Question

A. The elements Li, Na, and K, each having one valence electron, are in period 2, 3 and 4 respectively of modern periodic table.

(i) To which group of the modern periodic table these elements belong? [1 mark]
(ii)Which one of them is least reactive? [1 mark]
(iii) Which one of them has the largest atomic radius? [1 mark]

B. What is a chemical symbol of an element? What information does it convey? [3 marks]

C. (i) Explain the effect of temperature on solubility of solid solutes. [1 mark]
(ii) A student wants to prepare a solution of acetone and water having $30\%$ v/v. If $6mL$ of acetone has been taken then how much water must be added to get the desired concentration? [3 marks]

Answer 1

A.(i)The given elements belong to Group 1 of the modern periodic table as all of them have one valence electron. [1 mark]
(ii) Li is least reactive because its outermost orbit is very near to the nucleus. So the removal of electron is difficult. [1 mark]
(iii) K has the largest atomic radius because it has more number of shells as compared to Na and Li. [1 mark]

B.
(i) A chemical symbol is the short form which stands for an atom of a specific element or the abbreviations used for the names of elements. [1 mark]
(ii) It represents a specific element. [0.5 mark]
(iii) It represents one atom of an element. [0.5 mark]
(iv) A symbol represents how many atoms are present in one gram (gm) atom. [0.5 mark]
(v) The chemical symbol of an element represents the mass of an atom of that element. [0.5 mark]

C. (i) Solubility of most of the solid solutes is directly proportional to the temperature. If the temperature increases, the solubility of most of the solid solutes increases.
[0.5 Marks]

For example- If we heat a saturated solution containing excess undissolved solute in it at a particular temperature, the excess solute dissolves in it. Which means solubility increases with the increase in temperature.
[0.5 Marks]

(ii) Given: Volume of solute (acetone) $=6mL$
Concentration (v/v) of solution $=30\%$
To find: The amount of water (in mL) required to get the desired concentration of $30\%$ v/v.

Solution:
Let the volume of water (solvent) required to be x.
So, the volume of solution $=6+x$
We know that,
Concentration of a solution (v/v%) $=\frac{Volumeofsolute}{Volumeofsolution}\times 100$
[1 mark]

On putting the values in the formula, we get
$\Rightarrow \frac{6}{6+x}\times 100=30$
$\Rightarrow 600=180+30x$
$\Rightarrow 30x=600-180$
$\Rightarrow x=\frac{420}{30}$
$x=14mL$
[2 marks]