IAA ACADEMY – Lesson 8: Chemical Formula

IAA ACADEMY – Lesson 8: Chemical Formulas

4.1 Chemical Formula (کیمیائی فارمولا – Keemiyaai Formula)

Dear students, everything around us, whether it’s an element or a compound, needs a specific way to be written down and understood by chemists worldwide. This is where Chemical Formulas come in! They are like the shorthand language of chemistry.

Existence of Elements (عناصر کا وجود – Anāṣir kā Wujood)

Elements exist in different forms in this world.

  • Some elements exist as a collection or aggregate of atoms (ایٹموں کا مجموعہ – atomon kā majmū’ah). These elements are represented simply by their symbols alone (صرف ان کی علامتوں سے – sirf un kī alāmaton se), for example: Na (Sodium), Ca (Calcium), C (Carbon), Fe (Iron), etc. This means they don’t usually exist as individual, distinct molecules.

  • Molecular Elements (مالیکیولی عناصر – Mālikūlī Anāṣir)

    On the other hand, elements like Oxygen (O₂), Nitrogen (N₂), Hydrogen (H₂) exist as discrete molecules (الگ الگ مالیکیول – alag alag mālikūl) in which their atoms are chemically bonded to each other. The subscript number (like the ‘2’ in O₂) tells us how many atoms are bonded in one molecule.

    For example, in ozone (اوزون – Ozone), three atoms of oxygen are bonded to each other, so its chemical formula is O₃.

Existence of Chemical Compounds (کیمیائی مرکبات کا وجود – Keemiyaai Murakkabāt kā Wujood)

Similar to elements, chemical compounds also exist in different forms, and their formulas reflect this.

  • Ionic Compounds (آئنک مرکبات – Ionic Murakkabāt)

    Common salt, which is sodium chloride (سوڈیم کلورائیڈ – Sodium Chloride), exists in the form of ions which are bonded together in a continuous crystal lattice (کرسٹل لیٹس – crystal lattice). Since the ratio between its ions (Na⁺ and Cl⁻) is 1:1, sodium chloride is represented by a formula unit NaCl.

    Similarly, other ionic compounds are represented by their formula units (فارمولا یونٹ – Formula Unit) which show the minimum ratio (کم سے کم تناسب – kam se kam tanāsub) present between their ions. Examples: CaCl₂ (Calcium Chloride), KBr (Potassium Bromide), BaCl₂ (Barium Chloride), etc.

  • Covalent Compounds (کوویلنٹ مرکبات – Covalent Murakkabāt)

    Covalent compounds generally exist as discrete molecules (الگ الگ مالیکیول – alag alag mālikūl) in which atoms are bonded together.

    For example:

    • Water (پانی – Pānī) exists as molecules which are represented by the chemical formula H₂O. It shows that in one molecule of water, two atoms of hydrogen are bonded to one atom of oxygen.
    • Ammonia (امونیا – Ammonia) is represented by NH₃.
    • Methane gas (میتھین گیس – Methane Gas) is represented by CH₄.

    A chemical compound that exists as discrete molecules is thus represented by a chemical formula which is called the molecular formula (مالیکیولی فارمولا – Molecular Formula) of that compound. It shows all the types of atoms bonded together and their exact number in one molecule of that compound.

    Examples of covalent compounds: HCl (Hydrogen Chloride), HF (Hydrogen Fluoride), H₂S (Hydrogen Sulfide), PH₃ (Phosphine), H₂O₂ (Hydrogen Peroxide), H₂SO₄ (Sulphuric Acid), CO₂ (Carbon Dioxide), CO (Carbon Monoxide), C₆H₆ (Benzene), etc.

Importance of Stoichiometry (اسٹویچیومیٹری کی اہمیت – Stoichiometry kī Ahmiyat)

Understanding chemical formulas is crucial for something called Stoichiometry (اسٹویچیومیٹری – Stoichiometry). This is a branch of chemistry that deals with the quantitative relationships between reactants and products in chemical reactions.

Stoichiometry is used in industry quite often to determine the amount of raw materials required to produce the desired amount of the products. For example, a factory making fertilizers needs to know exactly how much nitrogen and hydrogen to use to make a specific amount of ammonia. Without correct formulas, this calculation would be impossible!

4.2 Empirical Formula (ایمپیریکل فارمولا – Empirical Formula)

Definition and Purpose (تعریف اور مقصد – Ta’rīf aur Maqṣad)

The Empirical Formula of a compound shows the minimum (یا سادہ ترین) ratio between atoms present in that compound (اس کمپاؤنڈ میں موجود ایٹموں کے درمیان کم سے کم تناسب – us compound mein maujood atomon ke darmiyān kam se kam tanāsub).

All the ionic compounds are represented by their empirical formulas because they don’t exist as discrete molecules, but as a continuous lattice where the formula unit already shows the simplest ratio.

Example: The empirical formula of calcium fluoride (کیلشیم فلورائیڈ – Calcium Fluoride) is CaF₂, which shows that for every one calcium ion, there are two fluoride ions in its crystal. This is already the simplest ratio.

Difference in Molecular and Empirical Formulas (مالیکیولی اور ایمپیریکل فارمولوں میں فرق – Mālikūlī aur Empirical Formulon mein Farq)

For covalent chemical compounds, which exist as molecules, the empirical formulas may be different from their molecular formulas.

