Topic 1 & 2 Flashcards
Define Relative Molecular Mass
Does this apply to molecules with covalent bonds (like H₂O, CO₂) or ionic compounds (like NaCl, MgCl₂)?
Give some examples.
Relative Molecular Mass (Mr):
Definition: The relative molecular mass is the sum of the relative atomic masses (Ar) of all the atoms in a molecule.
Applies to: molecules with covalent bonds (like H₂O, CO₂).
Example: For water (H₂O):
Ar of H = 1, and there are 2 H atoms.
Ar of O = 16.
Mr = (1 × 2) + 16 = 18.
Define Relative Formula Mass
Does this apply to molecules with covalent bonds (like H₂O, CO₂) or ionic compounds (like NaCl, MgCl₂) and Covalent networks (Silicon dioxide (SiO₂))?
Provide some examples
Relative Formula Mass (Mr):
Definition: The relative formula mass is the sum of the relative atomic masses (Ar) of all the ions in a formula unit.
Applies to: ionic compounds (like NaCl, MgCl₂) because they don’t form molecules but instead exist as a lattice of ions.
Example: For sodium chloride (NaCl):
Ar of Na = 23.
Ar of Cl = 35.5.
Mr = 23 + 35.5 = 58.5.
What are the differences between relative formula mass and relative molecular mass?
Key Differences:
Type of Compound:
Relative molecular mass is used for molecular compounds (covalent).
Relative formula mass is used for ionic compounds.
Structure:
Molecular mass considers discrete molecules.
Formula mass considers the formula unit in the giant ionic lattice.
Examples:
Molecular mass: H₂O, CO₂.
Formula mass: NaCl, MgCl₂.
Explain this learning objective: “the relative formula mass should be used for compounds with giant structures.”
This phrase means that “relative formula mass” is specifically used to describe the mass of compounds that form giant structures like ionic lattices or extended covalent networks, instead of discrete molecules. Here’s why:
Giant Structures Don’t Have Molecules:
Compounds with giant structures (like NaCl, SiO₂) are made of vast, repeating patterns of ions or atoms held together by strong forces.
These don’t form individual molecules, so the concept of relative molecular mass doesn’t apply. Instead, we focus on the mass of a formula unit, which represents the simplest ratio of ions or atoms in the structure.
Examples of Giant Structures:
Ionic compounds: Sodium chloride (NaCl) is a giant ionic lattice made of Na⁺ and Cl⁻ ions in a repeating pattern.
Covalent networks: Silicon dioxide (SiO₂) is a giant covalent structure with a 3D network of Si and O atoms.
Key Point:
For these compounds, the relative formula mass is calculated using the relative atomic masses of the ions or atoms in the simplest ratio shown by the chemical formula.
Example:
For NaCl:
NaCl exists as a giant lattice, not as individual NaCl molecules.
The formula unit is one Na⁺ ion and one Cl⁻ ion.
Relative formula mass = Ar(Na) + Ar(Cl) = 23 + 35.5 = 58.5.
What Does Parts Per Million (ppm) Mean?
What Does Parts Per Million (ppm) Mean?
Definition: “Parts per million” (ppm) is a unit that measures very small concentrations of a substance in a mixture or solution.
It indicates how many parts of a substance are present per million parts of the whole mixture.
Analogy: Think of it like saying, “1 out of 1,000,000.”
Where Is ppm Used?
Environmental Science:
To measure pollutants in air, water, or soil (e.g., CO₂ levels in the atmosphere).
Chemistry:
To express the concentration of solutions when amounts are very small (e.g., trace impurities).
Biology/Medicine:
To indicate the concentration of nutrients or contaminants in biological systems (e.g., fluoride in drinking water).
What is the equation for ppm?
ppm = (mass ofsolution(g) **DIVIDED BY ** mass ofsolute(g)) ×10*6
If the solution is in a liquid state, volume can sometimes replace mass if the density is approximately 1 g/cm³.
Who Developed ppm?
There isn’t a specific person credited with inventing ppm, as it is a simple ratio concept that emerged naturally in science and engineering.
It gained prominence as industries and environmental studies needed standardized ways to express tiny concentrations in measurable units.
What Do “mol dm⁻³” Mean?
What Do “mol dm⁻³” and “g dm⁻³” Mean?
mol dm⁻³ (Moles per cubic decimeter):
This is a unit of concentration that tells you how many moles of a substance are dissolved in 1 cubic decimeter (dm³) of solution.
1 dm³ is equal to 1 liter, so “mol dm⁻³” is the same as saying “moles per liter.”
Example: A solution with 2 mol dm⁻³ of NaCl has 2 moles of sodium chloride dissolved in 1 dm³ of the solution.
What does g dm⁻³ (Grams per cubic decimeter) mean?
This is another unit of concentration, but it tells you how many grams of a substance are dissolved in 1 dm³ of solution.
It’s useful when dealing with masses of solutes rather than moles.
Example: A solution with 10 g dm⁻³ of sugar has 10 grams of sugar dissolved in 1 dm³ of solution.
What is the difference between “mol dm⁻³” and “g dm⁻³”?
Key Difference:
mol dm⁻³ measures concentration in terms of number of particles (moles).
g dm⁻³ measures concentration in terms of mass (grams).
What Are Double Salts?
What Are Double Salts?
Double salts are salts that consist of two different cations or two different anions in a fixed ratio. These salts crystallize together and form a solid that contains both components. They are not the same as mixtures of salts—they have a specific, fixed chemical composition.
Example of Double Salt:
Ammonium iron(II) sulfate: (NH4)2Fe(SO4)2
This salt contains both ammonium ions (NH4⁺) and iron(II) ions (Fe²⁺), and sulfate ions (SO4²⁻).
Define relative mass
Relative mass refers to the mass of an object or a particle compared to the mass of a specific reference unit, usually the mass of a carbon-12 atom. It helps us compare the masses of atoms and molecules without needing to use very tiny units.
Define periodic property
A “periodic property” refers to any characteristic or trend in the elements of the periodic table that repeats in a predictable way at regular intervals as you move across a period (row) or down a group (column).
For example:
Atomic radius: As you move across a period from left to right, the atomic radius decreases due to increased nuclear charge pulling electrons closer. But as you move down a group, the atomic radius increases because additional electron shells are added, making the atom larger.
Ionisation energy: This is the energy required to remove an electron from an atom. It generally increases across a period and decreases down a group.
Electronegativity: This refers to an atom’s ability to attract electrons in a bond. It increases across a period and decreases down a group.
In summary, periodic properties are those that show regular, repeating patterns when elements are arranged by their atomic number in the periodic table.
What does PH mean, and what does it stand for?
pH stands for “potential of Hydrogen” and is a measure of how acidic or alkaline (basic) a solution is. It tells you the concentration of hydrogen ions (H⁺) or protons in a solution.
