Formula Mass

For ionic compounds, such as sodium chloride, there is no discrete individual molecule. Instead, these are the aggregates represented by a formula of the type, (A⁺ B^–)_n.

Solid sodium chloride is represented by the formula (Na⁺Cl^–)_n,or (NaCl)_n. Thus, the simplest formula for sodium chloride can be written as NaCl.

So, the concept of formula mass is used for ionic compounds.

The sum of the atomic masses of all atoms in a formula unit of a substance is called the formula mass of the substance.

Calculating the formula mass of an ionic substance

The formula mass of any ionic compound is expressed as the sum of the atomic masses of all the atoms present in its formula.

For example, for NaCl,

Formula mass of NaCl = (1 × Atomic mass of Na) + (1 × Atomic mass of Cl)

We know that

            Atomic mass of Na = 23 u               and          Atomic mass of Cl = 35.5 u

So, Formula mass of NaCl = (1 × 23 u) + (1 × 35.5 u) = (23 + 35.5) u = 58.5 u

Thus, the molecular mass (or empirical formula mass) of a substance is equal to the sum of the atomic masses of all the elements present in its molecular (or empirical) formula, taking into account the number of atoms of each element present in its molecular (or empirical) formula. 

For example,

Molecular mass of CO₂

               1 C = 1 × 12 u = 12 u

   2 O = 2 × 16 u = 32 u

 Molecular mass of CO₂ = 44 u

Formula mass of Al₂O₃

               2Al = 2 × 27 u = 54 u

              3 O = 3 × 16 u = 48 u

 Formula mass of Al₂O₃ = 102 u

Formula mass of Na₂SO₄.10H₂O

                            2 Na = 2 × 23 u = 46 u

       1 S = 1 × 32 u = 32 u

                              4 O = 4 × 16 u = 64 u

    20 H = 20 × 1 u = 20 u

  10 O = 10 × 16 u = 160 u

Formula mass of Na₂SO₄.10H₂O = 322 u

Formula mass of K₂Cr₂O₇

                 2 K = 2 × 39 u = 78 u

    2 Cr = 2 × 52 u = 104 u

                  7 O = 7 × l6 u = 112 u

Formula mass of K₂Cr₂O₇ = 294 u

The concept of formula mass is applicable only to the ionic substances.

Do It Yourself

Calculate the formula mass of:

1.  KBr
2.  Na₂S
3.  Ca(OH)₂
4.  MgCl₂
5.  NH₄NO₃
6. Cu(NO₃)₂
7.  Na₂CO₃.10H₂O
8.  C₆H₁₂O₆
9.  Fe₂O₃
10.  (NH₄)₂SO₄
11.  P₂O₅
12.  CaSO₄

Prefixes in SI Units

The metric system has been adopted almost around the world. 

But, over the time, a number of other units (multiples and submultiples of the SI units) have come into use. 

This has happened because the SI units of some physical quantities are either too big or too small. 

The units smaller or bigger than those based on metric system are related to the metric unit by the powers of 10. 

Different powers of 10 are indicated by prefixes.

For example, one hundredth part (10^–2) of a metre is called centimetre: centi means one-hundredth.

A list of standard prefixes is given here.

Formula of Oxoacids

The formula of an oxoacids can be written from the formula of the corresponding oxyanion and the sufficient number of hydrogen ions (H⁺) so as to neutralise the charge on it.

Formula of the oxoanion + n H⁺ →  H_n(oxoanion)

For example, the formula of nitric acid can be written as follows:

Nitric acid contains oxyanion nitrate (NO₃^–).

Nitrate ion (NO₃^–) carries one negative charge on it. So, it requires only one hydrogen ion to neutralise its charge.

Therefore, the formula of the oxoacid containing NO₃^– ion = H⁺ + NO₃^– ® HNO₃

*In the case of organic acids, H adds to the COO^– group at the end of the molecule.

Naming Oxoacids

The general formula of oxoacids is H_aX_bO_c where a, b and c are small whole numbers (e.g. 1, 2, 3 …., etc.) and X is an element other than hydrogen and oxygen. Thus, oxoacids contain hydrogen and the corresponding oxyanion. 

Some examples are given below:

The name of an oxoacid can be written as follows:

·         For an acid containing the most common polyatomic ion of its group, simply use the first part of the polyatomic name and follow with the suffix ic and add the term acid at the end.

For example,

Name of HNO₃    =     Nitr         + ic + acid ⇒ Nitric acid

Name of HClO₃   =     Chlor             + ic + acid ⇒ Chloric acid

Name of H₂SO₄   =     Sulphur     + ic + acid ⇒ Sulphuric acid

Name of H₃PO₄   =     Phosphor   + ic + acid ⇒ Phosphoric acid

·         For the acid containing the polyatomic ion with the less oxygen than the ic, use the suffix ous, and add the term acid at the end.