Let’s look at some examples:

  • Hydrogen peroxide (ہائیڈروجن پر آکسائیڈ – Hydrogen Peroxide) is represented by its molecular formula H₂O₂. This tells us there are 2 Hydrogen atoms and 2 Oxygen atoms in one molecule. Its empirical formula will be HO, because the simplest ratio of H:O is 1:1.
  • A benzene molecule (بینزین مالیکیول – Benzene Molecule) has C₆H₆ as its molecular formula. This means 6 Carbon and 6 Hydrogen atoms. So its empirical formula will be CH (simplest ratio 1:1).
  • For water (پانی – Pānī), the molecular formula and the empirical formula are both the same, i.e., H₂O. This is because there does not exist any simpler ratio between hydrogen and oxygen atoms in a water molecule.

Same Empirical Formula for Different Compounds (مختلف مرکبات کے لیے ایک ہی ایمپیریکل فارمولا – Mukhtalif Murakkabāt ke liye Ek Hī Empirical Formula)

Since an empirical formula does not tell us the actual number of atoms present in that compound, rather it represents only the simplest ratio between atoms (ایٹموں کے درمیان سادہ ترین تناسب – atomon ke darmiyān sādah tarīn tanāsub), it is possible that some compounds may have the same empirical formula.

Example: Both benzene (C₆H₆) and acetylene (C₂H₂) have the same empirical formula CH. This shows that while their molecular structures are very different, the simplest ratio of carbon to hydrogen atoms is the same (1:1).

4.3 Chemical Formula of Binary Ionic Compounds (دو عنصری آئنک مرکبات کا کیمیائی فارمولا – Do Anṣarī Ionic Murakkabāt kā Keemiyaai Formula)

Now, let’s learn a practical skill: how to write the chemical formula for binary ionic compounds (دو عنصری آئنک مرکبات – do anṣarī ionic murakkabāt), which are compounds made of only two different elements, one metal and one non-metal.

Steps to Write the Formula (فارمولا لکھنے کے مراحل – Formula Likhne ke Marāhil)

To write the formula of an ionic compound, follow these steps:

  1. First, identify the cations and anions (کیٹائنز اور اینائنز کی شناخت – cations aur anions kī shinaakht) and the number of charges present on them (ان پر موجود چارج کی تعداد – un par maujood charge kī ta’dād). (You can refer to Table 4.1 below for common ions and their charges).
  2. Finally, combine the two ions together to form an electrically neutral compound (دو آئنوں کو آپس میں ملا کر ایک برقی طور پر غیر جانبدار کمپاؤنڈ بنانا – do ionon ko āpas mein milā kar ek barqi tor par ghair jānibdār compound banānā). This means the total positive charge must equal the total negative charge.

Writing Formula from the Name (نام سے فارمولا لکھنا – Nām se Formula Likhna)

If you know the name of a binary ionic compound, you can write its chemical formula:

  • Start by writing the symbol of the cation (مثبت آئن کی علامت – musbat ion kī alāmat) with its charge.
  • Then write the symbol of the anion (منفی آئن کی علامت – manfī ion kī alāmat) with its charge.
  • Figure out how many of these ions are needed to give an electrically neutral compound (برقی طور پر غیر جانبدار کمپاؤنڈ – barqi tor par ghair jānibdār compound).

Example: Lithium Oxide (لیتھیم آکسائیڈ – Lithium Oxide)

Let’s write down the formula of lithium oxide:

  • The symbol of lithium cation with its single positive charge is Li⁺.
  • The symbol of the oxide anion is O²⁻.

Crisscross Method (کرس کراس طریقہ – Crisscross Tarīqah)

This is a simple trick to help you write the formula correctly:

  • In this method, the numerical value of each of the ion charges is crossed over (ہر آئن کے چارج کی عددی قیمت کو کراس کر کے دوسرے آئن کے سبسکرپٹ میں لکھنا – har ion ke charge kī adadī qīmat ko cross kar ke dūsrē ion ke subscript mein likhnā) to become the subscript (سبسکرپٹ – subscript – the small number written below) of the other ion.
  • The signs of the charges are then dropped (چارجز کی علامتیں ہٹا دی جاتی ہیں – charges kī alāmatain haṭā dī jātī hain).

Let’s apply this to Lithium Oxide: 

Cation    → Li¹⁺  (We usually don't write '1' as a subscript)
Anion     → O²⁻

Crisscross the numerical values:
Li₂O₁

Resulting formula → Li₂O

This means you need two Lithium ions (each with +1 charge) to balance one Oxide ion (with -2 charge) to make the compound electrically neutral.

Interesting Information! (دلچسپ معلومات – Dilchasp Ma’lūmāt)

The composition of all the chemical products we use in our lives, such as shampoos, perfumes, soaps, and fertilizers, are formed using stoichiometric calculations (اسٹویچیومیٹرک حسابات – stoichiometric hisābāt).

Without stoichiometry (and thus, correct chemical formulas), the chemical industry as we know it would not exist! It’s the backbone of chemical manufacturing.

Table 4.1 — Atoms and Their Ions (ایٹم اور ان کے آئن – Atom aur Unke Ions)

This table provides a handy reference for common atoms and the charges of the ions they typically form. This will be very useful when you are writing chemical formulas!

Exercise: Apply Your Knowledge!

Let’s practice what we’ve learned about chemical formulas. Click to reveal the answers!