Lows of Chemical Combinations

8/6/2019 Lows of Chemical Combinations

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Gram molecular Mass: The molecular mass of a substance expressed in

grams is called its gram molecular mass.

(iii) Molecular mass: The average mass of one mole of any material iscalled its molar mass.

Mathematically, Molar mass (M)

Molar mass of any substance = mass of 6.23 x 1023chemical units of the

substance.

(iv) Molar volume: The volume occupied by one mole of any substance iscalled its molar volume. Molar volume of a substance depends upon temperature

and pressure. Molar volume of gaseous substances changes appreciably with

temperature and pressure. The molar volume of all gaseous substances at 273 K

and under 1 atm pressure (NTP conditions) is found to be 22.4 liter or 22400 mL.

The unit of molar volume is liter per mol (L/mol) or milliliter per mol (mL/mol)

Percentage Composition and molecular formula: The mass percentage of

each constituent element present in any compound is called its percentage

composition. The mass percentage of each element can be calculated by using the

relationship.

Mass percentage of element i

The percentage composition of a compound may also be obtained from the

molecular formula of the compound. This is done as follows:

(a) Write down the molecular formula of the compound(b) Calculate the mass of each element present in the molecule of the

substance by multiplying the atomic mass of the element by the number of atomsin the molecule.

(c) Calculate the molecular mass of the compound by adding the massesof all the atoms present in its molecule.

(d) Calculate the mass percentage of each element by using therelationship

Mass percentage of element

Empirical and molecular formula:

Empirical formula: The simplest formula of a substance which gives the

relative number of atoms of each element present in the molecule of thatsubstance is called as the empirical formula.

Molecular Formula: The symbolic representation of a molecule of any

substance describing the actual number of atoms in it is called its molecular

formula.

Molecular formula = n Empirical formula

= (Empirical formula)

Where n may be 1, 2, 3...


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In SI system, MOLE (symbol, mol) was introduced as seventh base quantity

for the amount of a substance.

One mole is the amount of a substance that contains as many particles

or entities as there are atoms in exactly 12g (or 0.012 kg) of the 12C isotope.

Avogadros Constant

We can say that 1 mol of hydrogen atoms = 6.022 1023 atoms

1 mol of water molecules = 6.022 1023 water molecules1 mol of sodium chloride = 6.022 1023 formula units of sodium chloride.

The mass of one mol of a substance in grams is called its molar mass .

The molar mass in grams is numerically equal to atomic/molecular/formula mass

in amu.

Molar mass of water = 18.02 g

Molar mass of sodium chloride = 58.5 g

Chemical Stoichiometry:

(i) Chemical Equation: A short hand representation of a chemicalreaction in terms of symbols and formulae of the substances involved in the

reaction is called Chemical Equation.

Essentials of a Chemical Equation

(a)It should represent a true chemical change(b)It should be balanced.(c)It should be molecular.(ii) Limiting Reagent: The reacting substance which gets used up first in

the reaction is called the Limiting reagent. This is because the amount of the

limiting reagent limits the amount of the products formed. A p art of the other

reactants which are present in amounts greater than the stoichimetric amounts is

left behind as unconsumed reagents.

Stoichimetry ofReactions is Solutions:

(i) Mass percentage or percent by mass: it is defined as the mass ofsolute in gram per 100g of the solution. For example, a 10% solution of sodium

chloride means that 10g of NaCl are present in 100g of the solution.

Both, the mass of solute and that of the solution must be expressed in the same

mass units. viz. both in grams or both in kilograms, etc.

(ii) Volume percentage of percent by Volume: The number of units ofvolume of the solute per 100 units of volume of the solution is known as volume

percent. For example, a 10% ( /V) solution of ethyl alcohol contains 10 mL of

alcohol in 100 mL of the solution. Mathematically, volume percent is expressed as

(iii) Molarity (M): Molarity of a solution is defined as the number of molesof solute dissolved per dm3 (or liter, L) of the solution. Molarity of any solution

depends upon temperature. So, molarity of any solution is specified for a given

temperature. Mathematically, molarity is defined as:


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Where n is the number of moles of the solute.

V is the volume of the solution in liters (or dm3)

Since the number of moles of any substance is related to its mass and the

molar mass,

The molarity of a solution then can also be expressed as:

So, W grams of a substance having molar mass M, is dissolved in sufficient

solvent, so as to make the total volume of V liter of the substance, the molarity (M)

of the solution is given by

(ii) Formality: The number of gram formula mass dissolved per liter of

the solution is called formality of the solution.

(iii) Normality (N): Normality of a solution is defined as the number ofequivalents of the solute dissolved per liter (or dm3) of the solution at any specified

temperature. Thus,

Since, the number of equivalents of any solute can be obtained from its

mass and equivalents mass, hence

Or

Normality of a solution also varies with temperature.

(iv) Molality (m): Molality of a solution is defined as the number ofmoles of solute per kg of the solvent. If a solution is prepared by dissolving n

moles of a solute in W kg of the solvent then.


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(v) Mole fraction (X):The mole fraction of any component of a solution isdefined as the ratio of the number of moles of that component to the total

number of moles of all the components of the solution. Thus if a solution

contains moles of A and moles of B, then

If the mole fraction of one component in a binary solution is known, that of

the other can be determined.

i.e.

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