Question

Observe the reactions given below:​

$2Mg$ (s) +$O_2$ (g) $\to$ A​

CuO (s) + $2HCl$ (aq) $\to$ B + $H_{2}O$​ (l)

$S{O}_2$ (g) + $H_{2}O$ (l) $\to$ C​

$C{O}_2$ (g) + 2NaOH (aq) $\to$ D +$H_{2}O$ ​(l)

Which of the following are the correct options?

• A. C will have pH value greater than 7.
• B. A will turn phenolphthalein solution pink to colourless.
• C. B will have a pH value below 7.
• D. D will turn blue litmus to red.

Answer 1

The correct option is C B will have a pH value below 7.

In the first reaction,
$2Mg$ (s) +$O_2$ (g) $\to$ 2MgO (s)
A is MgO (s). MgO is metal oxide so, it is basic in nature. So it will not turn pink phenolphthalein solution to colourless.
Hence, option (a) is incorrect.

In the second reaction,
CuO (s) + $2HCl$ (aq) $\to$ ${CuCl}_2$ (s) + $H_{2}O$​ (l)
B is ${CuCl}_2$ (s). This salt is formed by strong acid and weak base so its pH value will be less than 7.
Hence, option (b) is correct.

In the third reaction,
$S{O}_2$ (g) + $H_{2}O$ (l) $\to$ $H_2{SO}_3$ (l)
C is $H_2{SO}_3$ (l). $H_2{SO}_3$ is a weak acid so its pH value will be lower than 7.
Hence, option (c) is incorrect.

In the fourth reaction,
$C{O}_2$ (g) + 2NaOH (aq) $\to$ ${Na}_2{CO}_3$ (aq) +$H_{2}O$ ​(l)
D is ${Na}_2{CO}_3$ (aq) which is a weak base. It will not turn blue litmus red.
Hence, option (d) is incorrect.