For example,

Name of HNO₂    =     Nitr         + ous + acid ⇒   Nitrous acid

Name of HClO₂   =     Chlor             + ous + acid ⇒   Chlorous acid

Name of H₂SO₃   =     Sulphur     + ous + acid ⇒   Sulphurous acid

Name of H₃PO₂   =     Phosphor   + ous + acid ⇒   Phosphorus acid

Notice that the whole name for sulphur, not just the root, sulph-, is found in the name sulphuric acid.

Similarly, although the usual root for phosphorous is phosph-, the root phosphor- is used for phosphorus-containing oxoacids, as in the name phosphoric acid.

·         For and acid containing the polyatomic with two less oxygens than the ic, use the prefix hypo and the suffix ous, and add the term acid at the end.

For example,

Name of HClO  = Hypo + chlor + ous + acid    ⇒    Hypochlorous acid

Name of HPO₂  = Hypo + phosphor + ous + acid ⇒   Hypophosphorous acid

·         For an acid containing the polyatomic with one more oxygen than the ic, use the prefix per and the suffix ic and add the term acid at the end.

For example,

Name of HClO₄ = Per + chlor + ic + acid ⇒    Perchloric acid

Name of H₂SO₅ = Per + sulphur + ic + acid ⇒   Persulphuric acid

.

ACIDS

A hydrogen-containing compound which gives hydrogen ions (H₃O⁺ or H⁺) when dissolved in water, is called an acid.

Acids can be classified into two groups:

❖ Hydracids: The compounds of hydrogen with highly electronegative

(nonmetallic) elements other than oxygen, are called hydracids.

Examples: HCl, H₂S, HBr, etc. are some typical hydracids.

❖ Oxoacids: The acids which contain an oxygen-containing anion (called

oxoanion), are called oxoacids (earlier called oxyacids). Thus, oxoacids are the

compounds which contain hydrogen, oxygen and another nonmetallic element.

Examples: H₂SO₄, HNO₃, H₃PO₄, H₂CO₃, etc. are some typical oxoacids.

Formula of Hydracids

The formula of a hydracid can be written as follows:

•   Write the symbol of hydrogen (H).
•   Write the symbol or formula of the anion alongwith its valency present in the acid on the right of H.
•   Criss-cross their valency numbers.
•   Divide these numbers (written as subscripts) by a common factor, if required.

For example, the formula of the hydrochloric acid can be written as follows:

Naming Hydracids

Hydracids are obtained by dissolving the corresponding gas in water. These gases are named as covalent compounds.

Name of the gaseous hydracid = Hydrogen + Name of the anion of the second                                               element

For example,

Name of HCl(g) = Hydrogen + Chloride → Hydrogen chloride

Hydracids are named by writing hydro followed by the root of the name of the

second element, then –ic, and finally acid with a space (blank) between ic and acid.

Hydro + Root of the second element + ic + acid

The only exception is HI (aq) in which case o in hydro is dropped while writing its

name.

Some typical hydracids

The gaseous HCl, HBr, H₂S, etc. do not bear the names of acids. They are named as covalent compounds. A compound that dissolves in water to form an acid is called an acid anhydride (acid without water). Only the aqueous solutions of acid anhydrides are named as acids. Therefore,

• HCl(g) is called hydrogen chloride while HCl(aq) is called hydrochloric acid
• HCN(g) is called hydrogen cyanide while HCN(aq) is called hydrocyanic acid.

The distinction is naming the anhydrides and the acids is not critical for oxoacids, because all their anhydrides are different molecules. For example, the anhydride of H₂SO₄ is SO₃, not gaseous H₂SO₄. Thus, H₂SO₄ is always called sulphuric acid, not hydrogen sulphate.

CONVENTIONS IN WRITING SI UNITS

The following convensions (rules) should be strictly followed while writing SI units.

      i.        While writing a unit, only its singular form is used.

For example, the mass of any substance should be written as kg not as kgs.

Distances should be expressed in km not in kms,

e.g., ten kilogram is written as

10 kg                          10 kgs

(correct)                      (incorrect)

    ii.        The abbreviation of any unit does not have full stop (.) at the end, unless it appears at the end of a sentence. Therefore, no full stop is placed either in between or at the end of the symbol of the unit.

For example,

kg                   kg.                                          cm                              cm.

(correct)          (incorrect)                              (correct)                      (incorrect)

   iii.        One space is left between the last digit of a numeral and the symbol of the unit.

For example,

10 m                                       10m                            5 g                                          5g

(correct) (incorrect) (correct) (incorrect)

   iv.        Words and Symbols should not be mixed.

For example,

kilogram per cubic metre           kg/m3             kilogram/m3              kg per cubic metre

(correct)                                  (correct)          (incorrect)                  (incorrect)

    v.        With numerals, the symbol of the unit should be written.

For example,

100 cm                       one hundred cm                   100 centimetres

(correct)                      (incorrect)                              (incorrect)

   vi.        The names of the units derived from people’s names are written in small letters.

For example, the unit named after Joule is written as joule, (symbol: J). The unit of force named after newton is written as newton, (symbol: N).

 vii.        The sign of degree (°) is not written when Kelvin scale is employed.

For example, temperature of 273 kelvin is written as 273 K and not 273°K.

273 K                          273°K

(correct)                      (incorrect)