## Q1. Write the names of symbols of five elements where the symbols are taken from their name in a language other than English.

Ans. The names of symbols of the following elements have been taken from their name in a language other than English: Silver (Ag), gold (Au), lead (Pb), sodium (Na), iron (Fe) etc.

## Q2. What is valency? Mention a use of valency.

Ans. Valency is the combining capacity of an element. It can be used to find out how many atoms of an element will combine with the other element to form a chemical formula. For example, hydrogen has a valency + 1 and chlorine has a valency —1, so one atom of hydrogen combines with one atom of chlorine to form hydrochloric acid (HC1). Oxygen has a valency —2, so one atom of oxygen combines with two atoms of hydrogen to form water molecule ($${ H }_{ 2 }O$$).

## Q3. How many atoms are present in 0.35 mol of pure copper metal?

Ans. 1 mol of copper metal = 6.022 x $${10}^{23}$$ atoms of Cu

Therefore 0.35 mol of copper metal = 0.35 x 6.022 x $${10}^{23}$$ atoms of Cu

= 2.1 x $${10}^{23}$$ atoms of Cu

## Q4. An atom of neon has a mass of 3.35 x $${10}^{-23}$$ g. How many atoms of neon are there in 20 g of the gas?

Ans. Number of atoms

= $$\frac{Total mass}{Mass of 1 atom}$$

= $$\frac { 20 g }{ 3.35\times { 10 }^{ 23 } g }$$

= 5.97 x $${10}^{23}$$

## (ii). 100 g of helium? (atomic mass of helium is 4 u)

Ans.

(i). Number of He atoms in 100 u = $$\frac{Total mass of helium}{Mass of 1 atom of helium}$$

= $$\frac{100 u}{4 u}$$

= 25

(ii). Number of He atoms in 1OO g = No. of moles of helium x Avogadro number

= $$\frac{Total mass}{Mass of 1 atom}$$ x 6.022 x $${10}^{23}$$
= $$\frac{100 g}{4 g}$$ x 6.022 x $${10}^{23}$$
= 25 x 6.022 x $${10}^{23}$$
= 1.5 x $${10}^{25}$$

## Q6. Calculate the number of moles in 5.75 g of sodium. (atomic mass of sodium is 23)

Ans. 1 mol of sodium atoms = Gram atomic mass of sodium = 23 g

23 g of sodium = 1 mol of sodium

5.75 g of sodium = (5.75/23) mol of sodium = 0.25 mol

## (i). Sodium             (ii). Chlorine         (iii). Copper

Ans. The mass of 1 mol of an element is its atomic mass expressed in grams. Since the atomic masses of sodium, chlorine and copper are 23 g, 35.5 g and 63.6 g respectively, we get

(i). 23 g             (ii). 35.5 g             (iii). 63.5 g

## One thought on “Atoms and Molecules – Short Answer Questions”

1. mahir says:

I need more questions like these. Tey helped me a lot in gaining knowledge.