Mass decrease indicates a gas product escapes from the vessel and can't be accounted for.
The Mole and Chemical Equations
Big Numbers in Chemical Equations:
Coefficients in front of chemical formulas indicate the number of moles of each substance in the reaction.
Steps to Balance Equations Using Reacting Masses:
Divide the mass of each substance by its relative formula mass to find the number of moles.
Divide the number of moles of each substance by the smallest number of moles in the reaction.
Convert to whole numbers if necessary by multiplying.
Write the balanced equation.
Limiting Reactants
Definition: Limiting reactant is the reactant that is completely consumed first in a reaction, halting the reaction.
Example Process:
When magnesium carbonate reacts with hydrochloric acid, the reaction fizzles out when one reactant is used up.
Product Formation:
The amount of product formed is directly proportional to the amount of limiting reactant present.
Calculating Mass of Product:
Write the balanced equation.
Determine relative molar mass of correct products and reactants.
Use the mole ratio from the balanced equation to find how many moles are needed.
Calculate the mass of the product using moles.
Yield of Reaction: This refers to the mass of actual product obtained compared to theoretical calculations.
Gases and Solutions
Molar Volume of a Gas:
At room temperature (20°C), 1 mole of any gas occupies 24 dm³.
Formula for Volume of Gas Calculation:
\text{Volume of gas (dm}^3) = \frac{\text{mass of gas (g)}}{\text{Mr of gas}} \times 24
Solution Terminology:
Solute: Substance that dissolves (solid) in the solvent.
Solvent: Liquid that dissolves the solid.
Concentration: Measure of the amount of solute in a given volume of solvent.
Higher concentration indicates more solute in the same volume.
Concentration Calculations:
For molarity: \text{Concentration (mol/dm}^3) = \frac{\text{number of moles of solute}}{\text{volume of solvent in dm}^3}
Atom Economy
Definition: Atom economy measures the proportion of reactants that become useful products, indicating how efficiently resources are used in chemical processes.
Formula for Atom Economy:
\text{Atom Economy} = \frac{\text{Relative Formula Mass of Desired Products}}{\text{Relative Formula Mass of Total Reactants}} \times 100
Implications of High Atom Economy:
A higher atom economy contributes to sustainability, reducing waste and balancing cost efficiency.
Low atom economy reactions lead to resource depletion and pollution as waste materials accumulate.
% Yield
Definition: The % yield is the ratio of the actual yield to the theoretical maximum yield possible in a chemical reaction.
Formula for % Yield Calculation:
ext{% Yield} = \frac{\text{mass of products actually made}}{\text{maximum theoretical mass of product}} \times 100 $$
Observations:
% yield can only range between 0% and 100%. A 100% yield indicates that there were no losses or inefficiencies in the process, while a lower percentage indicates losses due to side reactions, incomplete reactions, or inefficiencies during product separation.