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 \({10}^{23}\)

Q5. How many atoms are there in

(i). 100 u of helium,

(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

Q7. How many grams of each of the following elements must be taken to get 1 mol of the element?

        (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

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