The combustion of ethane at 25 o C can be represented as follows: 2 C 2 H 6 g + 7 O 2 g → 4 C O 2 g + 6 H 2 O ( g ) If X molecules of ethane undergo combustion, What is the vapour density of ethane? ( A t o m i c m a s s of H = 1 a n d C = 12 )

Vapour density of a compound = Molecular mass of the compound 2 Molecular mass of ethane = 2 × At . wt of C + 6 × At . wt of H = 2 × 12 + 6 × 1 = 30 g Vapour density of ethane = 30 2 = 15 Thus, vapour density of ethane= 15

Match the following: (i) Molecular weight (a) 22 . 4 litres at STP (ii) Number of molecules (b) 2 × Vapour density (iii) Mass of one atom (c) Mass of an element Atomic mass (iv) Moles of an element (d) Atomic mass 6 . 022 × 10 23 (v) One mole of an element (e) Moles × 6 . 022 × 10 23

(i) Molecular weight or more precisely relative molecular mass is twice the value of vapour density. Vapour density is the density of a vapour in relation to that of hydrogen. (ii) Number of molecules is equal to number of moles × 6 . 022 × 10 23 . This is according to Avogadro's law. (iii) Mass of one atom is equal to Atomic mass 6 . 022 × 10 23 . Atomic mass is the mass of an atom of a chemical element expressed in atomic mass units. It is approximately equivalent to the number of protons and neutrons in the atom (iv) Moles of an element is calculated as Mass of the element Atomic mass . The mole is the unit of measurement for amount of substance in the International System of Units. (v) One mole of an element has a volume of 22 . 4 litres at STP. It is called the molar volume. (i) Molecular weight (b) 2 × vapour density (ii) Number of molecules (e) Moles × 6 . 022 × 10 23 (iii) Mass of one atom (c) Atomic mass 6 . 022 × 10 23 (iv) Moles of an element (d) Mass of element Atomic mass (v) One mole of an element (a) 22 . 4 litres at STP

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Victor Meyer's method is used for the determination of molecular weight of a volatile compound.

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Important Questions on Some Basic Concepts of Chemistry

EASY

Chemistry>Physical Chemistry>Some Basic Concepts of Chemistry>Determination of Relative Molecular Masses

Explain why some gases show abnormal density.

HARD

Chemistry>Physical Chemistry>Some Basic Concepts of Chemistry>Determination of Relative Molecular Masses

Establish the relationship between molecular weight and vapour density with the help of Avogadro's hypothesis.

MEDIUM

Chemistry>Physical Chemistry>Some Basic Concepts of Chemistry>Determination of Relative Molecular Masses

The combustion of ethane at $25^{o} C$ can be represented as follows:
$2 C_{2} H_{6} (g) + 7 O_{2} (g) \to 4 C O_{2} (g) + 6 H_{2} O (g)$

If $X$ molecules of ethane undergo combustion,
What is the vapour density of ethane?
$(A t o m i c m a s s of H = 1 a n d C = 12)$

EASY

Chemistry>Physical Chemistry>Some Basic Concepts of Chemistry>Determination of Relative Molecular Masses

After inserting Hofmanns' bottle, why should Victor Meyer's tube is corked at once ?

EASY

Chemistry>Physical Chemistry>Some Basic Concepts of Chemistry>Determination of Relative Molecular Masses

Why should the Victor Meyer's tube be dried before start of the experiment ?

HARD

Chemistry>Physical Chemistry>Some Basic Concepts of Chemistry>Determination of Relative Molecular Masses

Derive the relationship between Relative molecular mass and Vapour density.

HARD

Chemistry>Physical Chemistry>Some Basic Concepts of Chemistry>Determination of Relative Molecular Masses

Describe Victor Meyer's method for determining the molecular mass of a substance.

0.1

gm of a volatile substance on vaporisation in Victor Meyer's experiment displaced

123

ml moist air at

15 ° C

and

740

mm pressure. Calculate the molecular mass of the substance. (Aqueous tension at

15 ° C = 12.7

mm)

MEDIUM

Chemistry>Physical Chemistry>Some Basic Concepts of Chemistry>Determination of Relative Molecular Masses

Represent the union of two sets by Venn diagram for each of the following.

$X = {x | x$ is a prime number between $80$ and $100}$

$Y = {y | y$ is an odd number between $90$ and $100}$

EASY

Chemistry>Physical Chemistry>Some Basic Concepts of Chemistry>Determination of Relative Molecular Masses

0.24 g

of a volatile compound upon vaporization gives

45 mL

vapour at

NTP

. Vapour density of the substance will be

MEDIUM

Chemistry>Physical Chemistry>Some Basic Concepts of Chemistry>Determination of Relative Molecular Masses

Correct the following statement:

Relative atomic mass of an element is the number of times one molecule of an element is heavier than $\frac{1}{12}$ the mass of an atom of carbon- $12$ .

EASY

Chemistry>Physical Chemistry>Some Basic Concepts of Chemistry>Determination of Relative Molecular Masses

Which method is suitable for determining the molecular mass of volatile substances?

EASY

Chemistry>Physical Chemistry>Some Basic Concepts of Chemistry>Determination of Relative Molecular Masses

Why is asbestos or glass wool placed in the Victor Meyer's cylindrical glass vessel ?

HARD

Chemistry>Physical Chemistry>Some Basic Concepts of Chemistry>Determination of Relative Molecular Masses

Match the following:

(i) Molecular weight	(a) $22.4$ litres at STP
(ii) Number of molecules	(b) $2 \times Vapour density$
(iii) Mass of one atom	(c) $\frac{Mass of an element}{Atomic mass}$
(iv) Moles of an element	(d) $\frac{Atomic mass}{6.022 \times 10^{23}}$
(v) One mole of an element	(e) $Moles \times 6.022 \times 10^{23}$